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 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 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 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 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 218 timeout(int sig __unused) 219 { 220 221 timed_out = 1; 222 } 223 224 static 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 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