/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/

/***

  This file is part of systemd.

  Now copied into libglnx:

    - Use GError

  Copyright 2010 Lennart Poettering

  Copyright 2015 Colin Walters <walters@verbum.org>

  SPDX-License-Identifier: LGPL-2.1-or-later

  systemd is free software; you can redistribute it and/or modify it

  under the terms of the GNU Lesser General Public License as published by

  the Free Software Foundation; either version 2.1 of the License, or

  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but

  WITHOUT ANY WARRANTY; without even the implied warranty of

  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU

  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License

  along with systemd; If not, see <http://www.gnu.org/licenses/>.

***/

#include "libglnx-config.h"

#include <stdlib.h>

#include <stdbool.h>

#include <errno.h>

#include <string.h>

#include <stdio.h>

#include <limits.h>

#include <unistd.h>

#include <sys/types.h>

#include <sys/file.h>

#include <sys/stat.h>

#include <fcntl.h>

#include "glnx-lockfile.h"

#include "glnx-errors.h"

#include "glnx-fdio.h"

#include "glnx-backport-autocleanups.h"

#include "glnx-local-alloc.h"

#define newa(t, n) ((t*) alloca(sizeof(t)*(n)))

/**

 * glnx_make_lock_file:

 * @dfd: Directory file descriptor (if not `AT_FDCWD`, must have lifetime `>=` @out_lock)

 * @p: Path

 * @operation: one of `LOCK_SH`, `LOCK_EX`, `LOCK_UN`, as passed to flock()

 * @out_lock: (out) (caller allocates): Return location for lock

 * @error: Error

 *

 * Block until a lock file named @p (relative to @dfd) can be created,

 * using the flags in @operation, returning the lock data in the

 * caller-allocated location @out_lock.

 *

 * This API wraps new-style process locking if available, otherwise

 * falls back to BSD locks.

 */

gboolean

glnx_make_lock_file(int dfd, const char *p, int operation, GLnxLockFile *out_lock, GError **error) {

        glnx_autofd int fd = -1;

        g_autofree char *t = NULL;

        int r;

        /*

         * We use UNPOSIX locks if they are available. They have nice

         * semantics, and are mostly compatible with NFS. However,

         * they are only available on new kernels. When we detect we

         * are running on an older kernel, then we fall back to good

         * old BSD locks. They also have nice semantics, but are

         * slightly problematic on NFS, where they are upgraded to

         * POSIX locks, even though locally they are orthogonal to

         * POSIX locks.

         */

        t = g_strdup(p);

        for (;;) {

#ifdef F_OFD_SETLK

                struct flock fl = {

                        .l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,

                        .l_whence = SEEK_SET,

                };

#endif

                struct stat st;

                fd = openat(dfd, p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);

                if (fd < 0)

                        return glnx_throw_errno(error);

                /* Unfortunately, new locks are not in RHEL 7.1 glibc */

#ifdef F_OFD_SETLK

                r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);

#else

                r = -1;

                errno = EINVAL;

#endif

                if (r < 0) {

                        /* If the kernel is too old, use good old BSD locks */

                        if (errno == EINVAL)

                                r = flock(fd, operation);

                        if (r < 0)

                                return glnx_throw_errno_prefix (error, "flock");

                }

                /* If we acquired the lock, let's check if the file

                 * still exists in the file system. If not, then the

                 * previous exclusive owner removed it and then closed

                 * it. In such a case our acquired lock is worthless,

                 * hence try again. */

                if (!glnx_fstat (fd, &st, error))

                        return FALSE;

                if (st.st_nlink > 0)

                        break;

                glnx_close_fd (&fd);

        }

        /* Note that if this is not AT_FDCWD, the caller takes responsibility

         * for the fd's lifetime being >= that of the lock.

         */

        out_lock->initialized = TRUE;

        out_lock->dfd = dfd;

        out_lock->path = g_steal_pointer (&t);

        out_lock->fd = g_steal_fd (&fd);

        out_lock->operation = operation;

        return TRUE;

}

void glnx_release_lock_file(GLnxLockFile *f) {

        int r;

        if (!(f && f->initialized))

                return;

        if (f->path) {

                /* If we are the exclusive owner we can safely delete

                 * the lock file itself. If we are not the exclusive

                 * owner, we can try becoming it. */

                if (f->fd >= 0 &&

                    (f->operation & ~LOCK_NB) == LOCK_SH) {

#ifdef F_OFD_SETLK

                        static const struct flock fl = {

                                .l_type = F_WRLCK,

                                .l_whence = SEEK_SET,

                        };

                        r = fcntl(f->fd, F_OFD_SETLK, &fl);

#else

                        r = -1;

                        errno = EINVAL;

#endif

                        if (r < 0 && errno == EINVAL)

                                r = flock(f->fd, LOCK_EX|LOCK_NB);

                        if (r >= 0)

                                f->operation = LOCK_EX|LOCK_NB;

                }

                if ((f->operation & ~LOCK_NB) == LOCK_EX) {

                        (void) unlinkat(f->dfd, f->path, 0);

                }

                g_free(f->path);

                f->path = NULL;

        }

        glnx_close_fd (&f->fd);

        f->operation = 0;

        f->initialized = FALSE;

}