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, silent, status, waitsec; 60 pid_t child; 61 62 silent = keep = 0; 63 waitsec = -1; /* Infinite. */ 64 while ((ch = getopt(argc, argv, "skt:")) != -1) { 65 switch (ch) { 66 case 'k': 67 keep = 1; 68 break; 69 case 's': 70 silent = 1; 71 break; 72 case 't': 73 { 74 char *endptr; 75 waitsec = strtol(optarg, &endptr, 0); 76 if (*optarg == '\0' || *endptr != '\0' || waitsec < 0) 77 errx(EX_USAGE, 78 "invalid timeout \"%s\"", optarg); 79 } 80 break; 81 default: 82 usage(); 83 } 84 } 85 if (argc - optind < 2) 86 usage(); 87 lockname = argv[optind++]; 88 argc -= optind; 89 argv += optind; 90 if (waitsec > 0) { /* Set up a timeout. */ 91 struct sigaction act; 92 93 act.sa_handler = timeout; 94 sigemptyset(&act.sa_mask); 95 act.sa_flags = 0; /* Note that we do not set SA_RESTART. */ 96 sigaction(SIGALRM, &act, NULL); 97 alarm(waitsec); 98 } 99 /* 100 * If the "-k" option is not given, then we must not block when 101 * acquiring the lock. If we did, then the lock holder would 102 * unlink the file upon releasing the lock, and we would acquire 103 * a lock on a file with no directory entry. Then another 104 * process could come along and acquire the same lock. To avoid 105 * this problem, we separate out the actions of waiting for the 106 * lock to be available and of actually acquiring the lock. 107 * 108 * That approach produces behavior that is technically correct; 109 * however, it causes some performance & ordering problems for 110 * locks that have a lot of contention. First, it is unfair in 111 * the sense that a released lock isn't necessarily granted to 112 * the process that has been waiting the longest. A waiter may 113 * be starved out indefinitely. Second, it creates a thundering 114 * herd situation each time the lock is released. 115 * 116 * When the "-k" option is used, the unlink race no longer 117 * exists. In that case we can block while acquiring the lock, 118 * avoiding the separate step of waiting for the lock. This 119 * yields fairness and improved performance. 120 */ 121 lockfd = acquire_lock(lockname, O_NONBLOCK); 122 while (lockfd == -1 && !timed_out && waitsec != 0) { 123 if (keep) 124 lockfd = acquire_lock(lockname, 0); 125 else { 126 wait_for_lock(lockname); 127 lockfd = acquire_lock(lockname, O_NONBLOCK); 128 } 129 } 130 if (waitsec > 0) 131 alarm(0); 132 if (lockfd == -1) { /* We failed to acquire the lock. */ 133 if (silent) 134 exit(EX_TEMPFAIL); 135 errx(EX_TEMPFAIL, "%s: already locked", lockname); 136 } 137 /* At this point, we own the lock. */ 138 if (atexit(cleanup) == -1) 139 err(EX_OSERR, "atexit failed"); 140 if ((child = fork()) == -1) 141 err(EX_OSERR, "cannot fork"); 142 if (child == 0) { /* The child process. */ 143 close(lockfd); 144 execvp(argv[0], argv); 145 warn("%s", argv[0]); 146 _exit(1); 147 } 148 /* This is the parent process. */ 149 signal(SIGINT, SIG_IGN); 150 signal(SIGQUIT, SIG_IGN); 151 signal(SIGTERM, killed); 152 if (waitpid(child, &status, 0) == -1) 153 err(EX_OSERR, "waitpid failed"); 154 return (WIFEXITED(status) ? WEXITSTATUS(status) : 1); 155 } 156 157 /* 158 * Try to acquire a lock on the given file, creating the file if 159 * necessary. The flags argument is O_NONBLOCK or 0, depending on 160 * whether we should wait for the lock. Returns an open file descriptor 161 * on success, or -1 on failure. 162 */ 163 static int 164 acquire_lock(const char *name, int flags) 165 { 166 int fd; 167 168 if ((fd = open(name, O_RDONLY|O_CREAT|O_EXLOCK|flags, 0666)) == -1) { 169 if (errno == EAGAIN || errno == EINTR) 170 return (-1); 171 err(EX_CANTCREAT, "cannot open %s", name); 172 } 173 return (fd); 174 } 175 176 /* 177 * Remove the lock file. 178 */ 179 static void 180 cleanup(void) 181 { 182 183 if (keep) 184 flock(lockfd, LOCK_UN); 185 else 186 unlink(lockname); 187 } 188 189 /* 190 * Signal handler for SIGTERM. Cleans up the lock file, then re-raises 191 * the signal. 192 */ 193 static void 194 killed(int sig) 195 { 196 197 cleanup(); 198 signal(sig, SIG_DFL); 199 if (kill(getpid(), sig) == -1) 200 err(EX_OSERR, "kill failed"); 201 } 202 203 /* 204 * Signal handler for SIGALRM. 205 */ 206 static void 207 timeout(int sig __unused) 208 { 209 210 timed_out = 1; 211 } 212 213 static void 214 usage(void) 215 { 216 217 fprintf(stderr, 218 "usage: lockf [-ks] [-t seconds] file command [arguments]\n"); 219 exit(EX_USAGE); 220 } 221 222 /* 223 * Wait until it might be possible to acquire a lock on the given file. 224 * If the file does not exist, return immediately without creating it. 225 */ 226 static void 227 wait_for_lock(const char *name) 228 { 229 int fd; 230 231 if ((fd = open(name, O_RDONLY|O_EXLOCK, 0666)) == -1) { 232 if (errno == ENOENT || errno == EINTR) 233 return; 234 err(EX_CANTCREAT, "cannot open %s", name); 235 } 236 close(fd); 237 } 238