usr.bin/lockf/lockf.c

   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 }