1 /*- 2 * SPDX-License-Identifier: BSD-2-Clause 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 #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) __dead2; 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 | O_RDONLY; 64 waitsec = -1; /* Infinite. */ 65 while ((ch = getopt(argc, argv, "sknt:w")) != -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 case 'w': 86 flags = (flags & ~O_RDONLY) | O_WRONLY; 87 break; 88 default: 89 usage(); 90 } 91 } 92 if (argc - optind < 2) 93 usage(); 94 lockname = argv[optind++]; 95 argc -= optind; 96 argv += optind; 97 if (waitsec > 0) { /* Set up a timeout. */ 98 struct sigaction act; 99 100 act.sa_handler = timeout; 101 sigemptyset(&act.sa_mask); 102 act.sa_flags = 0; /* Note that we do not set SA_RESTART. */ 103 sigaction(SIGALRM, &act, NULL); 104 alarm(waitsec); 105 } 106 /* 107 * If the "-k" option is not given, then we must not block when 108 * acquiring the lock. If we did, then the lock holder would 109 * unlink the file upon releasing the lock, and we would acquire 110 * a lock on a file with no directory entry. Then another 111 * process could come along and acquire the same lock. To avoid 112 * this problem, we separate out the actions of waiting for the 113 * lock to be available and of actually acquiring the lock. 114 * 115 * That approach produces behavior that is technically correct; 116 * however, it causes some performance & ordering problems for 117 * locks that have a lot of contention. First, it is unfair in 118 * the sense that a released lock isn't necessarily granted to 119 * the process that has been waiting the longest. A waiter may 120 * be starved out indefinitely. Second, it creates a thundering 121 * herd situation each time the lock is released. 122 * 123 * When the "-k" option is used, the unlink race no longer 124 * exists. In that case we can block while acquiring the lock, 125 * avoiding the separate step of waiting for the lock. This 126 * yields fairness and improved performance. 127 */ 128 lockfd = acquire_lock(lockname, flags | O_NONBLOCK); 129 while (lockfd == -1 && !timed_out && waitsec != 0) { 130 if (keep) 131 lockfd = acquire_lock(lockname, flags); 132 else { 133 wait_for_lock(lockname); 134 lockfd = acquire_lock(lockname, flags | O_NONBLOCK); 135 } 136 } 137 if (waitsec > 0) 138 alarm(0); 139 if (lockfd == -1) { /* We failed to acquire the lock. */ 140 if (silent) 141 exit(EX_TEMPFAIL); 142 errx(EX_TEMPFAIL, "%s: already locked", lockname); 143 } 144 /* At this point, we own the lock. */ 145 if (atexit(cleanup) == -1) 146 err(EX_OSERR, "atexit failed"); 147 if ((child = fork()) == -1) 148 err(EX_OSERR, "cannot fork"); 149 if (child == 0) { /* The child process. */ 150 close(lockfd); 151 execvp(argv[0], argv); 152 warn("%s", argv[0]); 153 _exit(1); 154 } 155 /* This is the parent process. */ 156 signal(SIGINT, SIG_IGN); 157 signal(SIGQUIT, SIG_IGN); 158 signal(SIGTERM, killed); 159 if (waitpid(child, &status, 0) == -1) 160 err(EX_OSERR, "waitpid failed"); 161 return (WIFEXITED(status) ? WEXITSTATUS(status) : EX_SOFTWARE); 162 } 163 164 /* 165 * Try to acquire a lock on the given file, creating the file if 166 * necessary. The flags argument is O_NONBLOCK or 0, depending on 167 * whether we should wait for the lock. Returns an open file descriptor 168 * on success, or -1 on failure. 169 */ 170 static int 171 acquire_lock(const char *name, int flags) 172 { 173 int fd; 174 175 if ((fd = open(name, O_EXLOCK|flags, 0666)) == -1) { 176 if (errno == EAGAIN || errno == EINTR) 177 return (-1); 178 else if (errno == ENOENT && (flags & O_CREAT) == 0) 179 err(EX_UNAVAILABLE, "%s", name); 180 err(EX_CANTCREAT, "cannot open %s", name); 181 } 182 return (fd); 183 } 184 185 /* 186 * Remove the lock file. 187 */ 188 static void 189 cleanup(void) 190 { 191 192 if (keep) 193 flock(lockfd, LOCK_UN); 194 else 195 unlink(lockname); 196 } 197 198 /* 199 * Signal handler for SIGTERM. Cleans up the lock file, then re-raises 200 * the signal. 201 */ 202 static void 203 killed(int sig) 204 { 205 206 cleanup(); 207 signal(sig, SIG_DFL); 208 if (kill(getpid(), sig) == -1) 209 err(EX_OSERR, "kill failed"); 210 } 211 212 /* 213 * Signal handler for SIGALRM. 214 */ 215 static void 216 timeout(int sig __unused) 217 { 218 219 timed_out = 1; 220 } 221 222 static void 223 usage(void) 224 { 225 226 fprintf(stderr, 227 "usage: lockf [-kns] [-t seconds] file command [arguments]\n"); 228 exit(EX_USAGE); 229 } 230 231 /* 232 * Wait until it might be possible to acquire a lock on the given file. 233 * If the file does not exist, return immediately without creating it. 234 */ 235 static void 236 wait_for_lock(const char *name) 237 { 238 int fd; 239 240 if ((fd = open(name, O_RDONLY|O_EXLOCK, 0666)) == -1) { 241 if (errno == ENOENT || errno == EINTR) 242 return; 243 err(EX_CANTCREAT, "cannot open %s", name); 244 } 245 close(fd); 246 } 247