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