src/lib/file-lock.c

/* Copyright (c) 2002-2018 Dovecot authors, see the included COPYING file */

#include "lib.h"
#include "istream.h"
#include "file-lock.h"
#include "file-dotlock.h"
#include "time-util.h"

#include <time.h>
#include <sys/stat.h>
#ifdef HAVE_FLOCK
#  include <sys/file.h>
#endif

struct file_lock {
    int fd;
    char *path;
    struct dotlock *dotlock;

    struct timeval locked_time;
    int lock_type;
    enum file_lock_method lock_method;
    bool unlink_on_free;
    bool close_on_free;
};

static struct timeval lock_wait_start;
static uint64_t file_lock_wait_usecs = 0;
static long long file_lock_slow_warning_usecs = -1;

static void file_lock_log_warning_if_slow(struct file_lock *lock);

bool file_lock_method_parse(const char *name, enum file_lock_method *method_r)
{
    if (strcasecmp(name, "fcntl") == 0)
        *method_r = FILE_LOCK_METHOD_FCNTL;
    else if (strcasecmp(name, "flock") == 0)
        *method_r = FILE_LOCK_METHOD_FLOCK;
    else if (strcasecmp(name, "dotlock") == 0)
        *method_r = FILE_LOCK_METHOD_DOTLOCK;
    else
        return FALSE;
    return TRUE;
}

const char *file_lock_method_to_str(enum file_lock_method method)
{
    switch (method) {
    case FILE_LOCK_METHOD_FCNTL:
        return "fcntl";
    case FILE_LOCK_METHOD_FLOCK:
        return "flock";
    case FILE_LOCK_METHOD_DOTLOCK:
        return "dotlock";
    }
    i_unreached();
}

int file_try_lock(int fd, const char *path, int lock_type,
          enum file_lock_method lock_method,
          struct file_lock **lock_r)
{
    return file_wait_lock(fd, path, lock_type, lock_method, 0, lock_r);
}

int file_try_lock_error(int fd, const char *path, int lock_type,
            enum file_lock_method lock_method,
            struct file_lock **lock_r, const char **error_r)
{
    return file_wait_lock_error(fd, path, lock_type, lock_method, 0,
                    lock_r, error_r);
}

static const char *
file_lock_find_fcntl(int lock_fd, int lock_type)
{
    struct flock fl;

    i_zero(&fl);
    fl.l_type = lock_type;
    fl.l_whence = SEEK_SET;
    fl.l_start = 0;
    fl.l_len = 0;

    if (fcntl(lock_fd, F_GETLK, &fl) < 0 ||
        fl.l_type == F_UNLCK || fl.l_pid == -1 || fl.l_pid == 0)
        return "";
    return t_strdup_printf(" (%s lock held by pid %ld)",
        fl.l_type == F_RDLCK ? "READ" : "WRITE", (long)fl.l_pid);
}

static const char *
file_lock_find_proc_locks(int lock_fd ATTR_UNUSED)
{
    /* do anything except Linux support this? don't bother trying it for
       OSes we don't know about. */
#ifdef __linux__
    static bool have_proc_locks = TRUE;
    struct stat st;
    char node_buf[MAX_INT_STRLEN*3 + 2 + 1];
    struct istream *input;
    const char *line, *lock_type = "";
    pid_t pid = 0;
    int fd;

    if (!have_proc_locks)
        return NULL;

    if (fstat(lock_fd, &st) < 0)
        return "";
    i_snprintf(node_buf, sizeof(node_buf), "%02x:%02x:%llu",
           major(st.st_dev), minor(st.st_dev),
           (unsigned long long)st.st_ino);
    fd = open("/proc/locks", O_RDONLY);
    if (fd == -1) {
        have_proc_locks = FALSE;
        return "";
    }
    input = i_stream_create_fd_autoclose(&fd, 512);
    while (pid == 0 && (line = i_stream_read_next_line(input)) != NULL) T_BEGIN {
        const char *const *args = t_strsplit_spaces(line, " ");

        /* number: FLOCK/POSIX ADVISORY READ/WRITE pid
           major:minor:inode region-start region-end */
        if (str_array_length(args) < 8) {
            ; /* don't continue from within a T_BEGIN {...} T_END */
        } else if (strcmp(args[5], node_buf) == 0) {
            lock_type = strcmp(args[3], "READ") == 0 ?
                "READ" : "WRITE";
            if (str_to_pid(args[4], &pid) < 0)
                pid = 0;
        }
    } T_END;
    i_stream_destroy(&input);
    if (pid == 0) {
        /* not found */
        return "";
    }
    if (pid == getpid())
        return " (BUG: lock is held by our own process)";
    return t_strdup_printf(" (%s lock held by pid %ld)", lock_type, (long)pid);
#else
    return "";
#endif
}

const char *file_lock_find(int lock_fd, enum file_lock_method lock_method,
               int lock_type)
{
    const char *ret;

    if (lock_method == FILE_LOCK_METHOD_FCNTL) {
        ret = file_lock_find_fcntl(lock_fd, lock_type);
        if (ret[0] != '\0')
            return ret;
    }
    return file_lock_find_proc_locks(lock_fd);
}

static bool err_is_lock_timeout(time_t started, unsigned int timeout_secs)
{
    /* if EINTR took at least timeout_secs-1 number of seconds,
       assume it was the alarm. otherwise log EINTR failure.
       (We most likely don't want to retry EINTR since a signal
       means somebody wants us to stop blocking). */
    return errno == EINTR &&
        (unsigned long)(time(NULL) - started + 1) >= timeout_secs;
}

static int file_lock_do(int fd, const char *path, int lock_type,
            enum file_lock_method lock_method,
            unsigned int timeout_secs, const char **error_r)
{
    const char *lock_type_str;
    time_t started = time(NULL);
    int ret;

    i_assert(fd != -1);

    if (timeout_secs != 0) {
        alarm(timeout_secs);
        file_lock_wait_start();
    }

    lock_type_str = lock_type == F_UNLCK ? "unlock" :
        (lock_type == F_RDLCK ? "read-lock" : "write-lock");

    switch (lock_method) {
    case FILE_LOCK_METHOD_FCNTL: {
#ifndef HAVE_FCNTL
        *error_r = t_strdup_printf(
            "Can't lock file %s: fcntl() locks not supported", path);
        return -1;
#else
        struct flock fl;

        fl.l_type = lock_type;
        fl.l_whence = SEEK_SET;
        fl.l_start = 0;
        fl.l_len = 0;

        ret = fcntl(fd, timeout_secs != 0 ? F_SETLKW : F_SETLK, &fl);
        if (timeout_secs != 0) {
            alarm(0);
            file_lock_wait_end(path);
        }

        if (ret == 0)
            break;

        if (timeout_secs == 0 &&
            (errno == EACCES || errno == EAGAIN)) {
            /* locked by another process */
            *error_r = t_strdup_printf(
                "fcntl(%s, %s, F_SETLK) locking failed: %m "
                "(File is already locked)", path, lock_type_str);
            return 0;
        }

        if (err_is_lock_timeout(started, timeout_secs)) {
            errno = EAGAIN;
            *error_r = t_strdup_printf(
                "fcntl(%s, %s, F_SETLKW) locking failed: "
                "Timed out after %u seconds%s",
                path, lock_type_str, timeout_secs,
                file_lock_find(fd, lock_method, lock_type));
            return 0;
        }
        *error_r = t_strdup_printf("fcntl(%s, %s, %s) locking failed: %m",
            path, lock_type_str, timeout_secs == 0 ? "F_SETLK" : "F_SETLKW");
        if (errno == EDEADLK)
            i_panic("%s%s", *error_r, file_lock_find(fd, lock_method, lock_type));
        return -1;
#endif
    }
    case FILE_LOCK_METHOD_FLOCK: {
#ifndef HAVE_FLOCK
        *error_r = t_strdup_printf(
            "Can't lock file %s: flock() not supported", path);
        return -1;
#else
        int operation = timeout_secs != 0 ? 0 : LOCK_NB;

        switch (lock_type) {
        case F_RDLCK:
            operation |= LOCK_SH;
            break;
        case F_WRLCK:
            operation |= LOCK_EX;
            break;
        case F_UNLCK:
            operation |= LOCK_UN;
            break;
        }

        ret = flock(fd, operation);
        if (timeout_secs != 0) {
            alarm(0);
            file_lock_wait_end(path);
        }

        if (ret == 0)
            break;

        if (timeout_secs == 0 && errno == EWOULDBLOCK) {
            /* locked by another process */
            *error_r = t_strdup_printf(
                "flock(%s, %s) failed: %m "
                "(File is already locked)", path, lock_type_str);
            return 0;
        }
        if (err_is_lock_timeout(started, timeout_secs)) {
            errno = EAGAIN;
            *error_r = t_strdup_printf("flock(%s, %s) failed: "
                "Timed out after %u seconds%s",
                path, lock_type_str, timeout_secs,
                file_lock_find(fd, lock_method, lock_type));
            return 0;
        }
        *error_r = t_strdup_printf("flock(%s, %s) failed: %m",
                       path, lock_type_str);
        if (errno == EDEADLK)
            i_panic("%s%s", *error_r, file_lock_find(fd, lock_method, lock_type));
        return -1;
#endif
    }
    case FILE_LOCK_METHOD_DOTLOCK:
        /* we shouldn't get here */
        i_unreached();
    }

    return 1;
}

int file_wait_lock(int fd, const char *path, int lock_type,
           enum file_lock_method lock_method,
           unsigned int timeout_secs,
           struct file_lock **lock_r)
{
    const char *error;
    int ret;

    ret = file_wait_lock_error(fd, path, lock_type, lock_method,
                   timeout_secs, lock_r, &error);
    if (ret < 0)
        i_error("%s", error);
    return ret;
}

int file_wait_lock_error(int fd, const char *path, int lock_type,
             enum file_lock_method lock_method,
             unsigned int timeout_secs,
             struct file_lock **lock_r, const char **error_r)
{
    struct file_lock *lock;
    int ret;

    ret = file_lock_do(fd, path, lock_type, lock_method, timeout_secs, error_r);
    if (ret <= 0)
        return ret;

    lock = i_new(struct file_lock, 1);
    lock->fd = fd;
    lock->path = i_strdup(path);
    lock->lock_type = lock_type;
    lock->lock_method = lock_method;
    if (gettimeofday(&lock->locked_time, NULL) < 0)
        i_fatal("gettimeofday() failed: %m");
    *lock_r = lock;
    return 1;
}

int file_lock_try_update(struct file_lock *lock, int lock_type)
{
    const char *error;
    int ret;

    ret = file_lock_do(lock->fd, lock->path, lock_type,
               lock->lock_method, 0, &error);
    if (ret <= 0)
        return ret;
    file_lock_log_warning_if_slow(lock);
    lock->lock_type = lock_type;
    return 1;
}

void file_lock_set_unlink_on_free(struct file_lock *lock, bool set)
{
    lock->unlink_on_free = set;
}

void file_lock_set_close_on_free(struct file_lock *lock, bool set)
{
    lock->close_on_free = set;
}

struct file_lock *file_lock_from_dotlock(struct dotlock **dotlock)
{
    struct file_lock *lock;

    lock = i_new(struct file_lock, 1);
    lock->fd = -1;
    lock->path = i_strdup(file_dotlock_get_lock_path(*dotlock));
    lock->lock_type = F_WRLCK;
    lock->lock_method = FILE_LOCK_METHOD_DOTLOCK;
    if (gettimeofday(&lock->locked_time, NULL) < 0)
        i_fatal("gettimeofday() failed: %m");
    lock->dotlock = *dotlock;

    *dotlock = NULL;
    return lock;
}

static void file_unlock_real(struct file_lock *lock)
{
    const char *error;

    if (file_lock_do(lock->fd, lock->path, F_UNLCK,
             lock->lock_method, 0, &error) == 0) {
        /* this shouldn't happen */
        i_error("file_unlock(%s) failed: %m", lock->path);
    }
}

void file_unlock(struct file_lock **_lock)
{
    struct file_lock *lock = *_lock;

    *_lock = NULL;

    /* unlocking is unnecessary when the file is unlinked. or alternatively
       the unlink() must be done before unlocking, because otherwise it
       could be deleting the new lock. */
    i_assert(!lock->unlink_on_free);

    if (lock->dotlock == NULL)
        file_unlock_real(lock);
    file_lock_free(&lock);
}

static void file_try_unlink_locked(struct file_lock *lock)
{
    struct file_lock *temp_lock = NULL;
    struct stat st1, st2;
    const char *error;
    int ret;

    file_unlock_real(lock);
    ret = file_try_lock_error(lock->fd, lock->path, F_WRLCK,
                  lock->lock_method, &temp_lock, &error);
    if (ret < 0) {
        i_error("file_lock_free(): Unexpectedly failed to retry locking %s: %s",
            lock->path, error);
    } else if (ret == 0) {
        /* already locked by someone else */
    } else if (fstat(lock->fd, &st1) < 0) {
        /* not expected to happen */
        i_error("file_lock_free(): fstat(%s) failed: %m", lock->path);
    } else if (stat(lock->path, &st2) < 0) {
        if (errno != ENOENT)
            i_error("file_lock_free(): stat(%s) failed: %m", lock->path);
    } else if (st1.st_ino != st2.st_ino ||
           !CMP_DEV_T(st1.st_dev, st2.st_dev)) {
        /* lock file was recreated already - don't delete it */
    } else {
        /* nobody was waiting on the lock - unlink it */
        i_unlink(lock->path);
    }
    file_lock_free(&temp_lock);
}

void file_lock_free(struct file_lock **_lock)
{
    struct file_lock *lock = *_lock;

    if (lock == NULL)
        return;

    *_lock = NULL;

    if (lock->dotlock != NULL)
        file_dotlock_delete(&lock->dotlock);
    if (lock->unlink_on_free)
        file_try_unlink_locked(lock);
    if (lock->close_on_free)
        i_close_fd(&lock->fd);

    file_lock_log_warning_if_slow(lock);
    i_free(lock->path);
    i_free(lock);
}

const char *file_lock_get_path(struct file_lock *lock)
{
    return lock->path;
}

void file_lock_set_path(struct file_lock *lock, const char *path)
{
    if (path != lock->path) {
        i_free(lock->path);
        lock->path = i_strdup(path);
    }
}

void file_lock_wait_start(void)
{
    i_assert(lock_wait_start.tv_sec == 0);

    if (gettimeofday(&lock_wait_start, NULL) < 0)
        i_fatal("gettimeofday() failed: %m");
}

static void file_lock_wait_init_warning(void)
{
    const char *value;

    i_assert(file_lock_slow_warning_usecs == -1);

    value = getenv("FILE_LOCK_SLOW_WARNING_MSECS");
    if (value == NULL)
        file_lock_slow_warning_usecs = LLONG_MAX;
    else if (str_to_llong(value, &file_lock_slow_warning_usecs) == 0 &&
         file_lock_slow_warning_usecs > 0) {
        file_lock_slow_warning_usecs *= 1000;
    } else {
        i_error("FILE_LOCK_SLOW_WARNING_MSECS: "
            "Invalid value '%s' - ignoring", value);
        file_lock_slow_warning_usecs = LLONG_MAX;
    }
}

static void file_lock_log_warning_if_slow(struct file_lock *lock)
{
    if (file_lock_slow_warning_usecs < 0)
        file_lock_wait_init_warning();
    if (file_lock_slow_warning_usecs == LLONG_MAX) {
        /* slowness checking is disabled */
        return;
    }
    if (lock->lock_type != F_WRLCK) {
        /* some shared locks can legitimately be kept for a long time.
           don't warn about them. */
        return;
    }

    struct timeval now;
    if (gettimeofday(&now, NULL) < 0)
        i_fatal("gettimeofday() failed: %m");

    int diff = timeval_diff_msecs(&now, &lock->locked_time);
    if (diff > file_lock_slow_warning_usecs/1000) {
        i_warning("Lock %s kept for %d.%03d secs", lock->path,
              diff / 1000, diff % 1000);
    }
}

void file_lock_wait_end(const char *lock_name)
{
    struct timeval now;

    i_assert(lock_wait_start.tv_sec != 0);

    if (gettimeofday(&now, NULL) < 0)
        i_fatal("gettimeofday() failed: %m");
    long long diff = timeval_diff_usecs(&now, &lock_wait_start);
    if (diff > file_lock_slow_warning_usecs) {
        if (file_lock_slow_warning_usecs < 0)
            file_lock_wait_init_warning();
        if (diff > file_lock_slow_warning_usecs) {
            int diff_msecs = (diff + 999) / 1000;
            i_warning("Locking %s took %d.%03d secs", lock_name,
                  diff_msecs / 1000, diff_msecs % 1000);
        }
    }
    file_lock_wait_usecs += diff;
    lock_wait_start.tv_sec = 0;
}

uint64_t file_lock_wait_get_total_usecs(void)
{
    return file_lock_wait_usecs;
}