/* vi: set sw=4 ts=4: */
/*
 * circular buffer syslog implementation for busybox
 *
 * Copyright (C) 2000 by Gennady Feldman <gfeldman@gena01.com>
 *
 * Maintainer: Gennady Feldman <gfeldman@gena01.com> as of Mar 12, 2001
 *
 * Licensed under GPLv2 or later, see file LICENSE in this source tree.
 */
//config:config LOGREAD
//config:       bool "logread (4.8 kb)"
//config:       default y
//WRONG: it should be compilable without SYSLOG=y:
//WRONG:        depends on FEATURE_IPC_SYSLOG
//config:       help
//config:       If you enabled Circular Buffer support, you almost
//config:       certainly want to enable this feature as well. This
//config:       utility will allow you to read the messages that are
//config:       stored in the syslogd circular buffer.
//config:
//config:config FEATURE_LOGREAD_REDUCED_LOCKING
//config:       bool "Double buffering"
//config:       default y
//config:       depends on LOGREAD
//config:       help
//config:       'logread' output to slow serial terminals can have
//config:       side effects on syslog because of the semaphore.
//config:       This option make logread to double buffer copy
//config:       from circular buffer, minimizing semaphore
//config:       contention at some minor memory expense.
//config:

//applet:IF_LOGREAD(APPLET(logread, BB_DIR_SBIN, BB_SUID_DROP))

//kbuild:lib-$(CONFIG_LOGREAD) += logread.o

//usage:#define logread_trivial_usage
//usage:       "[-fF]"
//usage:#define logread_full_usage "\n\n"
//usage:       "Show messages in syslogd's circular buffer\n"
//usage:     "\n        -f      Output data as log grows"
//usage:     "\n        -F      Same as -f, but dump buffer first"

#include "libbb.h"
#include "common_bufsiz.h"
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>

#define DEBUG 0

/* our shared key (syslogd.c and logread.c must be in sync) */
enum { KEY_ID = 0x414e4547 }; /* "GENA" */

struct shbuf_ds {
        int32_t size;           // size of data - 1
        int32_t tail;           // end of message list
        char data[1];           // messages
};

static const struct sembuf init_sem[3] = {
        {0, -1, IPC_NOWAIT | SEM_UNDO},
        {1, 0}, {0, +1, SEM_UNDO}
};

struct globals {
        struct sembuf SMrup[1]; // {0, -1, IPC_NOWAIT | SEM_UNDO},
        struct sembuf SMrdn[2]; // {1, 0}, {0, +1, SEM_UNDO}
        struct shbuf_ds *shbuf;
} FIX_ALIASING;
#define G (*(struct globals*)bb_common_bufsiz1)
#define SMrup (G.SMrup)
#define SMrdn (G.SMrdn)
#define shbuf (G.shbuf)
#define INIT_G() do { \
        setup_common_bufsiz(); \
        memcpy(SMrup, init_sem, sizeof(init_sem)); \
} while (0)

#if 0
static void error_exit(const char *str) NORETURN;
static void error_exit(const char *str)
{
        /* Release all acquired resources */
        shmdt(shbuf);
        bb_perror_msg_and_die(str);
}
#else
/* On Linux, shmdt is not mandatory on exit */
# define error_exit(str) bb_simple_perror_msg_and_die(str)
#endif

/*
 * sem_up - up()'s a semaphore.
 */
static void sem_up(int semid)
{
        if (semop(semid, SMrup, 1) == -1)
                error_exit("semop[SMrup]");
}

static void interrupted(int sig)
{
        /* shmdt(shbuf); - on Linux, shmdt is not mandatory on exit */
        kill_myself_with_sig(sig);
}

int logread_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int logread_main(int argc UNUSED_PARAM, char **argv)
{
        unsigned cur;
        int log_semid; /* ipc semaphore id */
        int log_shmid; /* ipc shared memory id */
        int follow = getopt32(argv, "fF");

        INIT_G();

        log_shmid = shmget(KEY_ID, 0, 0);
        if (log_shmid == -1)
                bb_perror_msg_and_die("can't %s syslogd buffer", "find");

        /* Attach shared memory to our char* */
        shbuf = shmat(log_shmid, NULL, SHM_RDONLY);
        if (shbuf == NULL)
                bb_perror_msg_and_die("can't %s syslogd buffer", "access");

        log_semid = semget(KEY_ID, 0, 0);
        if (log_semid == -1)
                error_exit("can't get access to semaphores for syslogd buffer");

        bb_signals(BB_FATAL_SIGS, interrupted);

        /* Suppose atomic memory read */
        /* Max possible value for tail is shbuf->size - 1 */
        cur = shbuf->tail;

        /* Loop for -f or -F, one pass otherwise */
        do {
                unsigned shbuf_size;
                unsigned shbuf_tail;
                const char *shbuf_data;
#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
                int i;
                int len_first_part;
                int len_total = len_total; /* for gcc */
                char *copy = copy; /* for gcc */
#endif
                if (semop(log_semid, SMrdn, 2) == -1)
                        error_exit("semop[SMrdn]");

                /* Copy the info, helps gcc to realize that it doesn't change */
                shbuf_size = shbuf->size;
                shbuf_tail = shbuf->tail;
                shbuf_data = shbuf->data; /* pointer! */

                if (DEBUG)
                        printf("cur:%u tail:%u size:%u\n",
                                        cur, shbuf_tail, shbuf_size);

                if (!(follow & 1)) { /* not -f */
                        /* if -F, "convert" it to -f, so that we don't
                         * dump the entire buffer on each iteration
                         */
                        follow >>= 1;

                        /* advance to oldest complete message */
                        /* find NUL */
                        cur += strlen(shbuf_data + cur);
                        if (cur >= shbuf_size) { /* last byte in buffer? */
                                cur = strnlen(shbuf_data, shbuf_tail);
                                if (cur == shbuf_tail)
                                        goto unlock; /* no complete messages */
                        }
                        /* advance to first byte of the message */
                        cur++;
                        if (cur >= shbuf_size) /* last byte in buffer? */
                                cur = 0;
                } else { /* -f */
                        if (cur == shbuf_tail) {
                                sem_up(log_semid);
                                fflush_all();
                                sleep1(); /* TODO: replace me with a sleep_on */
                                continue;
                        }
                }

                /* Read from cur to tail */
#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
                len_first_part = len_total = shbuf_tail - cur;
                if (len_total < 0) {
                        /* message wraps: */
                        /* [SECOND PART.........FIRST PART] */
                        /*  ^data      ^tail    ^cur      ^size */
                        len_total += shbuf_size;
                }
                copy = xmalloc(len_total + 1);
                if (len_first_part < 0) {
                        /* message wraps (see above) */
                        len_first_part = shbuf_size - cur;
                        memcpy(copy + len_first_part, shbuf_data, shbuf_tail);
                }
                memcpy(copy, shbuf_data + cur, len_first_part);
                copy[len_total] = '\0';
                cur = shbuf_tail;
#else
                while (cur != shbuf_tail) {
                        fputs_stdout(shbuf_data + cur);
                        cur += strlen(shbuf_data + cur) + 1;
                        if (cur >= shbuf_size)
                                cur = 0;
                }
#endif
 unlock:
                /* release the lock on the log chain */
                sem_up(log_semid);

#if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
                for (i = 0; i < len_total; i += strlen(copy + i) + 1) {
                        fputs_stdout(copy + i);
                }
                free(copy);
#endif
                fflush_all();
        } while (follow);

        /* shmdt(shbuf); - on Linux, shmdt is not mandatory on exit */

        fflush_stdout_and_exit(EXIT_SUCCESS);
}