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;
66 waitsec = -1; /* Infinite. */
67 while ((ch = getopt(argc, argv, "sknt:")) != -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 default:
88 usage();
89 }
90 }
91 if (argc - optind < 2)
92 usage();
93 lockname = argv[optind++];
94 argc -= optind;
95 argv += optind;
96 if (waitsec > 0) { /* Set up a timeout. */
97 struct sigaction act;
98
99 act.sa_handler = timeout;
100 sigemptyset(&act.sa_mask);
101 act.sa_flags = 0; /* Note that we do not set SA_RESTART. */
102 sigaction(SIGALRM, &act, NULL);
103 alarm(waitsec);
104 }
105 /*
106 * If the "-k" option is not given, then we must not block when
107 * acquiring the lock. If we did, then the lock holder would
108 * unlink the file upon releasing the lock, and we would acquire
109 * a lock on a file with no directory entry. Then another
110 * process could come along and acquire the same lock. To avoid
111 * this problem, we separate out the actions of waiting for the
112 * lock to be available and of actually acquiring the lock.
113 *
114 * That approach produces behavior that is technically correct;
115 * however, it causes some performance & ordering problems for
116 * locks that have a lot of contention. First, it is unfair in
117 * the sense that a released lock isn't necessarily granted to
118 * the process that has been waiting the longest. A waiter may
119 * be starved out indefinitely. Second, it creates a thundering
120 * herd situation each time the lock is released.
121 *
122 * When the "-k" option is used, the unlink race no longer
123 * exists. In that case we can block while acquiring the lock,
124 * avoiding the separate step of waiting for the lock. This
125 * yields fairness and improved performance.
126 */
127 lockfd = acquire_lock(lockname, flags | O_NONBLOCK);
128 while (lockfd == -1 && !timed_out && waitsec != 0) {
129 if (keep)
130 lockfd = acquire_lock(lockname, flags);
131 else {
132 wait_for_lock(lockname);
133 lockfd = acquire_lock(lockname, flags | O_NONBLOCK);
134 }
135 }
136 if (waitsec > 0)
137 alarm(0);
138 if (lockfd == -1) { /* We failed to acquire the lock. */
139 if (silent)
140 exit(EX_TEMPFAIL);
141 errx(EX_TEMPFAIL, "%s: already locked", lockname);
142 }
143 /* At this point, we own the lock. */
144 if (atexit(cleanup) == -1)
145 err(EX_OSERR, "atexit failed");
146 if ((child = fork()) == -1)
147 err(EX_OSERR, "cannot fork");
148 if (child == 0) { /* The child process. */
149 close(lockfd);
150 execvp(argv[0], argv);
151 warn("%s", argv[0]);
152 _exit(1);
153 }
154 /* This is the parent process. */
155 signal(SIGINT, SIG_IGN);
156 signal(SIGQUIT, SIG_IGN);
157 signal(SIGTERM, killed);
158 if (waitpid(child, &status, 0) == -1)
159 err(EX_OSERR, "waitpid failed");
160 return (WIFEXITED(status) ? WEXITSTATUS(status) : EX_SOFTWARE);
161 }
162
163 /*
164 * Try to acquire a lock on the given file, creating the file if
165 * necessary. The flags argument is O_NONBLOCK or 0, depending on
166 * whether we should wait for the lock. Returns an open file descriptor
167 * on success, or -1 on failure.
168 */
169 static int
acquire_lock(const char * name,int flags)170 acquire_lock(const char *name, int flags)
171 {
172 int fd;
173
174 if ((fd = open(name, O_RDONLY|O_EXLOCK|flags, 0666)) == -1) {
175 if (errno == EAGAIN || errno == EINTR)
176 return (-1);
177 else if (errno == ENOENT && (flags & O_CREAT) == 0)
178 err(EX_UNAVAILABLE, "%s", name);
179 err(EX_CANTCREAT, "cannot open %s", name);
180 }
181 return (fd);
182 }
183
184 /*
185 * Remove the lock file.
186 */
187 static void
cleanup(void)188 cleanup(void)
189 {
190
191 if (keep)
192 flock(lockfd, LOCK_UN);
193 else
194 unlink(lockname);
195 }
196
197 /*
198 * Signal handler for SIGTERM. Cleans up the lock file, then re-raises
199 * the signal.
200 */
201 static void
killed(int sig)202 killed(int sig)
203 {
204
205 cleanup();
206 signal(sig, SIG_DFL);
207 if (kill(getpid(), sig) == -1)
208 err(EX_OSERR, "kill failed");
209 }
210
211 /*
212 * Signal handler for SIGALRM.
213 */
214 static void
timeout(int sig __unused)215 timeout(int sig __unused)
216 {
217
218 timed_out = 1;
219 }
220
221 static void
usage(void)222 usage(void)
223 {
224
225 fprintf(stderr,
226 "usage: lockf [-kns] [-t seconds] file command [arguments]\n");
227 exit(EX_USAGE);
228 }
229
230 /*
231 * Wait until it might be possible to acquire a lock on the given file.
232 * If the file does not exist, return immediately without creating it.
233 */
234 static void
wait_for_lock(const char * name)235 wait_for_lock(const char *name)
236 {
237 int fd;
238
239 if ((fd = open(name, O_RDONLY|O_EXLOCK, 0666)) == -1) {
240 if (errno == ENOENT || errno == EINTR)
241 return;
242 err(EX_CANTCREAT, "cannot open %s", name);
243 }
244 close(fd);
245 }
246