lockfile-util.c revision a1e58e8ee1c84b633d6d6d651d5328d4dd4eba5b
/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
/***
This file is part of systemd.
Copyright 2010 Lennart Poettering
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 <stdlib.h>
#include <stdbool.h>
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <limits.h>
#include <sys/file.h>
#include "util.h"
#include "lockfile-util.h"
#include "fileio.h"
int make_lock_file(const char *p, int operation, LockFile *ret) {
_cleanup_close_ int fd = -1;
_cleanup_free_ 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 = strdup(p);
if (!t)
return -ENOMEM;
for (;;) {
struct flock fl = {
.l_type = (operation & ~LOCK_NB) == LOCK_EX ? F_WRLCK : F_RDLCK,
.l_whence = SEEK_SET,
};
struct stat st;
fd = open(p, O_CREAT|O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NOCTTY, 0600);
if (fd < 0)
return -errno;
r = fcntl(fd, (operation & LOCK_NB) ? F_OFD_SETLK : F_OFD_SETLKW, &fl);
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 errno == EAGAIN ? -EBUSY : -errno;
}
/* 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 -errno;
if (st.st_nlink > 0)
break;
fd = safe_close(fd);
}
ret->path = t;
ret->fd = fd;
ret->operation = operation;
fd = -1;
t = NULL;
return r;
}
int make_lock_file_for(const char *p, int operation, LockFile *ret) {
const char *fn;
char *t;
assert(p);
assert(ret);
fn = basename(p);
if (!filename_is_valid(fn))
return -EINVAL;
t = newa(char, strlen(p) + 2 + 4 + 1);
stpcpy(stpcpy(stpcpy(mempcpy(t, p, fn - p), ".#"), fn), ".lck");
return make_lock_file(t, operation, ret);
}
void release_lock_file(LockFile *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) {
static const struct flock fl = {
.l_type = F_WRLCK,
.l_whence = SEEK_SET,
};
r = fcntl(f->fd, F_OFD_SETLK, &fl);
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)
unlink_noerrno(f->path);
f->path = mfree(f->path);
}
f->fd = safe_close(f->fd);
f->operation = 0;
}