/* vi: set sw=4 ts=4: */
/*
 * xreadlink.c - safe implementation of readlink.
 * Returns a NULL on failure.
 *
 * Licensed under GPLv2, see file LICENSE in this source tree.
 */
#include "libbb.h"

/* Some systems (eg Hurd) do not have MAXSYMLINKS definition,
 * set it to some reasonable value if it isn't defined */
#ifndef MAXSYMLINKS
# define MAXSYMLINKS 20
#endif

/*
 * NOTE: This function returns a malloced char* that you will have to free
 * yourself.
 */
char* FAST_FUNC xmalloc_readlink(const char *path)
{
        enum { GROWBY = 80 }; /* how large we will grow strings by */

        char *buf = NULL;
        int bufsize = 0, readsize = 0;

        do {
                bufsize += GROWBY;
                buf = xrealloc(buf, bufsize);
                readsize = readlink(path, buf, bufsize);
                if (readsize == -1) {
                        free(buf);
                        return NULL;
                }
        } while (bufsize < readsize + 1);

        buf[readsize] = '\0';

        return buf;
}

/*
 * This routine is not the same as realpath(), which
 * canonicalizes the given path completely. This routine only
 * follows trailing symlinks until a real file is reached and
 * returns its name. If the path ends in a dangling link or if
 * the target doesn't exist, the path is returned in any case.
 * Intermediate symlinks in the path are not expanded -- only
 * those at the tail.
 * A malloced char* is returned, which must be freed by the caller.
 */
char* FAST_FUNC xmalloc_follow_symlinks(const char *path)
{
        char *buf;
        char *lpc;
        char *linkpath;
        int bufsize;
        int looping = MAXSYMLINKS + 1;

        buf = xstrdup(path);
        goto jump_in;

        while (1) {
                linkpath = xmalloc_readlink(buf);
                if (!linkpath) {
                        /* not a symlink, or doesn't exist */
                        if (errno == EINVAL || errno == ENOENT)
                                return buf;
                        goto free_buf_ret_null;
                }

                if (!--looping) {
                        free(linkpath);
 free_buf_ret_null:
                        free(buf);
                        return NULL;
                }

                if (*linkpath != '/') {
                        bufsize += strlen(linkpath);
                        buf = xrealloc(buf, bufsize);
                        lpc = bb_get_last_path_component_strip(buf);
                        strcpy(lpc, linkpath);
                        free(linkpath);
                } else {
                        free(buf);
                        buf = linkpath;
 jump_in:
                        bufsize = strlen(buf) + 1;
                }
        }
}

char* FAST_FUNC xmalloc_readlink_or_warn(const char *path)
{
        char *buf = xmalloc_readlink(path);
        if (!buf) {
                /* EINVAL => "file: Invalid argument" => puzzled user */
                const char *errmsg = "not a symlink";
                int err = errno;
                if (err != EINVAL)
                        errmsg = strerror(err);
                bb_error_msg("%s: cannot read link: %s", path, errmsg);
        }
        return buf;
}

char* FAST_FUNC xmalloc_realpath(const char *path)
{
/* NB: uclibc also defines __GLIBC__
 * Therefore the test "if glibc, or uclibc >= 0.9.31" looks a bit weird:
 */
#if defined(__GLIBC__) && \
    (!defined(__UCLIBC__) || UCLIBC_VERSION >= KERNEL_VERSION(0, 9, 31))
        /* glibc provides a non-standard extension */
        /* new: POSIX.1-2008 specifies this behavior as well */
        return realpath(path, NULL);
#else
        char buf[PATH_MAX+1];

        /* on error returns NULL (xstrdup(NULL) == NULL) */
        return xstrdup(realpath(path, buf));
#endif
}

char* FAST_FUNC xmalloc_realpath_coreutils(const char *path)
{
        char *buf;

        errno = 0;
        buf = xmalloc_realpath(path);
        /*
         * There is one case when "readlink -f" and
         * "realpath" from coreutils succeed,
         * even though file does not exist, such as:
         *     /tmp/file_does_not_exist
         * (the directory must exist).
         */
        if (!buf && errno == ENOENT) {
                char *last_slash = strrchr(path, '/');
                if (last_slash) {
                        *last_slash++ = '\0';
                        buf = xmalloc_realpath(path);
                        if (buf) {
                                unsigned len = strlen(buf);
                                buf = xrealloc(buf, len + strlen(last_slash) + 2);
                                buf[len++] = '/';
                                strcpy(buf + len, last_slash);
                        }
                } else {
                        char *target = xmalloc_readlink(path);
                        if (target) {
                                char *cwd;
                                if (target[0] == '/') {
                                        /*
                                         * $ ln -s /bin/qwe symlink  # note: /bin is a link to /usr/bin
                                         * $ readlink -f symlink
                                         * /usr/bin/qwe/target_does_not_exist
                                         * $ realpath symlink
                                         * /usr/bin/qwe/target_does_not_exist
                                         */
                                        buf = xmalloc_realpath_coreutils(target);
                                        free(target);
                                        return buf;
                                }
                                /*
                                 * $ ln -s target_does_not_exist symlink
                                 * $ readlink -f symlink
                                 * /CURDIR/target_does_not_exist
                                 * $ realpath symlink
                                 * /CURDIR/target_does_not_exist
                                 */
                                cwd = xrealloc_getcwd_or_warn(NULL);
                                buf = concat_path_file(cwd, target);
                                free(cwd);
                                free(target);
                                return buf;
                        }
                }
        }

        return buf;
}