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/*-*- 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>
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 "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_fd_close 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(error);
}
/* 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. */
r = fstat(fd, &st);
if (r < 0)
return glnx_throw_errno(error);
if (st.st_nlink > 0)
break;
(void) close(fd);
fd = -1;
}
/* Note that if this is not AT_FDCWD, the caller takes responsibility
* for the fd's lifetime being >= that of the lock.
*/
out_lock->dfd = dfd;
out_lock->path = t;
out_lock->fd = fd;
out_lock->operation = operation;
fd = -1;
t = NULL;
return TRUE;
}
void glnx_release_lock_file(GLnxLockFile *f) {
int r;
if (!f)
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;
}
if (f->fd != -1)
(void) close (f->fd);
f->fd = -1;
f->operation = 0;
}
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