/*
 * QEMU System Emulator block driver
 *
 * Copyright (c) 2003 Fabrice Bellard
 * Copyright (c) 2020 Virtuozzo International GmbH.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include "qemu/osdep.h"
#include "block/trace.h"
#include "block/block_int.h"
#include "block/blockjob.h"
#include "block/dirty-bitmap.h"
#include "block/fuse.h"
#include "block/nbd.h"
#include "block/qdict.h"
#include "qemu/error-report.h"
#include "block/module_block.h"
#include "qemu/main-loop.h"
#include "qemu/module.h"
#include "qapi/error.h"
#include "qobject/qdict.h"
#include "qobject/qjson.h"
#include "qobject/qnull.h"
#include "qobject/qstring.h"
#include "qapi/qobject-output-visitor.h"
#include "qapi/qapi-visit-block-core.h"
#include "system/block-backend.h"
#include "qemu/notify.h"
#include "qemu/option.h"
#include "qemu/coroutine.h"
#include "block/qapi.h"
#include "qemu/timer.h"
#include "qemu/cutils.h"
#include "qemu/id.h"
#include "qemu/range.h"
#include "qemu/rcu.h"
#include "block/coroutines.h"

#ifdef CONFIG_BSD
#include <sys/ioctl.h>
#include <sys/queue.h>
#if defined(HAVE_SYS_DISK_H)
#include <sys/disk.h>
#endif
#endif

#ifdef _WIN32
#include <windows.h>
#endif

#define NOT_DONE 0x7fffffff /* used while emulated sync operation in progress */

/* Protected by BQL */
static QTAILQ_HEAD(, BlockDriverState) graph_bdrv_states =
    QTAILQ_HEAD_INITIALIZER(graph_bdrv_states);

/* Protected by BQL */
static QTAILQ_HEAD(, BlockDriverState) all_bdrv_states =
    QTAILQ_HEAD_INITIALIZER(all_bdrv_states);

/* Protected by BQL */
static QLIST_HEAD(, BlockDriver) bdrv_drivers =
    QLIST_HEAD_INITIALIZER(bdrv_drivers);

static BlockDriverState *bdrv_open_inherit(const char *filename,
                                           const char *reference,
                                           QDict *options, int flags,
                                           BlockDriverState *parent,
                                           const BdrvChildClass *child_class,
                                           BdrvChildRole child_role,
                                           bool parse_filename,
                                           Error **errp);

static bool bdrv_recurse_has_child(BlockDriverState *bs,
                                   BlockDriverState *child);

static void GRAPH_WRLOCK
bdrv_replace_child_noperm(BdrvChild *child, BlockDriverState *new_bs);

static void GRAPH_WRLOCK
bdrv_remove_child(BdrvChild *child, Transaction *tran);

static int bdrv_reopen_prepare(BDRVReopenState *reopen_state,
                               BlockReopenQueue *queue,
                               Transaction *change_child_tran, Error **errp);
static void bdrv_reopen_commit(BDRVReopenState *reopen_state);
static void bdrv_reopen_abort(BDRVReopenState *reopen_state);

static bool bdrv_backing_overridden(BlockDriverState *bs);

static bool GRAPH_RDLOCK
bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
                        GHashTable *visited, Transaction *tran, Error **errp);

/* If non-zero, use only whitelisted block drivers */
static int use_bdrv_whitelist;

#ifdef _WIN32
static int is_windows_drive_prefix(const char *filename)
{
    return (((filename[0] >= 'a' && filename[0] <= 'z') ||
             (filename[0] >= 'A' && filename[0] <= 'Z')) &&
            filename[1] == ':');
}

int is_windows_drive(const char *filename)
{
    if (is_windows_drive_prefix(filename) &&
        filename[2] == '\0')
        return 1;
    if (strstart(filename, "\\\\.\\", NULL) ||
        strstart(filename, "//./", NULL))
        return 1;
    return 0;
}
#endif

size_t bdrv_opt_mem_align(BlockDriverState *bs)
{
    if (!bs || !bs->drv) {
        /* page size or 4k (hdd sector size) should be on the safe side */
        return MAX(4096, qemu_real_host_page_size());
    }
    IO_CODE();

    return bs->bl.opt_mem_alignment;
}

size_t bdrv_min_mem_align(BlockDriverState *bs)
{
    if (!bs || !bs->drv) {
        /* page size or 4k (hdd sector size) should be on the safe side */
        return MAX(4096, qemu_real_host_page_size());
    }
    IO_CODE();

    return bs->bl.min_mem_alignment;
}

/* check if the path starts with "<protocol>:" */
int path_has_protocol(const char *path)
{
    const char *p;

#ifdef _WIN32
    if (is_windows_drive(path) ||
        is_windows_drive_prefix(path)) {
        return 0;
    }
    p = path + strcspn(path, ":/\\");
#else
    p = path + strcspn(path, ":/");
#endif

    return *p == ':';
}

int path_is_absolute(const char *path)
{
#ifdef _WIN32
    /* specific case for names like: "\\.\d:" */
    if (is_windows_drive(path) || is_windows_drive_prefix(path)) {
        return 1;
    }
    return (*path == '/' || *path == '\\');
#else
    return (*path == '/');
#endif
}

/* if filename is absolute, just return its duplicate. Otherwise, build a
   path to it by considering it is relative to base_path. URL are
   supported. */
char *path_combine(const char *base_path, const char *filename)
{
    const char *protocol_stripped = NULL;
    const char *p, *p1;
    char *result;
    int len;

    if (path_is_absolute(filename)) {
        return g_strdup(filename);
    }

    if (path_has_protocol(base_path)) {
        protocol_stripped = strchr(base_path, ':');
        if (protocol_stripped) {
            protocol_stripped++;
        }
    }
    p = protocol_stripped ?: base_path;

    p1 = strrchr(base_path, '/');
#ifdef _WIN32
    {
        const char *p2;
        p2 = strrchr(base_path, '\\');
        if (!p1 || p2 > p1) {
            p1 = p2;
        }
    }
#endif
    if (p1) {
        p1++;
    } else {
        p1 = base_path;
    }
    if (p1 > p) {
        p = p1;
    }
    len = p - base_path;

    result = g_malloc(len + strlen(filename) + 1);
    memcpy(result, base_path, len);
    strcpy(result + len, filename);

    return result;
}

/*
 * Helper function for bdrv_parse_filename() implementations to remove optional
 * protocol prefixes (especially "file:") from a filename and for putting the
 * stripped filename into the options QDict if there is such a prefix.
 */
void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
                                      QDict *options)
{
    if (strstart(filename, prefix, &filename)) {
        /* Stripping the explicit protocol prefix may result in a protocol
         * prefix being (wrongly) detected (if the filename contains a colon) */
        if (path_has_protocol(filename)) {
            GString *fat_filename;

            /* This means there is some colon before the first slash; therefore,
             * this cannot be an absolute path */
            assert(!path_is_absolute(filename));

            /* And we can thus fix the protocol detection issue by prefixing it
             * by "./" */
            fat_filename = g_string_new("./");
            g_string_append(fat_filename, filename);

            assert(!path_has_protocol(fat_filename->str));

            qdict_put(options, "filename",
                      qstring_from_gstring(fat_filename));
        } else {
            /* If no protocol prefix was detected, we can use the shortened
             * filename as-is */
            qdict_put_str(options, "filename", filename);
        }
    }
}


/* Returns whether the image file is opened as read-only. Note that this can
 * return false and writing to the image file is still not possible because the
 * image is inactivated. */
bool bdrv_is_read_only(BlockDriverState *bs)
{
    IO_CODE();
    return !(bs->open_flags & BDRV_O_RDWR);
}

static int GRAPH_RDLOCK
bdrv_can_set_read_only(BlockDriverState *bs, bool read_only,
                       bool ignore_allow_rdw, Error **errp)
{
    IO_CODE();

    /* Do not set read_only if copy_on_read is enabled */
    if (bs->copy_on_read && read_only) {
        error_setg(errp, "Can't set node '%s' to r/o with copy-on-read enabled",
                   bdrv_get_device_or_node_name(bs));
        return -EINVAL;
    }

    /* Do not clear read_only if it is prohibited */
    if (!read_only && !(bs->open_flags & BDRV_O_ALLOW_RDWR) &&
        !ignore_allow_rdw)
    {
        error_setg(errp, "Node '%s' is read only",
                   bdrv_get_device_or_node_name(bs));
        return -EPERM;
    }

    return 0;
}

/*
 * Called by a driver that can only provide a read-only image.
 *
 * Returns 0 if the node is already read-only or it could switch the node to
 * read-only because BDRV_O_AUTO_RDONLY is set.
 *
 * Returns -EACCES if the node is read-write and BDRV_O_AUTO_RDONLY is not set
 * or bdrv_can_set_read_only() forbids making the node read-only. If @errmsg
 * is not NULL, it is used as the error message for the Error object.
 */
int bdrv_apply_auto_read_only(BlockDriverState *bs, const char *errmsg,
                              Error **errp)
{
    int ret = 0;
    IO_CODE();

    if (!(bs->open_flags & BDRV_O_RDWR)) {
        return 0;
    }
    if (!(bs->open_flags & BDRV_O_AUTO_RDONLY)) {
        goto fail;
    }

    ret = bdrv_can_set_read_only(bs, true, false, NULL);
    if (ret < 0) {
        goto fail;
    }

    bs->open_flags &= ~BDRV_O_RDWR;

    return 0;

fail:
    error_setg(errp, "%s", errmsg ?: "Image is read-only");
    return -EACCES;
}

/*
 * If @backing is empty, this function returns NULL without setting
 * @errp.  In all other cases, NULL will only be returned with @errp
 * set.
 *
 * Therefore, a return value of NULL without @errp set means that
 * there is no backing file; if @errp is set, there is one but its
 * absolute filename cannot be generated.
 */
char *bdrv_get_full_backing_filename_from_filename(const char *backed,
                                                   const char *backing,
                                                   Error **errp)
{
    if (backing[0] == '\0') {
        return NULL;
    } else if (path_has_protocol(backing) || path_is_absolute(backing)) {
        return g_strdup(backing);
    } else if (backed[0] == '\0' || strstart(backed, "json:", NULL)) {
        error_setg(errp, "Cannot use relative backing file names for '%s'",
                   backed);
        return NULL;
    } else {
        return path_combine(backed, backing);
    }
}

/*
 * If @filename is empty or NULL, this function returns NULL without
 * setting @errp.  In all other cases, NULL will only be returned with
 * @errp set.
 */
static char * GRAPH_RDLOCK
bdrv_make_absolute_filename(BlockDriverState *relative_to,
                            const char *filename, Error **errp)
{
    char *dir, *full_name;

    if (!filename || filename[0] == '\0') {
        return NULL;
    } else if (path_has_protocol(filename) || path_is_absolute(filename)) {
        return g_strdup(filename);
    }

    dir = bdrv_dirname(relative_to, errp);
    if (!dir) {
        return NULL;
    }

    full_name = g_strconcat(dir, filename, NULL);
    g_free(dir);
    return full_name;
}

char *bdrv_get_full_backing_filename(BlockDriverState *bs, Error **errp)
{
    GLOBAL_STATE_CODE();
    return bdrv_make_absolute_filename(bs, bs->backing_file, errp);
}

void bdrv_register(BlockDriver *bdrv)
{
    assert(bdrv->format_name);
    GLOBAL_STATE_CODE();
    QLIST_INSERT_HEAD(&bdrv_drivers, bdrv, list);
}

BlockDriverState *bdrv_new(void)
{
    BlockDriverState *bs;
    int i;

    GLOBAL_STATE_CODE();

    bs = g_new0(BlockDriverState, 1);
    QLIST_INIT(&bs->dirty_bitmaps);
    for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
        QLIST_INIT(&bs->op_blockers[i]);
    }
    qemu_mutex_init(&bs->reqs_lock);
    qemu_mutex_init(&bs->dirty_bitmap_mutex);
    bs->refcnt = 1;
    bs->aio_context = qemu_get_aio_context();

    qemu_co_queue_init(&bs->flush_queue);

    qemu_co_mutex_init(&bs->bsc_modify_lock);
    bs->block_status_cache = g_new0(BdrvBlockStatusCache, 1);

    for (i = 0; i < bdrv_drain_all_count; i++) {
        bdrv_do_drained_begin_quiesce(bs, NULL);
    }

    QTAILQ_INSERT_TAIL(&all_bdrv_states, bs, bs_list);

    return bs;
}

static BlockDriver *bdrv_do_find_format(const char *format_name)
{
    BlockDriver *drv1;
    GLOBAL_STATE_CODE();

    QLIST_FOREACH(drv1, &bdrv_drivers, list) {
        if (!strcmp(drv1->format_name, format_name)) {
            return drv1;
        }
    }

    return NULL;
}

BlockDriver *bdrv_find_format(const char *format_name)
{
    BlockDriver *drv1;
    int i;

    GLOBAL_STATE_CODE();

    drv1 = bdrv_do_find_format(format_name);
    if (drv1) {
        return drv1;
    }

    /* The driver isn't registered, maybe we need to load a module */
    for (i = 0; i < (int)ARRAY_SIZE(block_driver_modules); ++i) {
        if (!strcmp(block_driver_modules[i].format_name, format_name)) {
            Error *local_err = NULL;
            int rv = block_module_load(block_driver_modules[i].library_name,
                                       &local_err);
            if (rv > 0) {
                return bdrv_do_find_format(format_name);
            } else if (rv < 0) {
                error_report_err(local_err);
            }
            break;
        }
    }
    return NULL;
}

static int bdrv_format_is_whitelisted(const char *format_name, bool read_only)
{
    static const char *whitelist_rw[] = {
        CONFIG_BDRV_RW_WHITELIST
        NULL
    };
    static const char *whitelist_ro[] = {
        CONFIG_BDRV_RO_WHITELIST
        NULL
    };
    const char **p;

    if (!whitelist_rw[0] && !whitelist_ro[0]) {
        return 1;               /* no whitelist, anything goes */
    }

    for (p = whitelist_rw; *p; p++) {
        if (!strcmp(format_name, *p)) {
            return 1;
        }
    }
    if (read_only) {
        for (p = whitelist_ro; *p; p++) {
            if (!strcmp(format_name, *p)) {
                return 1;
            }
        }
    }
    return 0;
}

int bdrv_is_whitelisted(BlockDriver *drv, bool read_only)
{
    GLOBAL_STATE_CODE();
    return bdrv_format_is_whitelisted(drv->format_name, read_only);
}

bool bdrv_uses_whitelist(void)
{
    return use_bdrv_whitelist;
}

typedef struct CreateCo {
    BlockDriver *drv;
    char *filename;
    QemuOpts *opts;
    int ret;
    Error *err;
} CreateCo;

int coroutine_fn bdrv_co_create(BlockDriver *drv, const char *filename,
                                QemuOpts *opts, Error **errp)
{
    ERRP_GUARD();
    int ret;
    GLOBAL_STATE_CODE();

    if (!drv->bdrv_co_create_opts) {
        error_setg(errp, "Driver '%s' does not support image creation",
                   drv->format_name);
        return -ENOTSUP;
    }

    ret = drv->bdrv_co_create_opts(drv, filename, opts, errp);
    if (ret < 0 && !*errp) {
        error_setg_errno(errp, -ret, "Could not create image");
    }

    return ret;
}

/**
 * Helper function for bdrv_create_file_fallback(): Resize @blk to at
 * least the given @minimum_size.
 *
 * On success, return @blk's actual length.
 * Otherwise, return -errno.
 */
static int64_t coroutine_fn GRAPH_UNLOCKED
create_file_fallback_truncate(BlockBackend *blk, int64_t minimum_size,
                              Error **errp)
{
    Error *local_err = NULL;
    int64_t size;
    int ret;

    GLOBAL_STATE_CODE();

    ret = blk_co_truncate(blk, minimum_size, false, PREALLOC_MODE_OFF, 0,
                          &local_err);
    if (ret < 0 && ret != -ENOTSUP) {
        error_propagate(errp, local_err);
        return ret;
    }

    size = blk_co_getlength(blk);
    if (size < 0) {
        error_free(local_err);
        error_setg_errno(errp, -size,
                         "Failed to inquire the new image file's length");
        return size;
    }

    if (size < minimum_size) {
        /* Need to grow the image, but we failed to do that */
        error_propagate(errp, local_err);
        return -ENOTSUP;
    }

    error_free(local_err);
    local_err = NULL;

    return size;
}

/**
 * Helper function for bdrv_create_file_fallback(): Zero the first
 * sector to remove any potentially pre-existing image header.
 */
static int coroutine_fn
create_file_fallback_zero_first_sector(BlockBackend *blk,
                                       int64_t current_size,
                                       Error **errp)
{
    int64_t bytes_to_clear;
    int ret;

    GLOBAL_STATE_CODE();

    bytes_to_clear = MIN(current_size, BDRV_SECTOR_SIZE);
    if (bytes_to_clear) {
        ret = blk_co_pwrite_zeroes(blk, 0, bytes_to_clear, BDRV_REQ_MAY_UNMAP);
        if (ret < 0) {
            error_setg_errno(errp, -ret,
                             "Failed to clear the new image's first sector");
            return ret;
        }
    }

    return 0;
}

/**
 * Simple implementation of bdrv_co_create_opts for protocol drivers
 * which only support creation via opening a file
 * (usually existing raw storage device)
 */
int coroutine_fn bdrv_co_create_opts_simple(BlockDriver *drv,
                                            const char *filename,
                                            QemuOpts *opts,
                                            Error **errp)
{
    ERRP_GUARD();
    BlockBackend *blk;
    QDict *options;
    int64_t size = 0;
    char *buf = NULL;
    PreallocMode prealloc;
    Error *local_err = NULL;
    int ret;

    GLOBAL_STATE_CODE();

    size = qemu_opt_get_size_del(opts, BLOCK_OPT_SIZE, 0);
    buf = qemu_opt_get_del(opts, BLOCK_OPT_PREALLOC);
    prealloc = qapi_enum_parse(&PreallocMode_lookup, buf,
                               PREALLOC_MODE_OFF, &local_err);
    g_free(buf);
    if (local_err) {
        error_propagate(errp, local_err);
        return -EINVAL;
    }

    if (prealloc != PREALLOC_MODE_OFF) {
        error_setg(errp, "Unsupported preallocation mode '%s'",
                   PreallocMode_str(prealloc));
        return -ENOTSUP;
    }

    options = qdict_new();
    qdict_put_str(options, "driver", drv->format_name);

    blk = blk_co_new_open(filename, NULL, options,
                          BDRV_O_RDWR | BDRV_O_RESIZE, errp);
    if (!blk) {
        error_prepend(errp, "Protocol driver '%s' does not support creating "
                      "new images, so an existing image must be selected as "
                      "the target; however, opening the given target as an "
                      "existing image failed: ",
                      drv->format_name);
        return -EINVAL;
    }

    size = create_file_fallback_truncate(blk, size, errp);
    if (size < 0) {
        ret = size;
        goto out;
    }

    ret = create_file_fallback_zero_first_sector(blk, size, errp);
    if (ret < 0) {
        goto out;
    }

    ret = 0;
out:
    blk_co_unref(blk);
    return ret;
}

int coroutine_fn bdrv_co_create_file(const char *filename, QemuOpts *opts,
                                     Error **errp)
{
    QemuOpts *protocol_opts;
    BlockDriver *drv;
    QDict *qdict;
    int ret;

    GLOBAL_STATE_CODE();

    drv = bdrv_find_protocol(filename, true, errp);
    if (drv == NULL) {
        return -ENOENT;
    }

    if (!drv->create_opts) {
        error_setg(errp, "Driver '%s' does not support image creation",
                   drv->format_name);
        return -ENOTSUP;
    }

    /*
     * 'opts' contains a QemuOptsList with a combination of format and protocol
     * default values.
     *
     * The format properly removes its options, but the default values remain
     * in 'opts->list'.  So if the protocol has options with the same name
     * (e.g. rbd has 'cluster_size' as qcow2), it will see the default values
     * of the format, since for overlapping options, the format wins.
     *
     * To avoid this issue, lets convert QemuOpts to QDict, in this way we take
     * only the set options, and then convert it back to QemuOpts, using the
     * create_opts of the protocol. So the new QemuOpts, will contain only the
     * protocol defaults.
     */
    qdict = qemu_opts_to_qdict(opts, NULL);
    protocol_opts = qemu_opts_from_qdict(drv->create_opts, qdict, errp);
    if (protocol_opts == NULL) {
        ret = -EINVAL;
        goto out;
    }

    ret = bdrv_co_create(drv, filename, protocol_opts, errp);
out:
    qemu_opts_del(protocol_opts);
    qobject_unref(qdict);
    return ret;
}

int coroutine_fn bdrv_co_delete_file(BlockDriverState *bs, Error **errp)
{
    Error *local_err = NULL;
    int ret;

    IO_CODE();
    assert(bs != NULL);
    assert_bdrv_graph_readable();

    if (!bs->drv) {
        error_setg(errp, "Block node '%s' is not opened", bs->filename);
        return -ENOMEDIUM;
    }

    if (!bs->drv->bdrv_co_delete_file) {
        error_setg(errp, "Driver '%s' does not support image deletion",
                   bs->drv->format_name);
        return -ENOTSUP;
    }

    ret = bs->drv->bdrv_co_delete_file(bs, &local_err);
    if (ret < 0) {
        error_propagate(errp, local_err);
    }

    return ret;
}

void coroutine_fn bdrv_co_delete_file_noerr(BlockDriverState *bs)
{
    Error *local_err = NULL;
    int ret;
    IO_CODE();

    if (!bs) {
        return;
    }

    ret = bdrv_co_delete_file(bs, &local_err);
    /*
     * ENOTSUP will happen if the block driver doesn't support
     * the 'bdrv_co_delete_file' interface. This is a predictable
     * scenario and shouldn't be reported back to the user.
     */
    if (ret == -ENOTSUP) {
        error_free(local_err);
    } else if (ret < 0) {
        error_report_err(local_err);
    }
}

/**
 * Try to get @bs's logical and physical block size.
 * On success, store them in @bsz struct and return 0.
 * On failure return -errno.
 * @bs must not be empty.
 */
int bdrv_probe_blocksizes(BlockDriverState *bs, BlockSizes *bsz)
{
    BlockDriver *drv = bs->drv;
    BlockDriverState *filtered = bdrv_filter_bs(bs);
    GLOBAL_STATE_CODE();

    if (drv && drv->bdrv_probe_blocksizes) {
        return drv->bdrv_probe_blocksizes(bs, bsz);
    } else if (filtered) {
        return bdrv_probe_blocksizes(filtered, bsz);
    }

    return -ENOTSUP;
}

/**
 * Try to get @bs's geometry (cyls, heads, sectors).
 * On success, store them in @geo struct and return 0.
 * On failure return -errno.
 * @bs must not be empty.
 */
int bdrv_probe_geometry(BlockDriverState *bs, HDGeometry *geo)
{
    BlockDriver *drv = bs->drv;
    BlockDriverState *filtered;

    GLOBAL_STATE_CODE();
    GRAPH_RDLOCK_GUARD_MAINLOOP();

    if (drv && drv->bdrv_probe_geometry) {
        return drv->bdrv_probe_geometry(bs, geo);
    }

    filtered = bdrv_filter_bs(bs);
    if (filtered) {
        return bdrv_probe_geometry(filtered, geo);
    }

    return -ENOTSUP;
}

/*
 * Create a uniquely-named empty temporary file.
 * Return the actual file name used upon success, otherwise NULL.
 * This string should be freed with g_free() when not needed any longer.
 *
 * Note: creating a temporary file for the caller to (re)open is
 * inherently racy. Use g_file_open_tmp() instead whenever practical.
 */
char *create_tmp_file(Error **errp)
{
    int fd;
    const char *tmpdir;
    g_autofree char *filename = NULL;

    tmpdir = g_get_tmp_dir();
#ifndef _WIN32
    /*
     * See commit 69bef79 ("block: use /var/tmp instead of /tmp for -snapshot")
     *
     * This function is used to create temporary disk images (like -snapshot),
     * so the files can become very large. /tmp is often a tmpfs where as
     * /var/tmp is usually on a disk, so more appropriate for disk images.
     */
    if (!g_strcmp0(tmpdir, "/tmp")) {
        tmpdir = "/var/tmp";
    }
#endif

    filename = g_strdup_printf("%s/vl.XXXXXX", tmpdir);
    fd = g_mkstemp(filename);
    if (fd < 0) {
        error_setg_errno(errp, errno, "Could not open temporary file '%s'",
                         filename);
        return NULL;
    }
    close(fd);

    return g_steal_pointer(&filename);
}

/*
 * Detect host devices. By convention, /dev/cdrom[N] is always
 * recognized as a host CDROM.
 */
static BlockDriver *find_hdev_driver(const char *filename)
{
    int score_max = 0, score;
    BlockDriver *drv = NULL, *d;
    GLOBAL_STATE_CODE();

    QLIST_FOREACH(d, &bdrv_drivers, list) {
        if (d->bdrv_probe_device) {
            score = d->bdrv_probe_device(filename);
            if (score > score_max) {
                score_max = score;
                drv = d;
            }
        }
    }

    return drv;
}

static BlockDriver *bdrv_do_find_protocol(const char *protocol)
{
    BlockDriver *drv1;
    GLOBAL_STATE_CODE();

    QLIST_FOREACH(drv1, &bdrv_drivers, list) {
        if (drv1->protocol_name && !strcmp(drv1->protocol_name, protocol)) {
            return drv1;
        }
    }

    return NULL;
}

BlockDriver *bdrv_find_protocol(const char *filename,
                                bool allow_protocol_prefix,
                                Error **errp)
{
    BlockDriver *drv1;
    char protocol[128];
    int len;
    const char *p;
    int i;

    GLOBAL_STATE_CODE();

    /*
     * XXX(hch): we really should not let host device detection
     * override an explicit protocol specification, but moving this
     * later breaks access to device names with colons in them.
     * Thanks to the brain-dead persistent naming schemes on udev-
     * based Linux systems those actually are quite common.
     */
    drv1 = find_hdev_driver(filename);
    if (drv1) {
        return drv1;
    }

    if (!path_has_protocol(filename) || !allow_protocol_prefix) {
        return &bdrv_file;
    }

    p = strchr(filename, ':');
    assert(p != NULL);
    len = p - filename;
    if (len > sizeof(protocol) - 1)
        len = sizeof(protocol) - 1;
    memcpy(protocol, filename, len);
    protocol[len] = '\0';

    drv1 = bdrv_do_find_protocol(protocol);
    if (drv1) {
        return drv1;
    }

    for (i = 0; i < (int)ARRAY_SIZE(block_driver_modules); ++i) {
        if (block_driver_modules[i].protocol_name &&
            !strcmp(block_driver_modules[i].protocol_name, protocol)) {
            int rv = block_module_load(block_driver_modules[i].library_name, errp);
            if (rv > 0) {
                drv1 = bdrv_do_find_protocol(protocol);
            } else if (rv < 0) {
                return NULL;
            }
            break;
        }
    }

    if (!drv1) {
        error_setg(errp, "Unknown protocol '%s'", protocol);
    }
    return drv1;
}

/*
 * Guess image format by probing its contents.
 * This is not a good idea when your image is raw (CVE-2008-2004), but
 * we do it anyway for backward compatibility.
 *
 * @buf         contains the image's first @buf_size bytes.
 * @buf_size    is the buffer size in bytes (generally BLOCK_PROBE_BUF_SIZE,
 *              but can be smaller if the image file is smaller)
 * @filename    is its filename.
 *
 * For all block drivers, call the bdrv_probe() method to get its
 * probing score.
 * Return the first block driver with the highest probing score.
 */
BlockDriver *bdrv_probe_all(const uint8_t *buf, int buf_size,
                            const char *filename)
{
    int score_max = 0, score;
    BlockDriver *drv = NULL, *d;
    IO_CODE();

    QLIST_FOREACH(d, &bdrv_drivers, list) {

        if (d->bdrv_probe) {
            score = d->bdrv_probe(buf, buf_size, filename);
            if (score > score_max) {
                score_max = score;
                drv = d;
            }
        }
    }

    return drv;
}

static int find_image_format(BlockBackend *file, const char *filename,
                             BlockDriver **pdrv, Error **errp)
{
    BlockDriver *drv;
    uint8_t buf[BLOCK_PROBE_BUF_SIZE];
    int ret = 0;

    GLOBAL_STATE_CODE();

    /* Return the raw BlockDriver * to scsi-generic devices or empty drives */
    if (blk_is_sg(file) || !blk_is_inserted(file) || blk_getlength(file) == 0) {
        *pdrv = &bdrv_raw;
        return ret;
    }

    ret = blk_pread(file, 0, sizeof(buf), buf, 0);
    if (ret < 0) {
        error_setg_errno(errp, -ret, "Could not read image for determining its "
                         "format");
        *pdrv = NULL;
        return ret;
    }

    drv = bdrv_probe_all(buf, sizeof(buf), filename);
    if (!drv) {
        error_setg(errp, "Could not determine image format: No compatible "
                   "driver found");
        *pdrv = NULL;
        return -ENOENT;
    }

    *pdrv = drv;
    return 0;
}

/**
 * Set the current 'total_sectors' value
 * Return 0 on success, -errno on error.
 */
int coroutine_fn bdrv_co_refresh_total_sectors(BlockDriverState *bs,
                                               int64_t hint)
{
    BlockDriver *drv = bs->drv;
    IO_CODE();
    assert_bdrv_graph_readable();

    if (!drv) {
        return -ENOMEDIUM;
    }

    /* Do not attempt drv->bdrv_co_getlength() on scsi-generic devices */
    if (bdrv_is_sg(bs))
        return 0;

    /* query actual device if possible, otherwise just trust the hint */
    if (drv->bdrv_co_getlength) {
        int64_t length = drv->bdrv_co_getlength(bs);
        if (length < 0) {
            return length;
        }
        hint = DIV_ROUND_UP(length, BDRV_SECTOR_SIZE);
    }

    bs->total_sectors = hint;

    if (bs->total_sectors * BDRV_SECTOR_SIZE > BDRV_MAX_LENGTH) {
        return -EFBIG;
    }

    return 0;
}

/**
 * Combines a QDict of new block driver @options with any missing options taken
 * from @old_options, so that leaving out an option defaults to its old value.
 */
static void bdrv_join_options(BlockDriverState *bs, QDict *options,
                              QDict *old_options)
{
    GLOBAL_STATE_CODE();
    if (bs->drv && bs->drv->bdrv_join_options) {
        bs->drv->bdrv_join_options(options, old_options);
    } else {
        qdict_join(options, old_options, false);
    }
}

static BlockdevDetectZeroesOptions bdrv_parse_detect_zeroes(QemuOpts *opts,
                                                            int open_flags,
                                                            Error **errp)
{
    Error *local_err = NULL;
    char *value = qemu_opt_get_del(opts, "detect-zeroes");
    BlockdevDetectZeroesOptions detect_zeroes =
        qapi_enum_parse(&BlockdevDetectZeroesOptions_lookup, value,
                        BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF, &local_err);
    GLOBAL_STATE_CODE();
    g_free(value);
    if (local_err) {
        error_propagate(errp, local_err);
        return detect_zeroes;
    }

    if (detect_zeroes == BLOCKDEV_DETECT_ZEROES_OPTIONS_UNMAP &&
        !(open_flags & BDRV_O_UNMAP))
    {
        error_setg(errp, "setting detect-zeroes to unmap is not allowed "
                   "without setting discard operation to unmap");
    }

    return detect_zeroes;
}

/**
 * Set open flags for aio engine
 *
 * Return 0 on success, -1 if the engine specified is invalid
 */
int bdrv_parse_aio(const char *mode, int *flags)
{
    if (!strcmp(mode, "threads")) {
        /* do nothing, default */
    } else if (!strcmp(mode, "native")) {
        *flags |= BDRV_O_NATIVE_AIO;
#ifdef CONFIG_LINUX_IO_URING
    } else if (!strcmp(mode, "io_uring")) {
        *flags |= BDRV_O_IO_URING;
#endif
    } else {
        return -1;
    }

    return 0;
}

/**
 * Set open flags for a given discard mode
 *
 * Return 0 on success, -1 if the discard mode was invalid.
 */
int bdrv_parse_discard_flags(const char *mode, int *flags)
{
    *flags &= ~BDRV_O_UNMAP;

    if (!strcmp(mode, "off") || !strcmp(mode, "ignore")) {
        /* do nothing */
    } else if (!strcmp(mode, "on") || !strcmp(mode, "unmap")) {
        *flags |= BDRV_O_UNMAP;
    } else {
        return -1;
    }

    return 0;
}

/**
 * Set open flags for a given cache mode
 *
 * Return 0 on success, -1 if the cache mode was invalid.
 */
int bdrv_parse_cache_mode(const char *mode, int *flags, bool *writethrough)
{
    *flags &= ~BDRV_O_CACHE_MASK;

    if (!strcmp(mode, "off") || !strcmp(mode, "none")) {
        *writethrough = false;
        *flags |= BDRV_O_NOCACHE;
    } else if (!strcmp(mode, "directsync")) {
        *writethrough = true;
        *flags |= BDRV_O_NOCACHE;
    } else if (!strcmp(mode, "writeback")) {
        *writethrough = false;
    } else if (!strcmp(mode, "unsafe")) {
        *writethrough = false;
        *flags |= BDRV_O_NO_FLUSH;
    } else if (!strcmp(mode, "writethrough")) {
        *writethrough = true;
    } else {
        return -1;
    }

    return 0;
}

static char *bdrv_child_get_parent_desc(BdrvChild *c)
{
    BlockDriverState *parent = c->opaque;
    return g_strdup_printf("node '%s'", bdrv_get_node_name(parent));
}

static void GRAPH_RDLOCK bdrv_child_cb_drained_begin(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;
    bdrv_do_drained_begin_quiesce(bs, NULL);
}

static bool GRAPH_RDLOCK bdrv_child_cb_drained_poll(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;
    return bdrv_drain_poll(bs, NULL, false);
}

static void GRAPH_RDLOCK bdrv_child_cb_drained_end(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;
    bdrv_drained_end(bs);
}

static int bdrv_child_cb_inactivate(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;
    GLOBAL_STATE_CODE();
    assert(bs->open_flags & BDRV_O_INACTIVE);
    return 0;
}

static bool GRAPH_RDLOCK
bdrv_child_cb_change_aio_ctx(BdrvChild *child, AioContext *ctx,
                             GHashTable *visited, Transaction *tran,
                             Error **errp)
{
    BlockDriverState *bs = child->opaque;
    return bdrv_change_aio_context(bs, ctx, visited, tran, errp);
}

/*
 * Returns the options and flags that a temporary snapshot should get, based on
 * the originally requested flags (the originally requested image will have
 * flags like a backing file)
 */
static void bdrv_temp_snapshot_options(int *child_flags, QDict *child_options,
                                       int parent_flags, QDict *parent_options)
{
    GLOBAL_STATE_CODE();
    *child_flags = (parent_flags & ~BDRV_O_SNAPSHOT) | BDRV_O_TEMPORARY;

    /* For temporary files, unconditional cache=unsafe is fine */
    qdict_set_default_str(child_options, BDRV_OPT_CACHE_DIRECT, "off");
    qdict_set_default_str(child_options, BDRV_OPT_CACHE_NO_FLUSH, "on");

    /* Copy the read-only and discard options from the parent */
    qdict_copy_default(child_options, parent_options, BDRV_OPT_READ_ONLY);
    qdict_copy_default(child_options, parent_options, BDRV_OPT_DISCARD);

    /* aio=native doesn't work for cache.direct=off, so disable it for the
     * temporary snapshot */
    *child_flags &= ~BDRV_O_NATIVE_AIO;
}

static void GRAPH_WRLOCK bdrv_backing_attach(BdrvChild *c)
{
    BlockDriverState *parent = c->opaque;
    BlockDriverState *backing_hd = c->bs;

    GLOBAL_STATE_CODE();
    assert(!parent->backing_blocker);
    error_setg(&parent->backing_blocker,
               "node is used as backing hd of '%s'",
               bdrv_get_device_or_node_name(parent));

    bdrv_refresh_filename(backing_hd);

    parent->open_flags &= ~BDRV_O_NO_BACKING;

    bdrv_op_block_all(backing_hd, parent->backing_blocker);
    /* Otherwise we won't be able to commit or stream */
    bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_COMMIT_TARGET,
                    parent->backing_blocker);
    bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_STREAM,
                    parent->backing_blocker);
    /*
     * We do backup in 3 ways:
     * 1. drive backup
     *    The target bs is new opened, and the source is top BDS
     * 2. blockdev backup
     *    Both the source and the target are top BDSes.
     * 3. internal backup(used for block replication)
     *    Both the source and the target are backing file
     *
     * In case 1 and 2, neither the source nor the target is the backing file.
     * In case 3, we will block the top BDS, so there is only one block job
     * for the top BDS and its backing chain.
     */
    bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_BACKUP_SOURCE,
                    parent->backing_blocker);
    bdrv_op_unblock(backing_hd, BLOCK_OP_TYPE_BACKUP_TARGET,
                    parent->backing_blocker);
}

static void bdrv_backing_detach(BdrvChild *c)
{
    BlockDriverState *parent = c->opaque;

    GLOBAL_STATE_CODE();
    assert(parent->backing_blocker);
    bdrv_op_unblock_all(c->bs, parent->backing_blocker);
    error_free(parent->backing_blocker);
    parent->backing_blocker = NULL;
}

static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,
                                        const char *filename,
                                        bool backing_mask_protocol,
                                        Error **errp)
{
    BlockDriverState *parent = c->opaque;
    bool read_only = bdrv_is_read_only(parent);
    int ret;
    const char *format_name;
    GLOBAL_STATE_CODE();

    if (read_only) {
        ret = bdrv_reopen_set_read_only(parent, false, errp);
        if (ret < 0) {
            return ret;
        }
    }

    if (base->drv) {
        /*
         * If the new base image doesn't have a format driver layer, which we
         * detect by the fact that @base is a protocol driver, we record
         * 'raw' as the format instead of putting the protocol name as the
         * backing format
         */
        if (backing_mask_protocol && base->drv->protocol_name) {
            format_name = "raw";
        } else {
            format_name = base->drv->format_name;
        }
    } else {
        format_name = "";
    }

    ret = bdrv_change_backing_file(parent, filename, format_name, false);
    if (ret < 0) {
        error_setg_errno(errp, -ret, "Could not update backing file link");
    }

    if (read_only) {
        bdrv_reopen_set_read_only(parent, true, NULL);
    }

    return ret;
}

/*
 * Returns the options and flags that a generic child of a BDS should
 * get, based on the given options and flags for the parent BDS.
 */
static void bdrv_inherited_options(BdrvChildRole role, bool parent_is_format,
                                   int *child_flags, QDict *child_options,
                                   int parent_flags, QDict *parent_options)
{
    int flags = parent_flags;
    GLOBAL_STATE_CODE();

    /*
     * First, decide whether to set, clear, or leave BDRV_O_PROTOCOL.
     * Generally, the question to answer is: Should this child be
     * format-probed by default?
     */

    /*
     * Pure and non-filtered data children of non-format nodes should
     * be probed by default (even when the node itself has BDRV_O_PROTOCOL
     * set).  This only affects a very limited set of drivers (namely
     * quorum and blkverify when this comment was written).
     * Force-clear BDRV_O_PROTOCOL then.
     */
    if (!parent_is_format &&
        (role & BDRV_CHILD_DATA) &&
        !(role & (BDRV_CHILD_METADATA | BDRV_CHILD_FILTERED)))
    {
        flags &= ~BDRV_O_PROTOCOL;
    }

    /*
     * All children of format nodes (except for COW children) and all
     * metadata children in general should never be format-probed.
     * Force-set BDRV_O_PROTOCOL then.
     */
    if ((parent_is_format && !(role & BDRV_CHILD_COW)) ||
        (role & BDRV_CHILD_METADATA))
    {
        flags |= BDRV_O_PROTOCOL;
    }

    /*
     * If the cache mode isn't explicitly set, inherit direct and no-flush from
     * the parent.
     */
    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_DIRECT);
    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_NO_FLUSH);
    qdict_copy_default(child_options, parent_options, BDRV_OPT_FORCE_SHARE);

    if (role & BDRV_CHILD_COW) {
        /* backing files are opened read-only by default */
        qdict_set_default_str(child_options, BDRV_OPT_READ_ONLY, "on");
        qdict_set_default_str(child_options, BDRV_OPT_AUTO_READ_ONLY, "off");
    } else {
        /* Inherit the read-only option from the parent if it's not set */
        qdict_copy_default(child_options, parent_options, BDRV_OPT_READ_ONLY);
        qdict_copy_default(child_options, parent_options,
                           BDRV_OPT_AUTO_READ_ONLY);
    }

    /*
     * bdrv_co_pdiscard() respects unmap policy for the parent, so we
     * can default to enable it on lower layers regardless of the
     * parent option.
     */
    qdict_set_default_str(child_options, BDRV_OPT_DISCARD, "unmap");

    /* Clear flags that only apply to the top layer */
    flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_COPY_ON_READ);

    if (role & BDRV_CHILD_METADATA) {
        flags &= ~BDRV_O_NO_IO;
    }
    if (role & BDRV_CHILD_COW) {
        flags &= ~BDRV_O_TEMPORARY;
    }

    *child_flags = flags;
}

static void GRAPH_WRLOCK bdrv_child_cb_attach(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;

    assert_bdrv_graph_writable();
    QLIST_INSERT_HEAD(&bs->children, child, next);
    if (bs->drv->is_filter || (child->role & BDRV_CHILD_FILTERED)) {
        /*
         * Here we handle filters and block/raw-format.c when it behave like
         * filter. They generally have a single PRIMARY child, which is also the
         * FILTERED child, and that they may have multiple more children, which
         * are neither PRIMARY nor FILTERED. And never we have a COW child here.
         * So bs->file will be the PRIMARY child, unless the PRIMARY child goes
         * into bs->backing on exceptional cases; and bs->backing will be
         * nothing else.
         */
        assert(!(child->role & BDRV_CHILD_COW));
        if (child->role & BDRV_CHILD_PRIMARY) {
            assert(child->role & BDRV_CHILD_FILTERED);
            assert(!bs->backing);
            assert(!bs->file);

            if (bs->drv->filtered_child_is_backing) {
                bs->backing = child;
            } else {
                bs->file = child;
            }
        } else {
            assert(!(child->role & BDRV_CHILD_FILTERED));
        }
    } else if (child->role & BDRV_CHILD_COW) {
        assert(bs->drv->supports_backing);
        assert(!(child->role & BDRV_CHILD_PRIMARY));
        assert(!bs->backing);
        bs->backing = child;
        bdrv_backing_attach(child);
    } else if (child->role & BDRV_CHILD_PRIMARY) {
        assert(!bs->file);
        bs->file = child;
    }
}

static void GRAPH_WRLOCK bdrv_child_cb_detach(BdrvChild *child)
{
    BlockDriverState *bs = child->opaque;

    if (child->role & BDRV_CHILD_COW) {
        bdrv_backing_detach(child);
    }

    assert_bdrv_graph_writable();
    QLIST_REMOVE(child, next);
    if (child == bs->backing) {
        assert(child != bs->file);
        bs->backing = NULL;
    } else if (child == bs->file) {
        bs->file = NULL;
    }
}

static int bdrv_child_cb_update_filename(BdrvChild *c, BlockDriverState *base,
                                         const char *filename,
                                         bool backing_mask_protocol,
                                         Error **errp)
{
    if (c->role & BDRV_CHILD_COW) {
        return bdrv_backing_update_filename(c, base, filename,
                                            backing_mask_protocol,
                                            errp);
    }
    return 0;
}

AioContext *child_of_bds_get_parent_aio_context(BdrvChild *c)
{
    BlockDriverState *bs = c->opaque;
    IO_CODE();

    return bdrv_get_aio_context(bs);
}

const BdrvChildClass child_of_bds = {
    .parent_is_bds   = true,
    .get_parent_desc = bdrv_child_get_parent_desc,
    .inherit_options = bdrv_inherited_options,
    .drained_begin   = bdrv_child_cb_drained_begin,
    .drained_poll    = bdrv_child_cb_drained_poll,
    .drained_end     = bdrv_child_cb_drained_end,
    .attach          = bdrv_child_cb_attach,
    .detach          = bdrv_child_cb_detach,
    .inactivate      = bdrv_child_cb_inactivate,
    .change_aio_ctx  = bdrv_child_cb_change_aio_ctx,
    .update_filename = bdrv_child_cb_update_filename,
    .get_parent_aio_context = child_of_bds_get_parent_aio_context,
};

AioContext *bdrv_child_get_parent_aio_context(BdrvChild *c)
{
    IO_CODE();
    return c->klass->get_parent_aio_context(c);
}

static int bdrv_open_flags(BlockDriverState *bs, int flags)
{
    int open_flags = flags;
    GLOBAL_STATE_CODE();

    /*
     * Clear flags that are internal to the block layer before opening the
     * image.
     */
    open_flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_PROTOCOL);

    return open_flags;
}

static void update_flags_from_options(int *flags, QemuOpts *opts)
{
    GLOBAL_STATE_CODE();

    *flags &= ~(BDRV_O_CACHE_MASK | BDRV_O_RDWR | BDRV_O_AUTO_RDONLY);

    if (qemu_opt_get_bool_del(opts, BDRV_OPT_CACHE_NO_FLUSH, false)) {
        *flags |= BDRV_O_NO_FLUSH;
    }

    if (qemu_opt_get_bool_del(opts, BDRV_OPT_CACHE_DIRECT, false)) {
        *flags |= BDRV_O_NOCACHE;
    }

    if (!qemu_opt_get_bool_del(opts, BDRV_OPT_READ_ONLY, false)) {
        *flags |= BDRV_O_RDWR;
    }

    if (qemu_opt_get_bool_del(opts, BDRV_OPT_AUTO_READ_ONLY, false)) {
        *flags |= BDRV_O_AUTO_RDONLY;
    }

    if (!qemu_opt_get_bool_del(opts, BDRV_OPT_ACTIVE, true)) {
        *flags |= BDRV_O_INACTIVE;
    }
}

static void update_options_from_flags(QDict *options, int flags)
{
    GLOBAL_STATE_CODE();
    if (!qdict_haskey(options, BDRV_OPT_CACHE_DIRECT)) {
        qdict_put_bool(options, BDRV_OPT_CACHE_DIRECT, flags & BDRV_O_NOCACHE);
    }
    if (!qdict_haskey(options, BDRV_OPT_CACHE_NO_FLUSH)) {
        qdict_put_bool(options, BDRV_OPT_CACHE_NO_FLUSH,
                       flags & BDRV_O_NO_FLUSH);
    }
    if (!qdict_haskey(options, BDRV_OPT_READ_ONLY)) {
        qdict_put_bool(options, BDRV_OPT_READ_ONLY, !(flags & BDRV_O_RDWR));
    }
    if (!qdict_haskey(options, BDRV_OPT_AUTO_READ_ONLY)) {
        qdict_put_bool(options, BDRV_OPT_AUTO_READ_ONLY,
                       flags & BDRV_O_AUTO_RDONLY);
    }
}

static void bdrv_assign_node_name(BlockDriverState *bs,
                                  const char *node_name,
                                  Error **errp)
{
    char *gen_node_name = NULL;
    GLOBAL_STATE_CODE();

    if (!node_name) {
        node_name = gen_node_name = id_generate(ID_BLOCK);
    } else if (!id_wellformed(node_name)) {
        /*
         * Check for empty string or invalid characters, but not if it is
         * generated (generated names use characters not available to the user)
         */
        error_setg(errp, "Invalid node-name: '%s'", node_name);
        return;
    }

    /* takes care of avoiding namespaces collisions */
    if (blk_by_name(node_name)) {
        error_setg(errp, "node-name=%s is conflicting with a device id",
                   node_name);
        goto out;
    }

    /* takes care of avoiding duplicates node names */
    if (bdrv_find_node(node_name)) {
        error_setg(errp, "Duplicate nodes with node-name='%s'", node_name);
        goto out;
    }

    /* Make sure that the node name isn't truncated */
    if (strlen(node_name) >= sizeof(bs->node_name)) {
        error_setg(errp, "Node name too long");
        goto out;
    }

    /* copy node name into the bs and insert it into the graph list */
    pstrcpy(bs->node_name, sizeof(bs->node_name), node_name);
    QTAILQ_INSERT_TAIL(&graph_bdrv_states, bs, node_list);
out:
    g_free(gen_node_name);
}

static int no_coroutine_fn GRAPH_UNLOCKED
bdrv_open_driver(BlockDriverState *bs, BlockDriver *drv, const char *node_name,
                 QDict *options, int open_flags, Error **errp)
{
    Error *local_err = NULL;
    int i, ret;
    GLOBAL_STATE_CODE();

    bdrv_assign_node_name(bs, node_name, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        return -EINVAL;
    }

    bs->drv = drv;
    bs->opaque = g_malloc0(drv->instance_size);

    assert(!drv->bdrv_needs_filename || bs->filename[0]);
    if (drv->bdrv_open) {
        ret = drv->bdrv_open(bs, options, open_flags, &local_err);
    } else {
        ret = 0;
    }

    if (ret < 0) {
        if (local_err) {
            error_propagate(errp, local_err);
        } else if (bs->filename[0]) {
            error_setg_errno(errp, -ret, "Could not open '%s'", bs->filename);
        } else {
            error_setg_errno(errp, -ret, "Could not open image");
        }
        goto open_failed;
    }

    assert(!(bs->supported_read_flags & ~BDRV_REQ_MASK));
    assert(!(bs->supported_write_flags & ~BDRV_REQ_MASK));

    /*
     * Always allow the BDRV_REQ_REGISTERED_BUF optimization hint. This saves
     * drivers that pass read/write requests through to a child the trouble of
     * declaring support explicitly.
     *
     * Drivers must not propagate this flag accidentally when they initiate I/O
     * to a bounce buffer. That case should be rare though.
     */
    bs->supported_read_flags |= BDRV_REQ_REGISTERED_BUF;
    bs->supported_write_flags |= BDRV_REQ_REGISTERED_BUF;

    ret = bdrv_refresh_total_sectors(bs, bs->total_sectors);
    if (ret < 0) {
        error_setg_errno(errp, -ret, "Could not refresh total sector count");
        return ret;
    }

    bdrv_graph_rdlock_main_loop();
    bdrv_refresh_limits(bs, NULL, &local_err);
    bdrv_graph_rdunlock_main_loop();

    if (local_err) {
        error_propagate(errp, local_err);
        return -EINVAL;
    }

    assert(bdrv_opt_mem_align(bs) != 0);
    assert(bdrv_min_mem_align(bs) != 0);
    assert(is_power_of_2(bs->bl.request_alignment));

    for (i = 0; i < bs->quiesce_counter; i++) {
        if (drv->bdrv_drain_begin) {
            drv->bdrv_drain_begin(bs);
        }
    }

    return 0;
open_failed:
    bs->drv = NULL;

    bdrv_graph_wrlock_drained();
    if (bs->file != NULL) {
        bdrv_unref_child(bs, bs->file);
        assert(!bs->file);
    }
    bdrv_graph_wrunlock();

    g_free(bs->opaque);
    bs->opaque = NULL;
    return ret;
}

/*
 * Create and open a block node.
 *
 * @options is a QDict of options to pass to the block drivers, or NULL for an
 * empty set of options. The reference to the QDict belongs to the block layer
 * after the call (even on failure), so if the caller intends to reuse the
 * dictionary, it needs to use qobject_ref() before calling bdrv_open.
 */
BlockDriverState *bdrv_new_open_driver_opts(BlockDriver *drv,
                                            const char *node_name,
                                            QDict *options, int flags,
                                            Error **errp)
{
    BlockDriverState *bs;
    int ret;

    GLOBAL_STATE_CODE();

    bs = bdrv_new();
    bs->open_flags = flags;
    bs->options = options ?: qdict_new();
    bs->explicit_options = qdict_clone_shallow(bs->options);
    bs->opaque = NULL;

    update_options_from_flags(bs->options, flags);

    ret = bdrv_open_driver(bs, drv, node_name, bs->options, flags, errp);
    if (ret < 0) {
        qobject_unref(bs->explicit_options);
        bs->explicit_options = NULL;
        qobject_unref(bs->options);
        bs->options = NULL;
        bdrv_unref(bs);
        return NULL;
    }

    return bs;
}

/* Create and open a block node. */
BlockDriverState *bdrv_new_open_driver(BlockDriver *drv, const char *node_name,
                                       int flags, Error **errp)
{
    GLOBAL_STATE_CODE();
    return bdrv_new_open_driver_opts(drv, node_name, NULL, flags, errp);
}

QemuOptsList bdrv_runtime_opts = {
    .name = "bdrv_common",
    .head = QTAILQ_HEAD_INITIALIZER(bdrv_runtime_opts.head),
    .desc = {
        {
            .name = "node-name",
            .type = QEMU_OPT_STRING,
            .help = "Node name of the block device node",
        },
        {
            .name = "driver",
            .type = QEMU_OPT_STRING,
            .help = "Block driver to use for the node",
        },
        {
            .name = BDRV_OPT_CACHE_DIRECT,
            .type = QEMU_OPT_BOOL,
            .help = "Bypass software writeback cache on the host",
        },
        {
            .name = BDRV_OPT_CACHE_NO_FLUSH,
            .type = QEMU_OPT_BOOL,
            .help = "Ignore flush requests",
        },
        {
            .name = BDRV_OPT_ACTIVE,
            .type = QEMU_OPT_BOOL,
            .help = "Node is activated",
        },
        {
            .name = BDRV_OPT_READ_ONLY,
            .type = QEMU_OPT_BOOL,
            .help = "Node is opened in read-only mode",
        },
        {
            .name = BDRV_OPT_AUTO_READ_ONLY,
            .type = QEMU_OPT_BOOL,
            .help = "Node can become read-only if opening read-write fails",
        },
        {
            .name = "detect-zeroes",
            .type = QEMU_OPT_STRING,
            .help = "try to optimize zero writes (off, on, unmap)",
        },
        {
            .name = BDRV_OPT_DISCARD,
            .type = QEMU_OPT_STRING,
            .help = "discard operation (ignore/off, unmap/on)",
        },
        {
            .name = BDRV_OPT_FORCE_SHARE,
            .type = QEMU_OPT_BOOL,
            .help = "always accept other writers (default: off)",
        },
        { /* end of list */ }
    },
};

QemuOptsList bdrv_create_opts_simple = {
    .name = "simple-create-opts",
    .head = QTAILQ_HEAD_INITIALIZER(bdrv_create_opts_simple.head),
    .desc = {
        {
            .name = BLOCK_OPT_SIZE,
            .type = QEMU_OPT_SIZE,
            .help = "Virtual disk size"
        },
        {
            .name = BLOCK_OPT_PREALLOC,
            .type = QEMU_OPT_STRING,
            .help = "Preallocation mode (allowed values: off)"
        },
        { /* end of list */ }
    }
};

/*
 * Common part for opening disk images and files
 *
 * Removes all processed options from *options.
 */
static int bdrv_open_common(BlockDriverState *bs, BlockBackend *file,
                            QDict *options, Error **errp)
{
    int ret, open_flags;
    const char *filename;
    const char *driver_name = NULL;
    const char *node_name = NULL;
    const char *discard;
    QemuOpts *opts;
    BlockDriver *drv;
    Error *local_err = NULL;
    bool ro;

    GLOBAL_STATE_CODE();

    bdrv_graph_rdlock_main_loop();
    assert(bs->file == NULL);
    assert(options != NULL && bs->options != options);
    bdrv_graph_rdunlock_main_loop();

    opts = qemu_opts_create(&bdrv_runtime_opts, NULL, 0, &error_abort);
    if (!qemu_opts_absorb_qdict(opts, options, errp)) {
        ret = -EINVAL;
        goto fail_opts;
    }

    update_flags_from_options(&bs->open_flags, opts);

    driver_name = qemu_opt_get(opts, "driver");
    drv = bdrv_find_format(driver_name);
    assert(drv != NULL);

    bs->force_share = qemu_opt_get_bool(opts, BDRV_OPT_FORCE_SHARE, false);

    if (bs->force_share && (bs->open_flags & BDRV_O_RDWR)) {
        error_setg(errp,
                   BDRV_OPT_FORCE_SHARE
                   "=on can only be used with read-only images");
        ret = -EINVAL;
        goto fail_opts;
    }

    if (file != NULL) {
        bdrv_graph_rdlock_main_loop();
        bdrv_refresh_filename(blk_bs(file));
        bdrv_graph_rdunlock_main_loop();

        filename = blk_bs(file)->filename;
    } else {
        /*
         * Caution: while qdict_get_try_str() is fine, getting
         * non-string types would require more care.  When @options
         * come from -blockdev or blockdev_add, its members are typed
         * according to the QAPI schema, but when they come from
         * -drive, they're all QString.
         */
        filename = qdict_get_try_str(options, "filename");
    }

    if (drv->bdrv_needs_filename && (!filename || !filename[0])) {
        error_setg(errp, "The '%s' block driver requires a file name",
                   drv->format_name);
        ret = -EINVAL;
        goto fail_opts;
    }

    trace_bdrv_open_common(bs, filename ?: "", bs->open_flags,
                           drv->format_name);

    ro = bdrv_is_read_only(bs);

    if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv, ro)) {
        if (!ro && bdrv_is_whitelisted(drv, true)) {
            bdrv_graph_rdlock_main_loop();
            ret = bdrv_apply_auto_read_only(bs, NULL, NULL);
            bdrv_graph_rdunlock_main_loop();
        } else {
            ret = -ENOTSUP;
        }
        if (ret < 0) {
            error_setg(errp,
                       !ro && bdrv_is_whitelisted(drv, true)
                       ? "Driver '%s' can only be used for read-only devices"
                       : "Driver '%s' is not whitelisted",
                       drv->format_name);
            goto fail_opts;
        }
    }

    /* bdrv_new() and bdrv_close() make it so */
    assert(qatomic_read(&bs->copy_on_read) == 0);

    if (bs->open_flags & BDRV_O_COPY_ON_READ) {
        if (!ro) {
            bdrv_enable_copy_on_read(bs);
        } else {
            error_setg(errp, "Can't use copy-on-read on read-only device");
            ret = -EINVAL;
            goto fail_opts;
        }
    }

    discard = qemu_opt_get(opts, BDRV_OPT_DISCARD);
    if (discard != NULL) {
        if (bdrv_parse_discard_flags(discard, &bs->open_flags) != 0) {
            error_setg(errp, "Invalid discard option");
            ret = -EINVAL;
            goto fail_opts;
        }
    }

    bs->detect_zeroes =
        bdrv_parse_detect_zeroes(opts, bs->open_flags, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        ret = -EINVAL;
        goto fail_opts;
    }

    if (filename != NULL) {
        pstrcpy(bs->filename, sizeof(bs->filename), filename);
    } else {
        bs->filename[0] = '\0';
    }
    pstrcpy(bs->exact_filename, sizeof(bs->exact_filename), bs->filename);

    /* Open the image, either directly or using a protocol */
    open_flags = bdrv_open_flags(bs, bs->open_flags);
    node_name = qemu_opt_get(opts, "node-name");

    assert(!drv->protocol_name || file == NULL);
    ret = bdrv_open_driver(bs, drv, node_name, options, open_flags, errp);
    if (ret < 0) {
        goto fail_opts;
    }

    qemu_opts_del(opts);

    return 0;

fail_opts:
    qemu_opts_del(opts);
    return ret;
}

static QDict *parse_json_filename(const char *filename, Error **errp)
{
    ERRP_GUARD();
    QObject *options_obj;
    QDict *options;
    int ret;
    GLOBAL_STATE_CODE();

    ret = strstart(filename, "json:", &filename);
    assert(ret);

    options_obj = qobject_from_json(filename, errp);
    if (!options_obj) {
        error_prepend(errp, "Could not parse the JSON options: ");
        return NULL;
    }

    options = qobject_to(QDict, options_obj);
    if (!options) {
        qobject_unref(options_obj);
        error_setg(errp, "Invalid JSON object given");
        return NULL;
    }

    qdict_flatten(options);

    return options;
}

static void parse_json_protocol(QDict *options, const char **pfilename,
                                Error **errp)
{
    QDict *json_options;
    Error *local_err = NULL;
    GLOBAL_STATE_CODE();

    /* Parse json: pseudo-protocol */
    if (!*pfilename || !g_str_has_prefix(*pfilename, "json:")) {
        return;
    }

    json_options = parse_json_filename(*pfilename, &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        return;
    }

    /* Options given in the filename have lower priority than options
     * specified directly */
    qdict_join(options, json_options, false);
    qobject_unref(json_options);
    *pfilename = NULL;
}

/*
 * Fills in default options for opening images and converts the legacy
 * filename/flags pair to option QDict entries.
 * The BDRV_O_PROTOCOL flag in *flags will be set or cleared accordingly if a
 * block driver has been specified explicitly.
 */
static int bdrv_fill_options(QDict **options, const char *filename,
                             int *flags, bool allow_parse_filename,
                             Error **errp)
{
    const char *drvname;
    bool protocol = *flags & BDRV_O_PROTOCOL;
    bool parse_filename = false;
    BlockDriver *drv = NULL;
    Error *local_err = NULL;

    GLOBAL_STATE_CODE();

    /*
     * Caution: while qdict_get_try_str() is fine, getting non-string
     * types would require more care.  When @options come from
     * -blockdev or blockdev_add, its members are typed according to
     * the QAPI schema, but when they come from -drive, they're all
     * QString.
     */
    drvname = qdict_get_try_str(*options, "driver");
    if (drvname) {
        drv = bdrv_find_format(drvname);
        if (!drv) {
            error_setg(errp, "Unknown driver '%s'", drvname);
            return -ENOENT;
        }
        /* If the user has explicitly specified the driver, this choice should
         * override the BDRV_O_PROTOCOL flag */
        protocol = drv->protocol_name;
    }

    if (protocol) {
        *flags |= BDRV_O_PROTOCOL;
    } else {
        *flags &= ~BDRV_O_PROTOCOL;
    }

    /* Translate cache options from flags into options */
    update_options_from_flags(*options, *flags);

    /* Fetch the file name from the options QDict if necessary */
    if (protocol && filename) {
        if (!qdict_haskey(*options, "filename")) {
            qdict_put_str(*options, "filename", filename);
            parse_filename = allow_parse_filename;
        } else {
            error_setg(errp, "Can't specify 'file' and 'filename' options at "
                             "the same time");
            return -EINVAL;
        }
    }

    /* Find the right block driver */
    /* See cautionary note on accessing @options above */
    filename = qdict_get_try_str(*options, "filename");

    if (!drvname && protocol) {
        if (filename) {
            drv = bdrv_find_protocol(filename, parse_filename, errp);
            if (!drv) {
                return -EINVAL;
            }

            drvname = drv->format_name;
            qdict_put_str(*options, "driver", drvname);
        } else {
            error_setg(errp, "Must specify either driver or file");
            return -EINVAL;
        }
    }

    assert(drv || !protocol);

    /* Driver-specific filename parsing */
    if (drv && drv->bdrv_parse_filename && parse_filename) {
        drv->bdrv_parse_filename(filename, *options, &local_err);
        if (local_err) {
            error_propagate(errp, local_err);
            return -EINVAL;
        }

        if (!drv->bdrv_needs_filename) {
            qdict_del(*options, "filename");
        }
    }

    return 0;
}

typedef struct BlockReopenQueueEntry {
     bool prepared;
     BDRVReopenState state;
     QTAILQ_ENTRY(BlockReopenQueueEntry) entry;
} BlockReopenQueueEntry;

/*
 * Return the flags that @bs will have after the reopens in @q have
 * successfully completed. If @q is NULL (or @bs is not contained in @q),
 * return the current flags.
 */
static int bdrv_reopen_get_flags(BlockReopenQueue *q, BlockDriverState *bs)
{
    BlockReopenQueueEntry *entry;

    if (q != NULL) {
        QTAILQ_FOREACH(entry, q, entry) {
            if (entry->state.bs == bs) {
                return entry->state.flags;
            }
        }
    }

    return bs->open_flags;
}

/* Returns whether the image file can be written to after the reopen queue @q
 * has been successfully applied, or right now if @q is NULL. */
static bool bdrv_is_writable_after_reopen(BlockDriverState *bs,
                                          BlockReopenQueue *q)
{
    int flags = bdrv_reopen_get_flags(q, bs);

    return (flags & (BDRV_O_RDWR | BDRV_O_INACTIVE)) == BDRV_O_RDWR;
}

/*
 * Return whether the BDS can be written to.  This is not necessarily
 * the same as !bdrv_is_read_only(bs), as inactivated images may not
 * be written to but do not count as read-only images.
 */
bool bdrv_is_writable(BlockDriverState *bs)
{
    IO_CODE();
    return bdrv_is_writable_after_reopen(bs, NULL);
}

static char *bdrv_child_user_desc(BdrvChild *c)
{
    GLOBAL_STATE_CODE();
    return c->klass->get_parent_desc(c);
}

/*
 * Check that @a allows everything that @b needs. @a and @b must reference same
 * child node.
 */
static bool bdrv_a_allow_b(BdrvChild *a, BdrvChild *b, Error **errp)
{
    const char *child_bs_name;
    g_autofree char *a_user = NULL;
    g_autofree char *b_user = NULL;
    g_autofree char *perms = NULL;

    assert(a->bs);
    assert(a->bs == b->bs);
    GLOBAL_STATE_CODE();

    if ((b->perm & a->shared_perm) == b->perm) {
        return true;
    }

    child_bs_name = bdrv_get_node_name(b->bs);
    a_user = bdrv_child_user_desc(a);
    b_user = bdrv_child_user_desc(b);
    perms = bdrv_perm_names(b->perm & ~a->shared_perm);

    error_setg(errp, "Permission conflict on node '%s': permissions '%s' are "
               "both required by %s (uses node '%s' as '%s' child) and "
               "unshared by %s (uses node '%s' as '%s' child).",
               child_bs_name, perms,
               b_user, child_bs_name, b->name,
               a_user, child_bs_name, a->name);

    return false;
}

static bool GRAPH_RDLOCK
bdrv_parent_perms_conflict(BlockDriverState *bs, Error **errp)
{
    BdrvChild *a, *b;
    GLOBAL_STATE_CODE();

    /*
     * During the loop we'll look at each pair twice. That's correct because
     * bdrv_a_allow_b() is asymmetric and we should check each pair in both
     * directions.
     */
    QLIST_FOREACH(a, &bs->parents, next_parent) {
        QLIST_FOREACH(b, &bs->parents, next_parent) {
            if (a == b) {
                continue;
            }

            if (!bdrv_a_allow_b(a, b, errp)) {
                return true;
            }
        }
    }

    return false;
}

static void GRAPH_RDLOCK
bdrv_child_perm(BlockDriverState *bs, BlockDriverState *child_bs,
                BdrvChild *c, BdrvChildRole role,
                BlockReopenQueue *reopen_queue,
                uint64_t parent_perm, uint64_t parent_shared,
                uint64_t *nperm, uint64_t *nshared)
{
    assert(bs->drv && bs->drv->bdrv_child_perm);
    GLOBAL_STATE_CODE();
    bs->drv->bdrv_child_perm(bs, c, role, reopen_queue,
                             parent_perm, parent_shared,
                             nperm, nshared);
    /* TODO Take force_share from reopen_queue */
    if (child_bs && child_bs->force_share) {
        *nshared = BLK_PERM_ALL;
    }
}

/*
 * Adds the whole subtree of @bs (including @bs itself) to the @list (except for
 * nodes that are already in the @list, of course) so that final list is
 * topologically sorted. Return the result (GSList @list object is updated, so
 * don't use old reference after function call).
 *
 * On function start @list must be already topologically sorted and for any node
 * in the @list the whole subtree of the node must be in the @list as well. The
 * simplest way to satisfy this criteria: use only result of
 * bdrv_topological_dfs() or NULL as @list parameter.
 */
static GSList * GRAPH_RDLOCK
bdrv_topological_dfs(GSList *list, GHashTable *found, BlockDriverState *bs)
{
    BdrvChild *child;
    g_autoptr(GHashTable) local_found = NULL;

    GLOBAL_STATE_CODE();

    if (!found) {
        assert(!list);
        found = local_found = g_hash_table_new(NULL, NULL);
    }

    if (g_hash_table_contains(found, bs)) {
        return list;
    }
    g_hash_table_add(found, bs);

    QLIST_FOREACH(child, &bs->children, next) {
        list = bdrv_topological_dfs(list, found, child->bs);
    }

    return g_slist_prepend(list, bs);
}

typedef struct BdrvChildSetPermState {
    BdrvChild *child;
    uint64_t old_perm;
    uint64_t old_shared_perm;
} BdrvChildSetPermState;

static void bdrv_child_set_perm_abort(void *opaque)
{
    BdrvChildSetPermState *s = opaque;

    GLOBAL_STATE_CODE();

    s->child->perm = s->old_perm;
    s->child->shared_perm = s->old_shared_perm;
}

static TransactionActionDrv bdrv_child_set_pem_drv = {
    .abort = bdrv_child_set_perm_abort,
    .clean = g_free,
};

static void bdrv_child_set_perm(BdrvChild *c, uint64_t perm,
                                uint64_t shared, Transaction *tran)
{
    BdrvChildSetPermState *s = g_new(BdrvChildSetPermState, 1);
    GLOBAL_STATE_CODE();

    *s = (BdrvChildSetPermState) {
        .child = c,
        .old_perm = c->perm,
        .old_shared_perm = c->shared_perm,
    };

    c->perm = perm;
    c->shared_perm = shared;

    tran_add(tran, &bdrv_child_set_pem_drv, s);
}

static void GRAPH_RDLOCK bdrv_drv_set_perm_commit(void *opaque)
{
    BlockDriverState *bs = opaque;
    uint64_t cumulative_perms, cumulative_shared_perms;
    GLOBAL_STATE_CODE();

    if (bs->drv->bdrv_set_perm) {
        bdrv_get_cumulative_perm(bs, &cumulative_perms,
                                 &cumulative_shared_perms);
        bs->drv->bdrv_set_perm(bs, cumulative_perms, cumulative_shared_perms);
    }
}

static void GRAPH_RDLOCK bdrv_drv_set_perm_abort(void *opaque)
{
    BlockDriverState *bs = opaque;
    GLOBAL_STATE_CODE();

    if (bs->drv->bdrv_abort_perm_update) {
        bs->drv->bdrv_abort_perm_update(bs);
    }
}

TransactionActionDrv bdrv_drv_set_perm_drv = {
    .abort = bdrv_drv_set_perm_abort,
    .commit = bdrv_drv_set_perm_commit,
};

/*
 * After calling this function, the transaction @tran may only be completed
 * while holding a reader lock for the graph.
 */
static int GRAPH_RDLOCK
bdrv_drv_set_perm(BlockDriverState *bs, uint64_t perm, uint64_t shared_perm,
                  Transaction *tran, Error **errp)
{
    GLOBAL_STATE_CODE();
    if (!bs->drv) {
        return 0;
    }

    if (bs->drv->bdrv_check_perm) {
        int ret = bs->drv->bdrv_check_perm(bs, perm, shared_perm, errp);
        if (ret < 0) {
            return ret;
        }
    }

    if (tran) {
        tran_add(tran, &bdrv_drv_set_perm_drv, bs);
    }

    return 0;
}

typedef struct BdrvReplaceChildState {
    BdrvChild *child;
    BlockDriverState *old_bs;
} BdrvReplaceChildState;

static void GRAPH_WRLOCK bdrv_replace_child_commit(void *opaque)
{
    BdrvReplaceChildState *s = opaque;
    GLOBAL_STATE_CODE();

    bdrv_schedule_unref(s->old_bs);
}

static void GRAPH_WRLOCK bdrv_replace_child_abort(void *opaque)
{
    BdrvReplaceChildState *s = opaque;
    BlockDriverState *new_bs = s->child->bs;

    GLOBAL_STATE_CODE();
    assert_bdrv_graph_writable();

    /* old_bs reference is transparently moved from @s to @s->child */
    if (!s->child->bs) {
        /*
         * The parents were undrained when removing old_bs from the child. New
         * requests can't have been made, though, because the child was empty.
         *
         * TODO Make bdrv_replace_child_noperm() transactionable to avoid
         * undraining the parent in the first place. Once this is done, having
         * new_bs drained when calling bdrv_replace_child_tran() is not a
         * requirement any more.
         */
        bdrv_parent_drained_begin_single(s->child);
        assert(!bdrv_parent_drained_poll_single(s->child));
    }
    assert(s->child->quiesced_parent);
    bdrv_replace_child_noperm(s->child, s->old_bs);

    bdrv_unref(new_bs);
}

static TransactionActionDrv bdrv_replace_child_drv = {
    .commit = bdrv_replace_child_commit,
    .abort = bdrv_replace_child_abort,
    .clean = g_free,
};

/*
 * bdrv_replace_child_tran
 *
 * Note: real unref of old_bs is done only on commit.
 *
 * Both @child->bs and @new_bs (if non-NULL) must be drained. @new_bs must be
 * kept drained until the transaction is completed.
 *
 * After calling this function, the transaction @tran may only be completed
 * while holding a writer lock for the graph.
 *
 * The function doesn't update permissions, caller is responsible for this.
 */
static void GRAPH_WRLOCK
bdrv_replace_child_tran(BdrvChild *child, BlockDriverState *new_bs,
                        Transaction *tran)
{
    BdrvReplaceChildState *s = g_new(BdrvReplaceChildState, 1);

    assert(child->quiesced_parent);
    assert(!new_bs || new_bs->quiesce_counter);

    *s = (BdrvReplaceChildState) {
        .child = child,
        .old_bs = child->bs,
    };
    tran_add(tran, &bdrv_replace_child_drv, s);

    if (new_bs) {
        bdrv_ref(new_bs);
    }

    bdrv_replace_child_noperm(child, new_bs);
    /* old_bs reference is transparently moved from @child to @s */
}

/*
 * Refresh permissions in @bs subtree. The function is intended to be called
 * after some graph modification that was done without permission update.
 *
 * After calling this function, the transaction @tran may only be completed
 * while holding a reader lock for the graph.
 */
static int GRAPH_RDLOCK
bdrv_node_refresh_perm(BlockDriverState *bs, BlockReopenQueue *q,
                       Transaction *tran, Error **errp)
{
    BlockDriver *drv = bs->drv;
    BdrvChild *c;
    int ret;
    uint64_t cumulative_perms, cumulative_shared_perms;
    GLOBAL_STATE_CODE();

    bdrv_get_cumulative_perm(bs, &cumulative_perms, &cumulative_shared_perms);

    /* Write permissions never work with read-only images */
    if ((cumulative_perms & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED)) &&
        !bdrv_is_writable_after_reopen(bs, q))
    {
        if (!bdrv_is_writable_after_reopen(bs, NULL)) {
            error_setg(errp, "Block node is read-only");
        } else {
            error_setg(errp, "Read-only block node '%s' cannot support "
                       "read-write users", bdrv_get_node_name(bs));
        }

        return -EPERM;
    }

    /*
     * Unaligned requests will automatically be aligned to bl.request_alignment
     * and without RESIZE we can't extend requests to write to space beyond the
     * end of the image, so it's required that the image size is aligned.
     */
    if ((cumulative_perms & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED)) &&
        !(cumulative_perms & BLK_PERM_RESIZE))
    {
        if ((bs->total_sectors * BDRV_SECTOR_SIZE) % bs->bl.request_alignment) {
            error_setg(errp, "Cannot get 'write' permission without 'resize': "
                             "Image size is not a multiple of request "
                             "alignment");
            return -EPERM;
        }
    }

    /* Check this node */
    if (!drv) {
        return 0;
    }

    ret = bdrv_drv_set_perm(bs, cumulative_perms, cumulative_shared_perms, tran,
                            errp);
    if (ret < 0) {
        return ret;
    }

    /* Drivers that never have children can omit .bdrv_child_perm() */
    if (!drv->bdrv_child_perm) {
        assert(QLIST_EMPTY(&bs->children));
        return 0;
    }

    /* Check all children */
    QLIST_FOREACH(c, &bs->children, next) {
        uint64_t cur_perm, cur_shared;

        bdrv_child_perm(bs, c->bs, c, c->role, q,
                        cumulative_perms, cumulative_shared_perms,
                        &cur_perm, &cur_shared);
        bdrv_child_set_perm(c, cur_perm, cur_shared, tran);
    }

    return 0;
}

/*
 * @list is a product of bdrv_topological_dfs() (may be called several times) -
 * a topologically sorted subgraph.
 *
 * After calling this function, the transaction @tran may only be completed
 * while holding a reader lock for the graph.
 */
static int GRAPH_RDLOCK
bdrv_do_refresh_perms(GSList *list, BlockReopenQueue *q, Transaction *tran,
                      Error **errp)
{
    int ret;
    BlockDriverState *bs;
    GLOBAL_STATE_CODE();

    for ( ; list; list = list->next) {
        bs = list->data;

        if (bdrv_parent_perms_conflict(bs, errp)) {
            return -EINVAL;
        }

        ret = bdrv_node_refresh_perm(bs, q, tran, errp);
        if (ret < 0) {
            return ret;
        }
    }

    return 0;
}

/*
 * @list is any list of nodes. List is completed by all subtrees and
 * topologically sorted. It's not a problem if some node occurs in the @list
 * several times.
 *
 * After calling this function, the transaction @tran may only be completed
 * while holding a reader lock for the graph.
 */
static int GRAPH_RDLOCK
bdrv_list_refresh_perms(GSList *list, BlockReopenQueue *q, Transaction *tran,
                        Error **errp)
{
    g_autoptr(GHashTable) found = g_hash_table_new(NULL, NULL);
    g_autoptr(GSList) refresh_list = NULL;

    for ( ; list; list = list->next) {
        refresh_list = bdrv_topological_dfs(refresh_list, found, list->data);
    }

    return bdrv_do_refresh_perms(refresh_list, q, tran, errp);
}

void bdrv_get_cumulative_perm(BlockDriverState *bs, uint64_t *perm,
                              uint64_t *shared_perm)
{
    BdrvChild *c;
    uint64_t cumulative_perms = 0;
    uint64_t cumulative_shared_perms = BLK_PERM_ALL;

    GLOBAL_STATE_CODE();

    QLIST_FOREACH(c, &bs->parents, next_parent) {
        cumulative_perms |= c->perm;
        cumulative_shared_perms &= c->shared_perm;
    }

    *perm = cumulative_perms;
    *shared_perm = cumulative_shared_perms;
}

char *bdrv_perm_names(uint64_t perm)
{
    struct perm_name {
        uint64_t perm;
        const char *name;
    } permissions[] = {
        { BLK_PERM_CONSISTENT_READ, "consistent read" },
        { BLK_PERM_WRITE,           "write" },
        { BLK_PERM_WRITE_UNCHANGED, "write unchanged" },
        { BLK_PERM_RESIZE,          "resize" },
        { 0, NULL }
    };

    GString *result = g_string_sized_new(30);
    struct perm_name *p;

    for (p = permissions; p->name; p++) {
        if (perm & p->perm) {
            if (result->len > 0) {
                g_string_append(result, ", ");
            }
            g_string_append(result, p->name);
        }
    }

    return g_string_free(result, FALSE);
}


/*
 * @tran is allowed to be NULL. In this case no rollback is possible.
 *
 * After calling this function, the transaction @tran may only be completed
 * while holding a reader lock for the graph.
 */
static int GRAPH_RDLOCK
bdrv_refresh_perms(BlockDriverState *bs, Transaction *tran, Error **errp)
{
    int ret;
    Transaction *local_tran = NULL;
    g_autoptr(GSList) list = bdrv_topological_dfs(NULL, NULL, bs);
    GLOBAL_STATE_CODE();

    if (!tran) {
        tran = local_tran = tran_new();
    }

    ret = bdrv_do_refresh_perms(list, NULL, tran, errp);

    if (local_tran) {
        tran_finalize(local_tran, ret);
    }

    return ret;
}

int bdrv_child_try_set_perm(BdrvChild *c, uint64_t perm, uint64_t shared,
                            Error **errp)
{
    Error *local_err = NULL;
    Transaction *tran = tran_new();
    int ret;

    GLOBAL_STATE_CODE();

    bdrv_child_set_perm(c, perm, shared, tran);

    ret = bdrv_refresh_perms(c->bs, tran, &local_err);

    tran_finalize(tran, ret);

    if (ret < 0) {
        if ((perm & ~c->perm) || (c->shared_perm & ~shared)) {
            /* tighten permissions */
            error_propagate(errp, local_err);
        } else {
            /*
             * Our caller may intend to only loosen restrictions and
             * does not expect this function to fail.  Errors are not
             * fatal in such a case, so we can just hide them from our
             * caller.
             */
            error_free(local_err);
            ret = 0;
        }
    }

    return ret;
}

int bdrv_child_refresh_perms(BlockDriverState *bs, BdrvChild *c, Error **errp)
{
    uint64_t parent_perms, parent_shared;
    uint64_t perms, shared;

    GLOBAL_STATE_CODE();

    bdrv_get_cumulative_perm(bs, &parent_perms, &parent_shared);
    bdrv_child_perm(bs, c->bs, c, c->role, NULL,
                    parent_perms, parent_shared, &perms, &shared);

    return bdrv_child_try_set_perm(c, perms, shared, errp);
}

/*
 * Default implementation for .bdrv_child_perm() for block filters:
 * Forward CONSISTENT_READ, WRITE, WRITE_UNCHANGED, and RESIZE to the
 * filtered child.
 */
static void bdrv_filter_default_perms(BlockDriverState *bs, BdrvChild *c,
                                      BdrvChildRole role,
                                      BlockReopenQueue *reopen_queue,
                                      uint64_t perm, uint64_t shared,
                                      uint64_t *nperm, uint64_t *nshared)
{
    GLOBAL_STATE_CODE();
    *nperm = perm & DEFAULT_PERM_PASSTHROUGH;
    *nshared = (shared & DEFAULT_PERM_PASSTHROUGH) | DEFAULT_PERM_UNCHANGED;
}

static void bdrv_default_perms_for_cow(BlockDriverState *bs, BdrvChild *c,
                                       BdrvChildRole role,
                                       BlockReopenQueue *reopen_queue,
                                       uint64_t perm, uint64_t shared,
                                       uint64_t *nperm, uint64_t *nshared)
{
    assert(role & BDRV_CHILD_COW);
    GLOBAL_STATE_CODE();

    /*
     * We want consistent read from backing files if the parent needs it.
     * No other operations are performed on backing files.
     */
    perm &= BLK_PERM_CONSISTENT_READ;

    /*
     * If the parent can deal with changing data, we're okay with a
     * writable and resizable backing file.
     * TODO Require !(perm & BLK_PERM_CONSISTENT_READ), too?
     */
    if (shared & BLK_PERM_WRITE) {
        shared = BLK_PERM_WRITE | BLK_PERM_RESIZE;
    } else {
        shared = 0;
    }

    shared |= BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED;

    if (bs->open_flags & BDRV_O_INACTIVE) {
        shared |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
    }

    *nperm = perm;
    *nshared = shared;
}

static void bdrv_default_perms_for_storage(BlockDriverState *bs, BdrvChild *c,
                                           BdrvChildRole role,
                                           BlockReopenQueue *reopen_queue,
                                           uint64_t perm, uint64_t shared,
                                           uint64_t *nperm, uint64_t *nshared)
{
    int flags;

    GLOBAL_STATE_CODE();
    assert(role & (BDRV_CHILD_METADATA | BDRV_CHILD_DATA));

    flags = bdrv_reopen_get_flags(reopen_queue, bs);

    /*
     * Apart from the modifications below, the same permissions are
     * forwarded and left alone as for filters
     */
    bdrv_filter_default_perms(bs, c, role, reopen_queue,
                              perm, shared, &perm, &shared);

    if (role & BDRV_CHILD_METADATA) {
        /* Format drivers may touch metadata even if the guest doesn't write */
        if (bdrv_is_writable_after_reopen(bs, reopen_queue)) {
            perm |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
        }

        /*
         * bs->file always needs to be consistent because of the
         * metadata. We can never allow other users to resize or write
         * to it.
         */
        if (!(flags & BDRV_O_NO_IO)) {
            perm |= BLK_PERM_CONSISTENT_READ;
        }
        shared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
    }

    if (role & BDRV_CHILD_DATA) {
        /*
         * Technically, everything in this block is a subset of the
         * BDRV_CHILD_METADATA path taken above, and so this could
         * be an "else if" branch.  However, that is not obvious, and
         * this function is not performance critical, therefore we let
         * this be an independent "if".
         */

        /*
         * We cannot allow other users to resize the file because the
         * format driver might have some assumptions about the size
         * (e.g. because it is stored in metadata, or because the file
         * is split into fixed-size data files).
         */
        shared &= ~BLK_PERM_RESIZE;

        /*
         * WRITE_UNCHANGED often cannot be performed as such on the
         * data file.  For example, the qcow2 driver may still need to
         * write copied clusters on copy-on-read.
         */
        if (perm & BLK_PERM_WRITE_UNCHANGED) {
            perm |= BLK_PERM_WRITE;
        }

        /*
         * If the data file is written to, the format driver may
         * expect to be able to resize it by writing beyond the EOF.
         */
        if (perm & BLK_PERM_WRITE) {
            perm |= BLK_PERM_RESIZE;
        }
    }

    if (bs->open_flags & BDRV_O_INACTIVE) {
        shared |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
    }

    *nperm = perm;
    *nshared = shared;
}

void bdrv_default_perms(BlockDriverState *bs, BdrvChild *c,
                        BdrvChildRole role, BlockReopenQueue *reopen_queue,
                        uint64_t perm, uint64_t shared,
                        uint64_t *nperm, uint64_t *nshared)
{
    GLOBAL_STATE_CODE();
    if (role & BDRV_CHILD_FILTERED) {
        assert(!(role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA |
                         BDRV_CHILD_COW)));
        bdrv_filter_default_perms(bs, c, role, reopen_queue,
                                  perm, shared, nperm, nshared);
    } else if (role & BDRV_CHILD_COW) {
        assert(!(role & (BDRV_CHILD_DATA | BDRV_CHILD_METADATA)));
        bdrv_default_perms_for_cow(bs, c, role, reopen_queue,
                                   perm, shared, nperm, nshared);
    } else if (role & (BDRV_CHILD_METADATA | BDRV_CHILD_DATA)) {
        bdrv_default_perms_for_storage(bs, c, role, reopen_queue,
                                       perm, shared, nperm, nshared);
    } else {
        g_assert_not_reached();
    }
}

uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm)
{
    static const uint64_t permissions[] = {
        [BLOCK_PERMISSION_CONSISTENT_READ]  = BLK_PERM_CONSISTENT_READ,
        [BLOCK_PERMISSION_WRITE]            = BLK_PERM_WRITE,
        [BLOCK_PERMISSION_WRITE_UNCHANGED]  = BLK_PERM_WRITE_UNCHANGED,
        [BLOCK_PERMISSION_RESIZE]           = BLK_PERM_RESIZE,
    };

    QEMU_BUILD_BUG_ON(ARRAY_SIZE(permissions) != BLOCK_PERMISSION__MAX);
    QEMU_BUILD_BUG_ON(1UL << ARRAY_SIZE(permissions) != BLK_PERM_ALL + 1);

    assert(qapi_perm < BLOCK_PERMISSION__MAX);

    return permissions[qapi_perm];
}

/*
 * Replaces the node that a BdrvChild points to without updating permissions.
 *
 * If @new_bs is non-NULL, the parent of @child must already be drained through
 * @child.
 */
static void GRAPH_WRLOCK
bdrv_replace_child_noperm(BdrvChild *child, BlockDriverState *new_bs)
{
    BlockDriverState *old_bs = child->bs;
    int new_bs_quiesce_counter;

    assert(!child->frozen);

    /*
     * If we want to change the BdrvChild to point to a drained node as its new
     * child->bs, we need to make sure that its new parent is drained, too. In
     * other words, either child->quiesce_parent must already be true or we must
     * be able to set it and keep the parent's quiesce_counter consistent with
     * that, but without polling or starting new requests (this function
     * guarantees that it doesn't poll, and starting new requests would be
     * against the invariants of drain sections).
     *
     * To keep things simple, we pick the first option (child->quiesce_parent
     * must already be true). We also generalise the rule a bit to make it
     * easier to verify in callers and more likely to be covered in test cases:
     * The parent must be quiesced through this child even if new_bs isn't
     * currently drained.
     *
     * The only exception is for callers that always pass new_bs == NULL. In
     * this case, we obviously never need to consider the case of a drained
     * new_bs, so we can keep the callers simpler by allowing them not to drain
     * the parent.
     */
    assert(!new_bs || child->quiesced_parent);
    assert(old_bs != new_bs);
    GLOBAL_STATE_CODE();

    if (old_bs && new_bs) {
        assert(bdrv_get_aio_context(old_bs) == bdrv_get_aio_context(new_bs));
    }

    if (old_bs) {
        if (child->klass->detach) {
            child->klass->detach(child);
        }
        QLIST_REMOVE(child, next_parent);
    }

    child->bs = new_bs;

    if (new_bs) {
        QLIST_INSERT_HEAD(&new_bs->parents, child, next_parent);
        if (child->klass->attach) {
            child->klass->attach(child);
        }
    }

    /*
     * If the parent was drained through this BdrvChild previously, but new_bs
     * is not drained, allow requests to come in only after the new node has
     * been attached.
     */
    new_bs_quiesce_counter = (new_bs ? new_bs->quiesce_counter : 0);
    if (!new_bs_quiesce_counter && child->quiesced_parent) {
        bdrv_parent_drained_end_single(child);
    }
}

/**
 * Free the given @child.
 *
 * The child must be empty (i.e. `child->bs == NULL`) and it must be
 * unused (i.e. not in a children list).

 */
static void bdrv_child_free(BdrvChild *child)
{
    assert(!child->bs);
    GLOBAL_STATE_CODE();
    GRAPH_RDLOCK_GUARD_MAINLOOP();

    assert(!child->next.le_prev); /* not in children list */

    g_free(child->name);
    g_free(child);
}

typedef struct BdrvAttachChildCommonState {
    BdrvChild *child;
    AioContext *old_parent_ctx;
    AioContext *old_child_ctx;
} BdrvAttachChildCommonState;

static void GRAPH_WRLOCK bdrv_attach_child_common_abort(void *opaque)
{
    BdrvAttachChildCommonState *s = opaque;
    BlockDriverState *bs = s->child->bs;

    GLOBAL_STATE_CODE();
    assert_bdrv_graph_writable();

    bdrv_replace_child_noperm(s->child, NULL);

    if (bdrv_get_aio_context(bs) != s->old_child_ctx) {
        bdrv_try_change_aio_context_locked(bs, s->old_child_ctx, NULL,
                                           &error_abort);
    }

    if (bdrv_child_get_parent_aio_context(s->child) != s->old_parent_ctx) {
        Transaction *tran;
        GHashTable *visited;
        bool ret;

        tran = tran_new();

        /* No need to visit `child`, because it has been detached already */
        visited = g_hash_table_new(NULL, NULL);
        ret = s->child->klass->change_aio_ctx(s->child, s->old_parent_ctx,
                                              visited, tran, &error_abort);
        g_hash_table_destroy(visited);

        /* transaction is supposed to always succeed */
        assert(ret == true);
        tran_commit(tran);
    }

    bdrv_schedule_unref(bs);
    bdrv_child_free(s->child);
}

static TransactionActionDrv bdrv_attach_child_common_drv = {
    .abort = bdrv_attach_child_common_abort,
    .clean = g_free,
};

/*
 * Common part of attaching bdrv child to bs or to blk or to job
 *
 * Function doesn't update permissions, caller is responsible for this.
 *
 * After calling this function, the transaction @tran may only be completed
 * while holding a writer lock for the graph.
 *
 * Returns new created child.
 *
 * Both @parent_bs and @child_bs can move to a different AioContext in this
 * function.
 *
 * All block nodes must be drained before this function is called until after
 * the transaction is finalized.
 */
static BdrvChild * GRAPH_WRLOCK
bdrv_attach_child_common(BlockDriverState *child_bs,
                         const char *child_name,
                         const BdrvChildClass *child_class,
                         BdrvChildRole child_role,
                         uint64_t perm, uint64_t shared_perm,
                         void *opaque,
                         Transaction *tran, Error **errp)
{
    BdrvChild *new_child;
    AioContext *parent_ctx;
    AioContext *child_ctx = bdrv_get_aio_context(child_bs);

    assert(child_class->get_parent_desc);
    GLOBAL_STATE_CODE();

    if (bdrv_is_inactive(child_bs) && (perm & ~BLK_PERM_CONSISTENT_READ)) {
        g_autofree char *perm_names = bdrv_perm_names(perm);
        error_setg(errp, "Permission '%s' unavailable on inactive node",
                   perm_names);
        return NULL;
    }

    new_child = g_new(BdrvChild, 1);
    *new_child = (BdrvChild) {
        .bs             = NULL,
        .name           = g_strdup(child_name),
        .klass          = child_class,
        .role           = child_role,
        .perm           = perm,
        .shared_perm    = shared_perm,
        .opaque         = opaque,
    };

    /*
     * If the AioContexts don't match, first try to move the subtree of
     * child_bs into the AioContext of the new parent. If this doesn't work,
     * try moving the parent into the AioContext of child_bs instead.
     */
    parent_ctx = bdrv_child_get_parent_aio_context(new_child);
    if (child_ctx != parent_ctx) {
        Error *local_err = NULL;
        int ret = bdrv_try_change_aio_context_locked(child_bs, parent_ctx, NULL,
                                                     &local_err);

        if (ret < 0 && child_class->change_aio_ctx) {
            Transaction *aio_ctx_tran = tran_new();
            GHashTable *visited = g_hash_table_new(NULL, NULL);
            bool ret_child;

            g_hash_table_add(visited, new_child);
            ret_child = child_class->change_aio_ctx(new_child, child_ctx,
                                                    visited, aio_ctx_tran,
                                                    NULL);
            if (ret_child == true) {
                error_free(local_err);
                ret = 0;
            }
            tran_finalize(aio_ctx_tran, ret_child == true ? 0 : -1);
            g_hash_table_destroy(visited);
        }

        if (ret < 0) {
            error_propagate(errp, local_err);
            bdrv_child_free(new_child);
            return NULL;
        }
    }

    bdrv_ref(child_bs);
    /*
     * Let every new BdrvChild start with a drained parent. Inserting the child
     * in the graph with bdrv_replace_child_noperm() will undrain it if
     * @child_bs is not drained.
     *
     * The child was only just created and is not yet visible in global state
     * until bdrv_replace_child_noperm() inserts it into the graph, so nobody
     * could have sent requests and polling is not necessary.
     *
     * Note that this means that the parent isn't fully drained yet, we only
     * stop new requests from coming in. This is fine, we don't care about the
     * old requests here, they are not for this child. If another place enters a
     * drain section for the same parent, but wants it to be fully quiesced, it
     * will not run most of the code in .drained_begin() again (which is not
     * a problem, we already did this), but it will still poll until the parent
     * is fully quiesced, so it will not be negatively affected either.
     */
    bdrv_parent_drained_begin_single(new_child);
    bdrv_replace_child_noperm(new_child, child_bs);

    BdrvAttachChildCommonState *s = g_new(BdrvAttachChildCommonState, 1);
    *s = (BdrvAttachChildCommonState) {
        .child = new_child,
        .old_parent_ctx = parent_ctx,
        .old_child_ctx = child_ctx,
    };
    tran_add(tran, &bdrv_attach_child_common_drv, s);

    return new_child;
}

/*
 * Function doesn't update permissions, caller is responsible for this.
 *
 * Both @parent_bs and @child_bs can move to a different AioContext in this
 * function.
 *
 * After calling this function, the transaction @tran may only be completed
 * while holding a writer lock for the graph.
 *
 * All block nodes must be drained before this function is called until after
 * the transaction is finalized.
 */
static BdrvChild * GRAPH_WRLOCK
bdrv_attach_child_noperm(BlockDriverState *parent_bs,
                         BlockDriverState *child_bs,
                         const char *child_name,
                         const BdrvChildClass *child_class,
                         BdrvChildRole child_role,
                         Transaction *tran,
                         Error **errp)
{
    uint64_t perm, shared_perm;

    assert(parent_bs->drv);
    GLOBAL_STATE_CODE();

    if (bdrv_recurse_has_child(child_bs, parent_bs)) {
        error_setg(errp, "Making '%s' a %s child of '%s' would create a cycle",
                   child_bs->node_name, child_name, parent_bs->node_name);
        return NULL;
    }
    if (bdrv_is_inactive(child_bs) && !bdrv_is_inactive(parent_bs)) {
        error_setg(errp, "Inactive '%s' can't be a %s child of active '%s'",
                   child_bs->node_name, child_name, parent_bs->node_name);
        return NULL;
    }

    bdrv_get_cumulative_perm(parent_bs, &perm, &shared_perm);
    bdrv_child_perm(parent_bs, child_bs, NULL, child_role, NULL,
                    perm, shared_perm, &perm, &shared_perm);

    return bdrv_attach_child_common(child_bs, child_name, child_class,
                                    child_role, perm, shared_perm, parent_bs,
                                    tran, errp);
}

/*
 * This function steals the reference to child_bs from the caller.
 * That reference is later dropped by bdrv_root_unref_child().
 *
 * On failure NULL is returned, errp is set and the reference to
 * child_bs is also dropped.
 *
 * All block nodes must be drained.
 */
BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                  const char *child_name,
                                  const BdrvChildClass *child_class,
                                  BdrvChildRole child_role,
                                  uint64_t perm, uint64_t shared_perm,
                                  void *opaque, Error **errp)
{
    int ret;
    BdrvChild *child;
    Transaction *tran = tran_new();

    GLOBAL_STATE_CODE();

    child = bdrv_attach_child_common(child_bs, child_name, child_class,
                                   child_role, perm, shared_perm, opaque,
                                   tran, errp);
    if (!child) {
        ret = -EINVAL;
        goto out;
    }

    ret = bdrv_refresh_perms(child_bs, tran, errp);

out:
    tran_finalize(tran, ret);

    bdrv_schedule_unref(child_bs);

    return ret < 0 ? NULL : child;
}

/*
 * This function transfers the reference to child_bs from the caller
 * to parent_bs. That reference is later dropped by parent_bs on
 * bdrv_close() or if someone calls bdrv_unref_child().
 *
 * On failure NULL is returned, errp is set and the reference to
 * child_bs is also dropped.
 *
 * All block nodes must be drained.
 */
BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
                             BlockDriverState *child_bs,
                             const char *child_name,
                             const BdrvChildClass *child_class,
                             BdrvChildRole child_role,
                             Error **errp)
{
    int ret;
    BdrvChild *child;
    Transaction *tran = tran_new();

    GLOBAL_STATE_CODE();

    child = bdrv_attach_child_noperm(parent_bs, child_bs, child_name,
                                     child_class, child_role, tran, errp);
    if (!child) {
        ret = -EINVAL;
        goto out;
    }

    ret = bdrv_refresh_perms(parent_bs, tran, errp);
    if (ret < 0) {
        goto out;
    }

out:
    tran_finalize(tran, ret);

    bdrv_schedule_unref(child_bs);

    return ret < 0 ? NULL : child;
}

/*
 * Callers must ensure that child->frozen is false.
 *
 * All block nodes must be drained.
 */
void bdrv_root_unref_child(BdrvChild *child)
{
    BlockDriverState *child_bs = child->bs;

    GLOBAL_STATE_CODE();
    bdrv_replace_child_noperm(child, NULL);
    bdrv_child_free(child);

    if (child_bs) {
        /*
         * Update permissions for old node. We're just taking a parent away, so
         * we're loosening restrictions. Errors of permission update are not
         * fatal in this case, ignore them.
         */
        bdrv_refresh_perms(child_bs, NULL, NULL);

        /*
         * When the parent requiring a non-default AioContext is removed, the
         * node moves back to the main AioContext
         */
        bdrv_try_change_aio_context_locked(child_bs, qemu_get_aio_context(),
                                           NULL, NULL);
    }

    bdrv_schedule_unref(child_bs);
}

typedef struct BdrvSetInheritsFrom {
    BlockDriverState *bs;
    BlockDriverState *old_inherits_from;
} BdrvSetInheritsFrom;

static void bdrv_set_inherits_from_abort(void *opaque)
{
    BdrvSetInheritsFrom *s = opaque;

    s->bs->inherits_from = s->old_inherits_from;
}

static TransactionActionDrv bdrv_set_inherits_from_drv = {
    .abort = bdrv_set_inherits_from_abort,
    .clean = g_free,
};

/* @tran is allowed to be NULL. In this case no rollback is possible */
static void bdrv_set_inherits_from(BlockDriverState *bs,
                                   BlockDriverState *new_inherits_from,
                                   Transaction *tran)
{
    if (tran) {
        BdrvSetInheritsFrom *s = g_new(BdrvSetInheritsFrom, 1);

        *s = (BdrvSetInheritsFrom) {
            .bs = bs,
            .old_inherits_from = bs->inherits_from,
        };

        tran_add(tran, &bdrv_set_inherits_from_drv, s);
    }

    bs->inherits_from = new_inherits_from;
}

/**
 * Clear all inherits_from pointers from children and grandchildren of
 * @root that point to @root, where necessary.
 * @tran is allowed to be NULL. In this case no rollback is possible
 */
static void GRAPH_WRLOCK
bdrv_unset_inherits_from(BlockDriverState *root, BdrvChild *child,
                         Transaction *tran)
{
    BdrvChild *c;

    if (child->bs->inherits_from == root) {
        /*
         * Remove inherits_from only when the last reference between root and
         * child->bs goes away.
         */
        QLIST_FOREACH(c, &root->children, next) {
            if (c != child && c->bs == child->bs) {
                break;
            }
        }
        if (c == NULL) {
            bdrv_set_inherits_from(child->bs, NULL, tran);
        }
    }

    QLIST_FOREACH(c, &child->bs->children, next) {
        bdrv_unset_inherits_from(root, c, tran);
    }
}

/*
 * Callers must ensure that child->frozen is false.
 *
 * All block nodes must be drained.
 */
void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child)
{
    GLOBAL_STATE_CODE();
    if (child == NULL) {
        return;
    }

    bdrv_unset_inherits_from(parent, child, NULL);
    bdrv_root_unref_child(child);
}


static void GRAPH_RDLOCK
bdrv_parent_cb_change_media(BlockDriverState *bs, bool load)
{
    BdrvChild *c;
    GLOBAL_STATE_CODE();
    QLIST_FOREACH(c, &bs->parents, next_parent) {
        if (c->klass->change_media) {
            c->klass->change_media(c, load);
        }
    }
}

/* Return true if you can reach parent going through child->inherits_from
 * recursively. If parent or child are NULL, return false */
static bool bdrv_inherits_from_recursive(BlockDriverState *child,
                                         BlockDriverState *parent)
{
    while (child && child != parent) {
        child = child->inherits_from;
    }

    return child != NULL;
}

/*
 * Return the BdrvChildRole for @bs's backing child.  bs->backing is
 * mostly used for COW backing children (role = COW), but also for
 * filtered children (role = FILTERED | PRIMARY).
 */
static BdrvChildRole bdrv_backing_role(BlockDriverState *bs)
{
    if (bs->drv && bs->drv->is_filter) {
        return BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY;
    } else {
        return BDRV_CHILD_COW;
    }
}

/*
 * Sets the bs->backing or bs->file link of a BDS. A new reference is created;
 * callers which don't need their own reference any more must call bdrv_unref().
 *
 * If the respective child is already present (i.e. we're detaching a node),
 * that child node must be drained.
 *
 * Function doesn't update permissions, caller is responsible for this.
 *
 * Both @parent_bs and @child_bs can move to a different AioContext in this
 * function.
 *
 * After calling this function, the transaction @tran may only be completed
 * while holding a writer lock for the graph.
 *
 * All block nodes must be drained before this function is called until after
 * the transaction is finalized.
 */
static int GRAPH_WRLOCK
bdrv_set_file_or_backing_noperm(BlockDriverState *parent_bs,
                                BlockDriverState *child_bs,
                                bool is_backing,
                                Transaction *tran, Error **errp)
{
    bool update_inherits_from =
        bdrv_inherits_from_recursive(child_bs, parent_bs);
    BdrvChild *child = is_backing ? parent_bs->backing : parent_bs->file;
    BdrvChildRole role;

    GLOBAL_STATE_CODE();

    if (!parent_bs->drv) {
        /*
         * Node without drv is an object without a class :/. TODO: finally fix
         * qcow2 driver to never clear bs->drv and implement format corruption
         * handling in other way.
         */
        error_setg(errp, "Node corrupted");
        return -EINVAL;
    }

    if (child && child->frozen) {
        error_setg(errp, "Cannot change frozen '%s' link from '%s' to '%s'",
                   child->name, parent_bs->node_name, child->bs->node_name);
        return -EPERM;
    }

    if (is_backing && !parent_bs->drv->is_filter &&
        !parent_bs->drv->supports_backing)
    {
        error_setg(errp, "Driver '%s' of node '%s' does not support backing "
                   "files", parent_bs->drv->format_name, parent_bs->node_name);
        return -EINVAL;
    }

    if (parent_bs->drv->is_filter) {
        role = BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY;
    } else if (is_backing) {
        role = BDRV_CHILD_COW;
    } else {
        /*
         * We only can use same role as it is in existing child. We don't have
         * infrastructure to determine role of file child in generic way
         */
        if (!child) {
            error_setg(errp, "Cannot set file child to format node without "
                       "file child");
            return -EINVAL;
        }
        role = child->role;
    }

    if (child) {
        assert(child->bs->quiesce_counter);
        bdrv_unset_inherits_from(parent_bs, child, tran);
        bdrv_remove_child(child, tran);
    }

    if (!child_bs) {
        goto out;
    }

    child = bdrv_attach_child_noperm(parent_bs, child_bs,
                                     is_backing ? "backing" : "file",
                                     &child_of_bds, role,
                                     tran, errp);
    if (!child) {
        return -EINVAL;
    }


    /*
     * If inherits_from pointed recursively to bs then let's update it to
     * point directly to bs (else it will become NULL).
     */
    if (update_inherits_from) {
        bdrv_set_inherits_from(child_bs, parent_bs, tran);
    }

out:
    bdrv_refresh_limits(parent_bs, tran, NULL);

    return 0;
}

/*
 * Both @bs and @backing_hd can move to a different AioContext in this
 * function.
 *
 * All block nodes must be drained.
 */
int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
                        Error **errp)
{
    int ret;
    Transaction *tran = tran_new();

    GLOBAL_STATE_CODE();
    assert(bs->quiesce_counter > 0);
    if (bs->backing) {
        assert(bs->backing->bs->quiesce_counter > 0);
    }

    ret = bdrv_set_file_or_backing_noperm(bs, backing_hd, true, tran, errp);
    if (ret < 0) {
        goto out;
    }

    ret = bdrv_refresh_perms(bs, tran, errp);
out:
    tran_finalize(tran, ret);
    return ret;
}

/*
 * Opens the backing file for a BlockDriverState if not yet open
 *
 * bdref_key specifies the key for the image's BlockdevRef in the options QDict.
 * That QDict has to be flattened; therefore, if the BlockdevRef is a QDict
 * itself, all options starting with "${bdref_key}." are considered part of the
 * BlockdevRef.
 *
 * TODO Can this be unified with bdrv_open_image()?
 */
int bdrv_open_backing_file(BlockDriverState *bs, QDict *parent_options,
                           const char *bdref_key, Error **errp)
{
    ERRP_GUARD();
    char *backing_filename = NULL;
    char *bdref_key_dot;
    const char *reference = NULL;
    int ret = 0;
    bool implicit_backing = false;
    BlockDriverState *backing_hd;
    QDict *options;
    QDict *tmp_parent_options = NULL;
    Error *local_err = NULL;

    GLOBAL_STATE_CODE();

    bdrv_graph_rdlock_main_loop();

    if (bs->backing != NULL) {
        goto free_exit;
    }

    /* NULL means an empty set of options */
    if (parent_options == NULL) {
        tmp_parent_options = qdict_new();
        parent_options = tmp_parent_options;
    }

    bs->open_flags &= ~BDRV_O_NO_BACKING;

    bdref_key_dot = g_strdup_printf("%s.", bdref_key);
    qdict_extract_subqdict(parent_options, &options, bdref_key_dot);
    g_free(bdref_key_dot);

    /*
     * Caution: while qdict_get_try_str() is fine, getting non-string
     * types would require more care.  When @parent_options come from
     * -blockdev or blockdev_add, its members are typed according to
     * the QAPI schema, but when they come from -drive, they're all
     * QString.
     */
    reference = qdict_get_try_str(parent_options, bdref_key);
    if (reference || qdict_haskey(options, "file.filename")) {
        /* keep backing_filename NULL */
    } else if (bs->backing_file[0] == '\0' && qdict_size(options) == 0) {
        qobject_unref(options);
        goto free_exit;
    } else {
        if (qdict_size(options) == 0) {
            /* If the user specifies options that do not modify the
             * backing file's behavior, we might still consider it the
             * implicit backing file.  But it's easier this way, and
             * just specifying some of the backing BDS's options is
             * only possible with -drive anyway (otherwise the QAPI
             * schema forces the user to specify everything). */
            implicit_backing = !strcmp(bs->auto_backing_file, bs->backing_file);
        }

        backing_filename = bdrv_get_full_backing_filename(bs, &local_err);
        if (local_err) {
            ret = -EINVAL;
            error_propagate(errp, local_err);
            qobject_unref(options);
            goto free_exit;
        }
    }

    if (!bs->drv || !bs->drv->supports_backing) {
        ret = -EINVAL;
        error_setg(errp, "Driver doesn't support backing files");
        qobject_unref(options);
        goto free_exit;
    }

    if (!reference &&
        bs->backing_format[0] != '\0' && !qdict_haskey(options, "driver")) {
        qdict_put_str(options, "driver", bs->backing_format);
    }

    backing_hd = bdrv_open_inherit(backing_filename, reference, options, 0, bs,
                                   &child_of_bds, bdrv_backing_role(bs), true,
                                   errp);
    if (!backing_hd) {
        bs->open_flags |= BDRV_O_NO_BACKING;
        error_prepend(errp, "Could not open backing file: ");
        ret = -EINVAL;
        goto free_exit;
    }

    if (implicit_backing) {
        bdrv_refresh_filename(backing_hd);
        pstrcpy(bs->auto_backing_file, sizeof(bs->auto_backing_file),
                backing_hd->filename);
    }

    /* Hook up the backing file link; drop our reference, bs owns the
     * backing_hd reference now */
    bdrv_graph_rdunlock_main_loop();
    bdrv_graph_wrlock_drained();
    ret = bdrv_set_backing_hd(bs, backing_hd, errp);
    bdrv_graph_wrunlock();
    bdrv_graph_rdlock_main_loop();
    bdrv_unref(backing_hd);

    if (ret < 0) {
        goto free_exit;
    }

    qdict_del(parent_options, bdref_key);

free_exit:
    g_free(backing_filename);
    qobject_unref(tmp_parent_options);
    bdrv_graph_rdunlock_main_loop();
    return ret;
}

static BlockDriverState *
bdrv_open_child_bs(const char *filename, QDict *options, const char *bdref_key,
                   BlockDriverState *parent, const BdrvChildClass *child_class,
                   BdrvChildRole child_role, bool allow_none,
                   bool parse_filename, Error **errp)
{
    BlockDriverState *bs = NULL;
    QDict *image_options;
    char *bdref_key_dot;
    const char *reference;

    assert(child_class != NULL);

    bdref_key_dot = g_strdup_printf("%s.", bdref_key);
    qdict_extract_subqdict(options, &image_options, bdref_key_dot);
    g_free(bdref_key_dot);

    /*
     * Caution: while qdict_get_try_str() is fine, getting non-string
     * types would require more care.  When @options come from
     * -blockdev or blockdev_add, its members are typed according to
     * the QAPI schema, but when they come from -drive, they're all
     * QString.
     */
    reference = qdict_get_try_str(options, bdref_key);
    if (!filename && !reference && !qdict_size(image_options)) {
        if (!allow_none) {
            error_setg(errp, "A block device must be specified for \"%s\"",
                       bdref_key);
        }
        qobject_unref(image_options);
        goto done;
    }

    bs = bdrv_open_inherit(filename, reference, image_options, 0,
                           parent, child_class, child_role, parse_filename,
                           errp);
    if (!bs) {
        goto done;
    }

done:
    qdict_del(options, bdref_key);
    return bs;
}

static BdrvChild * GRAPH_UNLOCKED
bdrv_open_child_common(const char *filename, QDict *options,
                       const char *bdref_key, BlockDriverState *parent,
                       const BdrvChildClass *child_class,
                       BdrvChildRole child_role, bool allow_none,
                       bool parse_filename, Error **errp)
{
    BlockDriverState *bs;
    BdrvChild *child;

    GLOBAL_STATE_CODE();

    bs = bdrv_open_child_bs(filename, options, bdref_key, parent, child_class,
                            child_role, allow_none, parse_filename, errp);
    if (bs == NULL) {
        return NULL;
    }

    bdrv_graph_wrlock_drained();
    child = bdrv_attach_child(parent, bs, bdref_key, child_class, child_role,
                              errp);
    bdrv_graph_wrunlock();

    return child;
}

/*
 * Opens a disk image whose options are given as BlockdevRef in another block
 * device's options.
 *
 * If allow_none is true, no image will be opened if filename is false and no
 * BlockdevRef is given. NULL will be returned, but errp remains unset.
 *
 * bdrev_key specifies the key for the image's BlockdevRef in the options QDict.
 * That QDict has to be flattened; therefore, if the BlockdevRef is a QDict
 * itself, all options starting with "${bdref_key}." are considered part of the
 * BlockdevRef.
 *
 * The BlockdevRef will be removed from the options QDict.
 *
 * @parent can move to a different AioContext in this function.
 */
BdrvChild *bdrv_open_child(const char *filename,
                           QDict *options, const char *bdref_key,
                           BlockDriverState *parent,
                           const BdrvChildClass *child_class,
                           BdrvChildRole child_role,
                           bool allow_none, Error **errp)
{
    return bdrv_open_child_common(filename, options, bdref_key, parent,
                                  child_class, child_role, allow_none, false,
                                  errp);
}

/*
 * This does mostly the same as bdrv_open_child(), but for opening the primary
 * child of a node. A notable difference from bdrv_open_child() is that it
 * enables filename parsing for protocol names (including json:).
 *
 * @parent can move to a different AioContext in this function.
 */
int bdrv_open_file_child(const char *filename,
                         QDict *options, const char *bdref_key,
                         BlockDriverState *parent, Error **errp)
{
    BdrvChildRole role;

    /* commit_top and mirror_top don't use this function */
    assert(!parent->drv->filtered_child_is_backing);
    role = parent->drv->is_filter ?
        (BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY) : BDRV_CHILD_IMAGE;

    if (!bdrv_open_child_common(filename, options, bdref_key, parent,
                                &child_of_bds, role, false, true, errp))
    {
        return -EINVAL;
    }

    return 0;
}

/*
 * TODO Future callers may need to specify parent/child_class in order for
 * option inheritance to work. Existing callers use it for the root node.
 */
BlockDriverState *bdrv_open_blockdev_ref(BlockdevRef *ref, Error **errp)
{
    BlockDriverState *bs = NULL;
    QObject *obj = NULL;
    QDict *qdict = NULL;
    const char *reference = NULL;
    Visitor *v = NULL;

    GLOBAL_STATE_CODE();

    if (ref->type == QTYPE_QSTRING) {
        reference = ref->u.reference;
    } else {
        BlockdevOptions *options = &ref->u.definition;
        assert(ref->type == QTYPE_QDICT);

        v = qobject_output_visitor_new(&obj);
        visit_type_BlockdevOptions(v, NULL, &options, &error_abort);
        visit_complete(v, &obj);

        qdict = qobject_to(QDict, obj);
        qdict_flatten(qdict);

        /* bdrv_open_inherit() defaults to the values in bdrv_flags (for
         * compatibility with other callers) rather than what we want as the
         * real defaults. Apply the defaults here instead. */
        qdict_set_default_str(qdict, BDRV_OPT_CACHE_DIRECT, "off");
        qdict_set_default_str(qdict, BDRV_OPT_CACHE_NO_FLUSH, "off");
        qdict_set_default_str(qdict, BDRV_OPT_READ_ONLY, "off");
        qdict_set_default_str(qdict, BDRV_OPT_AUTO_READ_ONLY, "off");

    }

    bs = bdrv_open_inherit(NULL, reference, qdict, 0, NULL, NULL, 0, false,
                           errp);
    obj = NULL;
    qobject_unref(obj);
    visit_free(v);
    return bs;
}

static BlockDriverState *bdrv_append_temp_snapshot(BlockDriverState *bs,
                                                   int flags,
                                                   QDict *snapshot_options,
                                                   Error **errp)
{
    ERRP_GUARD();
    g_autofree char *tmp_filename = NULL;
    int64_t total_size;
    QemuOpts *opts = NULL;
    BlockDriverState *bs_snapshot = NULL;
    int ret;

    GLOBAL_STATE_CODE();

    /* if snapshot, we create a temporary backing file and open it
       instead of opening 'filename' directly */

    /* Get the required size from the image */
    total_size = bdrv_getlength(bs);

    if (total_size < 0) {
        error_setg_errno(errp, -total_size, "Could not get image size");
        goto out;
    }

    /* Create the temporary image */
    tmp_filename = create_tmp_file(errp);
    if (!tmp_filename) {
        goto out;
    }

    opts = qemu_opts_create(bdrv_qcow2.create_opts, NULL, 0,
                            &error_abort);
    qemu_opt_set_number(opts, BLOCK_OPT_SIZE, total_size, &error_abort);
    ret = bdrv_create(&bdrv_qcow2, tmp_filename, opts, errp);
    qemu_opts_del(opts);
    if (ret < 0) {
        error_prepend(errp, "Could not create temporary overlay '%s': ",
                      tmp_filename);
        goto out;
    }

    /* Prepare options QDict for the temporary file */
    qdict_put_str(snapshot_options, "file.driver", "file");
    qdict_put_str(snapshot_options, "file.filename", tmp_filename);
    qdict_put_str(snapshot_options, "driver", "qcow2");

    bs_snapshot = bdrv_open(NULL, NULL, snapshot_options, flags, errp);
    snapshot_options = NULL;
    if (!bs_snapshot) {
        goto out;
    }

    ret = bdrv_append(bs_snapshot, bs, errp);
    if (ret < 0) {
        bs_snapshot = NULL;
        goto out;
    }

out:
    qobject_unref(snapshot_options);
    return bs_snapshot;
}

/*
 * Opens a disk image (raw, qcow2, vmdk, ...)
 *
 * options is a QDict of options to pass to the block drivers, or NULL for an
 * empty set of options. The reference to the QDict belongs to the block layer
 * after the call (even on failure), so if the caller intends to reuse the
 * dictionary, it needs to use qobject_ref() before calling bdrv_open.
 *
 * If *pbs is NULL, a new BDS will be created with a pointer to it stored there.
 * If it is not NULL, the referenced BDS will be reused.
 *
 * The reference parameter may be used to specify an existing block device which
 * should be opened. If specified, neither options nor a filename may be given,
 * nor can an existing BDS be reused (that is, *pbs has to be NULL).
 */
static BlockDriverState * no_coroutine_fn
bdrv_open_inherit(const char *filename, const char *reference, QDict *options,
                  int flags, BlockDriverState *parent,
                  const BdrvChildClass *child_class, BdrvChildRole child_role,
                  bool parse_filename, Error **errp)
{
    int ret;
    BlockBackend *file = NULL;
    BlockDriverState *bs;
    BlockDriver *drv = NULL;
    BdrvChild *child;
    const char *drvname;
    const char *backing;
    Error *local_err = NULL;
    QDict *snapshot_options = NULL;
    int snapshot_flags = 0;

    assert(!child_class || !flags);
    assert(!child_class == !parent);
    GLOBAL_STATE_CODE();
    assert(!qemu_in_coroutine());

    /* TODO We'll eventually have to take a writer lock in this function */
    GRAPH_RDLOCK_GUARD_MAINLOOP();

    if (reference) {
        bool options_non_empty = options ? qdict_size(options) : false;