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/*
* Copyright (C) 2010-2011 Joel Rosdahl
*
* This program is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License as published by the Free
* Software Foundation; either version 3 of the License, or (at your option)
* any later version.
*
* This program 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 General Public License for
* more details.
*
* You should have received a copy of the GNU General Public License along with
* this program; if not, write to the Free Software Foundation, Inc., 51
* Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#include "ccache.h"
/*
* This function acquires a lockfile for the given path. Returns true if the
* lock was acquired, otherwise false. If the lock has been considered stale
* for the number of microseconds specified by staleness_limit, the function
* will (if possible) break the lock and then try to acquire it again. The
* staleness limit should be reasonably larger than the longest time the lock
* can be expected to be held, and the updates of the locked path should
* probably be made with an atomic rename(2) to avoid corruption in the rare
* case that the lock is broken by another process.
*/
bool
lockfile_acquire(const char *path, unsigned staleness_limit)
{
char *lockfile = format("%s.lock", path);
char *my_content = NULL, *content = NULL, *initial_content = NULL;
const char *hostname = get_hostname();
bool acquired = false;
#ifdef _WIN32
const size_t bufsize = 1024;
int fd, len;
#else
int ret;
#endif
unsigned to_sleep = 1000, slept = 0; /* Microseconds. */
while (1) {
free(my_content);
my_content = format("%s:%d:%d", hostname, (int)getpid(), (int)time(NULL));
#ifdef _WIN32
fd = open(lockfile, O_WRONLY|O_CREAT|O_EXCL|O_BINARY, 0666);
if (fd == -1) {
cc_log("lockfile_acquire: open WRONLY %s: %s", lockfile, strerror(errno));
if (errno != EEXIST) {
/* Directory doesn't exist or isn't writable? */
goto out;
}
/* Someone else has the lock. */
fd = open(lockfile, O_RDONLY|O_BINARY);
if (fd == -1) {
if (errno == ENOENT) {
/*
* The file was removed after the open() call above, so retry
* acquiring it.
*/
continue;
} else {
cc_log("lockfile_acquire: open RDONLY %s: %s",
lockfile, strerror(errno));
goto out;
}
}
free(content);
content = x_malloc(bufsize);
if ((len = read(fd, content, bufsize - 1)) == -1) {
cc_log("lockfile_acquire: read %s: %s", lockfile, strerror(errno));
close(fd);
goto out;
}
close(fd);
content[len] = '\0';
} else {
/* We got the lock. */
if (write(fd, my_content, strlen(my_content)) == -1) {
cc_log("lockfile_acquire: write %s: %s", lockfile, strerror(errno));
close(fd);
x_unlink(lockfile);
goto out;
}
close(fd);
acquired = true;
goto out;
}
#else
ret = symlink(my_content, lockfile);
if (ret == 0) {
/* We got the lock. */
acquired = true;
goto out;
}
cc_log("lockfile_acquire: symlink %s: %s", lockfile, strerror(errno));
if (errno == EPERM) {
/*
* The file system does not support symbolic links. We have no choice but
* to grant the lock anyway.
*/
acquired = true;
goto out;
}
if (errno != EEXIST) {
/* Directory doesn't exist or isn't writable? */
goto out;
}
free(content);
content = x_readlink(lockfile);
if (!content) {
if (errno == ENOENT) {
/*
* The symlink was removed after the symlink() call above, so retry
* acquiring it.
*/
continue;
} else {
cc_log("lockfile_acquire: readlink %s: %s", lockfile, strerror(errno));
goto out;
}
}
#endif
if (str_eq(content, my_content)) {
/* Lost NFS reply? */
cc_log("lockfile_acquire: symlink %s failed but we got the lock anyway",
lockfile);
acquired = true;
goto out;
}
/*
* A possible improvement here would be to check if the process holding the
* lock is still alive and break the lock early if it isn't.
*/
cc_log("lockfile_acquire: lock info for %s: %s", lockfile, content);
if (!initial_content) {
initial_content = x_strdup(content);
}
if (slept > staleness_limit) {
if (str_eq(content, initial_content)) {
/* The lock seems to be stale -- break it. */
cc_log("lockfile_acquire: breaking %s", lockfile);
if (lockfile_acquire(lockfile, staleness_limit)) {
lockfile_release(path);
lockfile_release(lockfile);
to_sleep = 1000;
slept = 0;
continue;
}
}
cc_log("lockfile_acquire: gave up acquiring %s", lockfile);
goto out;
}
cc_log("lockfile_acquire: failed to acquire %s; sleeping %u microseconds",
lockfile, to_sleep);
usleep(to_sleep);
slept += to_sleep;
to_sleep *= 2;
}
out:
if (acquired) {
cc_log("Acquired lock %s", lockfile);
} else {
cc_log("Failed to acquire lock %s", lockfile);
}
free(lockfile);
free(my_content);
free(initial_content);
free(content);
return acquired;
}
/*
* Release the lockfile for the given path. Assumes that we are the legitimate
* owner.
*/
void
lockfile_release(const char *path)
{
char *lockfile = format("%s.lock", path);
cc_log("Releasing lock %s", lockfile);
tmp_unlink(lockfile);
free(lockfile);
}
#ifdef TEST_LOCKFILE
int
main(int argc, char **argv)
{
extern char *cache_logfile;
cache_logfile = "/dev/stdout";
if (argc == 4) {
unsigned staleness_limit = atoi(argv[1]);
if (str_eq(argv[2], "acquire")) {
return lockfile_acquire(argv[3], staleness_limit) == 0;
} else if (str_eq(argv[2], "release")) {
lockfile_release(argv[3]);
return 0;
}
}
fprintf(stderr,
"Usage: testlockfile <staleness_limit> <acquire|release> <path>\n");
return 1;
}
#endif
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