1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3 *
4 * Copyright (C) 1997 John D. Polstra. All rights reserved.
5 *
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
8 * are met:
9 * 1. Redistributions of source code must retain the above copyright
10 * notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY JOHN D. POLSTRA AND CONTRIBUTORS ``AS IS'' AND
16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18 * ARE DISCLAIMED. IN NO EVENT SHALL JOHN D. POLSTRA OR CONTRIBUTORS BE LIABLE
19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
25 * SUCH DAMAGE.
26 */
27
28 #include <sys/cdefs.h>
29 __FBSDID("$FreeBSD$");
30
31 #include <sys/types.h>
32 #include <sys/wait.h>
33
34 #include <err.h>
35 #include <errno.h>
36 #include <fcntl.h>
37 #include <signal.h>
38 #include <stdio.h>
39 #include <stdlib.h>
40 #include <sysexits.h>
41 #include <unistd.h>
42
43 static int acquire_lock(const char *name, int flags);
44 static void cleanup(void);
45 static void killed(int sig);
46 static void timeout(int sig);
47 static void usage(void);
48 static void wait_for_lock(const char *name);
49
50 static const char *lockname;
51 static int lockfd = -1;
52 static int keep;
53 static volatile sig_atomic_t timed_out;
54
55 /*
56 * Execute an arbitrary command while holding a file lock.
57 */
58 int
main(int argc,char ** argv)59 main(int argc, char **argv)
60 {
61 int ch, flags, silent, status, waitsec;
62 pid_t child;
63
64 silent = keep = 0;
65 flags = O_CREAT | O_RDONLY;
66 waitsec = -1; /* Infinite. */
67 while ((ch = getopt(argc, argv, "sknt:w")) != -1) {
68 switch (ch) {
69 case 'k':
70 keep = 1;
71 break;
72 case 'n':
73 flags &= ~O_CREAT;
74 break;
75 case 's':
76 silent = 1;
77 break;
78 case 't':
79 {
80 char *endptr;
81 waitsec = strtol(optarg, &endptr, 0);
82 if (*optarg == '\0' || *endptr != '\0' || waitsec < 0)
83 errx(EX_USAGE,
84 "invalid timeout \"%s\"", optarg);
85 }
86 break;
87 case 'w':
88 flags = (flags & ~O_RDONLY) | O_WRONLY;
89 break;
90 default:
91 usage();
92 }
93 }
94 if (argc - optind < 2)
95 usage();
96 lockname = argv[optind++];
97 argc -= optind;
98 argv += optind;
99 if (waitsec > 0) { /* Set up a timeout. */
100 struct sigaction act;
101
102 act.sa_handler = timeout;
103 sigemptyset(&act.sa_mask);
104 act.sa_flags = 0; /* Note that we do not set SA_RESTART. */
105 sigaction(SIGALRM, &act, NULL);
106 alarm(waitsec);
107 }
108 /*
109 * If the "-k" option is not given, then we must not block when
110 * acquiring the lock. If we did, then the lock holder would
111 * unlink the file upon releasing the lock, and we would acquire
112 * a lock on a file with no directory entry. Then another
113 * process could come along and acquire the same lock. To avoid
114 * this problem, we separate out the actions of waiting for the
115 * lock to be available and of actually acquiring the lock.
116 *
117 * That approach produces behavior that is technically correct;
118 * however, it causes some performance & ordering problems for
119 * locks that have a lot of contention. First, it is unfair in
120 * the sense that a released lock isn't necessarily granted to
121 * the process that has been waiting the longest. A waiter may
122 * be starved out indefinitely. Second, it creates a thundering
123 * herd situation each time the lock is released.
124 *
125 * When the "-k" option is used, the unlink race no longer
126 * exists. In that case we can block while acquiring the lock,
127 * avoiding the separate step of waiting for the lock. This
128 * yields fairness and improved performance.
129 */
130 lockfd = acquire_lock(lockname, flags | O_NONBLOCK);
131 while (lockfd == -1 && !timed_out && waitsec != 0) {
132 if (keep)
133 lockfd = acquire_lock(lockname, flags);
134 else {
135 wait_for_lock(lockname);
136 lockfd = acquire_lock(lockname, flags | O_NONBLOCK);
137 }
138 }
139 if (waitsec > 0)
140 alarm(0);
141 if (lockfd == -1) { /* We failed to acquire the lock. */
142 if (silent)
143 exit(EX_TEMPFAIL);
144 errx(EX_TEMPFAIL, "%s: already locked", lockname);
145 }
146 /* At this point, we own the lock. */
147 if (atexit(cleanup) == -1)
148 err(EX_OSERR, "atexit failed");
149 if ((child = fork()) == -1)
150 err(EX_OSERR, "cannot fork");
151 if (child == 0) { /* The child process. */
152 close(lockfd);
153 execvp(argv[0], argv);
154 warn("%s", argv[0]);
155 _exit(1);
156 }
157 /* This is the parent process. */
158 signal(SIGINT, SIG_IGN);
159 signal(SIGQUIT, SIG_IGN);
160 signal(SIGTERM, killed);
161 if (waitpid(child, &status, 0) == -1)
162 err(EX_OSERR, "waitpid failed");
163 return (WIFEXITED(status) ? WEXITSTATUS(status) : EX_SOFTWARE);
164 }
165
166 /*
167 * Try to acquire a lock on the given file, creating the file if
168 * necessary. The flags argument is O_NONBLOCK or 0, depending on
169 * whether we should wait for the lock. Returns an open file descriptor
170 * on success, or -1 on failure.
171 */
172 static int
acquire_lock(const char * name,int flags)173 acquire_lock(const char *name, int flags)
174 {
175 int fd;
176
177 if ((fd = open(name, O_EXLOCK|flags, 0666)) == -1) {
178 if (errno == EAGAIN || errno == EINTR)
179 return (-1);
180 else if (errno == ENOENT && (flags & O_CREAT) == 0)
181 err(EX_UNAVAILABLE, "%s", name);
182 err(EX_CANTCREAT, "cannot open %s", name);
183 }
184 return (fd);
185 }
186
187 /*
188 * Remove the lock file.
189 */
190 static void
cleanup(void)191 cleanup(void)
192 {
193
194 if (keep)
195 flock(lockfd, LOCK_UN);
196 else
197 unlink(lockname);
198 }
199
200 /*
201 * Signal handler for SIGTERM. Cleans up the lock file, then re-raises
202 * the signal.
203 */
204 static void
killed(int sig)205 killed(int sig)
206 {
207
208 cleanup();
209 signal(sig, SIG_DFL);
210 if (kill(getpid(), sig) == -1)
211 err(EX_OSERR, "kill failed");
212 }
213
214 /*
215 * Signal handler for SIGALRM.
216 */
217 static void
timeout(int sig __unused)218 timeout(int sig __unused)
219 {
220
221 timed_out = 1;
222 }
223
224 static void
usage(void)225 usage(void)
226 {
227
228 fprintf(stderr,
229 "usage: lockf [-kns] [-t seconds] file command [arguments]\n");
230 exit(EX_USAGE);
231 }
232
233 /*
234 * Wait until it might be possible to acquire a lock on the given file.
235 * If the file does not exist, return immediately without creating it.
236 */
237 static void
wait_for_lock(const char * name)238 wait_for_lock(const char *name)
239 {
240 int fd;
241
242 if ((fd = open(name, O_RDONLY|O_EXLOCK, 0666)) == -1) {
243 if (errno == ENOENT || errno == EINTR)
244 return;
245 err(EX_CANTCREAT, "cannot open %s", name);
246 }
247 close(fd);
248 }
249