lock_flock.c

/* lock_flock.c -- Lock files using flock()
 *
 * Copyright (c) 1994-2008 Carnegie Mellon University.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The name "Carnegie Mellon University" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For permission or any legal
 *    details, please contact
 *      Carnegie Mellon University
 *      Center for Technology Transfer and Enterprise Creation
 *      4615 Forbes Avenue
 *      Suite 302
 *      Pittsburgh, PA  15213
 *      (412) 268-7393, fax: (412) 268-7395
 *      innovation@andrew.cmu.edu
 *
 * 4. Redistributions of any form whatsoever must retain the following
 *    acknowledgment:
 *    "This product includes software developed by Computing Services
 *     at Carnegie Mellon University (http://www.cmu.edu/computing/)."
 *
 * CARNEGIE MELLON UNIVERSITY DISCLAIMS ALL WARRANTIES WITH REGARD TO
 * THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS, IN NO EVENT SHALL CARNEGIE MELLON UNIVERSITY BE LIABLE
 * FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN
 * AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
 * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <config.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#ifdef HAVE_UNISTD_H
#include <unistd.h>
#endif

#include "cyr_lock.h"

EXPORTED const char lock_method_desc[] = "flock";

EXPORTED double debug_locks_longer_than = 0.0;

/*
 * Block until we obtain an exclusive lock on the file descriptor 'fd',
 * opened for reading and writing on the file named 'filename'.  If
 * 'filename' is replaced, will re-open it as 'fd' and acquire a lock
 * on the new file.
 *
 * On success, returns 0.  If a pointer to a struct stat is given as
 * 'sbuf', it is filled in.
 *
 * On failure, returns -1 with an error code in errno.  If
 * 'failaction' is provided, it is filled in with a pointer to a fixed
 * string naming the action that failed.
 *
 */
EXPORTED int lock_reopen_ex(int fd, const char *filename,
                            struct stat *sbuf, const char **failaction,
                            int *changed)
{
    int r;
    struct stat sbuffile, sbufspare;
    int newfd;

    if (!sbuf) sbuf = &sbufspare;

    for (;;) {
        r = flock(fd, LOCK_EX);
        if (r == -1) {
            if (errno == EINTR) continue;
            if (failaction) *failaction = "locking";
            return -1;
        }

        r = fstat(fd, sbuf);
        if (!r) r = stat(filename, &sbuffile);
        if (r == -1) {
            if (failaction) *failaction = "stating";
            lock_unlock(fd, filename);
            return -1;
        }

        if (sbuf->st_ino == sbuffile.st_ino) return 0;

        if (changed) *changed = 1;

        newfd = open(filename, O_RDWR);
        if (newfd == -1) {
            if (failaction) *failaction = "opening";
            lock_unlock(fd, filename);
            return -1;
        }
        dup2(newfd, fd);
        close(newfd);
    }
}

/*
 * Obtain a lock on 'fd'.  The lock is exclusive if 'exclusive'
 * is true, otherwise shared.  Normally blocks until a lock is
 * obtained, but if 'nonblock' is true does not block and instead
 * fails with errno=EWOUDBLOCK if the lock cannot be obtained.
 *
 * Returns 0 for success, -1 for failure, with errno set to an
 * appropriate error code.
 */
EXPORTED int lock_setlock(int fd, int exclusive, int nonblock,
                          const char *filename __attribute__((unused)))
{
    int r;
    int op = (exclusive ? LOCK_EX : LOCK_SH);
    if (nonblock) op |= LOCK_NB;

    for (;;) {
        r = flock(fd, op);
        if (r != -1) return 0;
        if (errno == EINTR) continue;
        return -1;
    }
}

/*
 * Release any lock on 'fd'.  Always returns success.
 */
EXPORTED int lock_unlock(int fd, const char *filename __attribute__((unused)))
{
    int r;

    for (;;) {
        r = flock(fd, LOCK_UN);
        if (r != -1) return 0;
        if (errno == EINTR) continue;
        /* xxx help! */
        return -1;
    }
}