/* lckdo.c: run a program with a lock held,
 * to prevent multiple processes running in parallel.
 * Use just like `nice' or `nohup'.
 * Written by Michael Tokarev <mjt@tls.msk.ru>
 * Public domain.
 */

#define _GNU_SOURCE
#define _BSD_SOURCE
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <stdarg.h>
#include <sysexits.h>
#include <sys/file.h>
#include <sys/wait.h>
#include <signal.h>

/* compile with -DUSE_FLOCK to use flock() instead of fcntl() */

#if !defined(USE_FLOCK) && !defined(F_SETLKW)
# define USE_FLOCK
#endif

#ifndef __GNUC__
# ifndef __attribute__
#   define __attribute__(x)
# endif
#endif

static char *progname;
static void
__attribute__((format(printf,3,4)))
__attribute__((noreturn))
error(int errnum, int exitcode, const char *fmt, ...) {
  va_list ap;
  fprintf(stderr, "%s: ", progname);
  va_start(ap, fmt); vfprintf(stderr, fmt, ap); va_end(ap);
  if (errnum) fprintf(stderr, ": %s\n", strerror(errnum));
  else fputs("\n", stderr);
  exit(exitcode);
}

static const char *lckfile;
static int quiet;

static void sigalarm(int sig) {
  if (quiet)
    _exit(EX_TEMPFAIL);
  error(0, EX_TEMPFAIL,
        "lock file `%s' is already locked (timeout waiting)", lckfile);
  sig = sig;
}

int main(int argc, char **argv) {
  int fd;
  int c;
  int create = O_CREAT;
  int dofork = 1;
  int waittime = 0;
  int shared = 0;
  int test = 0;
  int fdn = -1;
#ifndef USE_FLOCK
  struct flock fl;
#endif

  if ((progname = strrchr(argv[0], '/')) == NULL) progname = argv[0];
  else argv[0] = ++progname;

  if (argc == 1) {
    printf(
"%s: execute a program with a lock set.\n"
"Usage: %s [options] lockfile program [arguments]\n"
"where options are:\n"
" -w - if the lock is already held by another process,\n"
"   wait for it to complete instead of failing immediately\n"
" -W sec - the same as -w but wait not more than sec secounds\n"
" -e - execute the program directly, no fork/wait\n"
"   (keeps extra open filedescriptor)\n"
" -E nnn - set the fd# to keep open in -e case (implies -e)\n"
" -n - do not create the lock file if it does not exist\n"
" -q - produce no output if lock is already held\n"
" -s - lock in shared (read) mode\n"
" -x - lock in exclusive (write) mode (default)\n"
" -t - test for lock existance"
#ifndef USE_FLOCK
                              " (just prints pid if any with -q)\n"
#endif
"   (implies -n)\n"
            , progname, progname);
    return 0;
  }

  while((c = getopt(argc, argv, "+wW:neE:sxtq")) != EOF)
    switch(c) {
      case 'w':
        if (!waittime)
          waittime = -1;
        break;
      case 'W':
        if ((waittime = atoi(optarg)) < 1)
          error(0, EX_USAGE, "invalid wait time `%s'", optarg);
        break;
      case 't':
        test = 1;
        /* fall */
      case 'n':
        create = 0;
        break;
      case 'E':
        if ((fdn = atoi(optarg)) < 0 || fdn == 2)
          error(0, EX_USAGE, "invalid fd# `%s'", optarg);
        /* fall */
      case 'e':
        dofork = 0;
        break;
      case 's':
        shared = 1;
        break;
      case 'x':
        shared = 0;
        break;
      case 'q':
        quiet = 1;
        break;
      default:
        return EX_USAGE;
    }

  argc -= optind; argv += optind;
  if (!argc || (!test && argc < 2))
    error(0, EX_USAGE, "too few arguments given");

  lckfile = *argv++;

#ifdef USE_FLOCK
  create |= O_RDONLY;
#else
  if (!test)
    create |= shared ? O_RDONLY : O_WRONLY;
#endif
  fd = open(lckfile, create, 0666);
  if (fd < 0) {
    if (test && errno == ENOENT) {
      if (!quiet)
        printf("lockfile `%s' is not locked\n", lckfile);
      return 0;
    }
    error(errno, EX_CANTCREAT, "unable to open `%s'", lckfile);
  }

  if (!test && fdn >= 0) {
    /* dup it early to comply with stupid POSIX fcntl locking semantics */
    if (dup2(fd, fdn) < 0)
      error(errno, EX_OSERR, "dup2(%d,%d) failed", fd, fdn);
    close(fd);
    fd = fdn;
  }

  if (test)
    waittime = 0;
  else if (waittime > 0) {
    alarm(waittime);
    signal(SIGALRM, sigalarm);
  }
#ifdef USE_FLOCK
  c = flock(fd, (shared ? LOCK_SH : LOCK_EX) | (waittime ? 0 : LOCK_NB));
  if (test && c < 0 &&
      (errno == EWOULDBLOCK || errno == EAGAIN || errno == EACCES)) {
    if (!quiet)
      printf("lockfile `%s' is locked\n", lckfile);
    else
      printf("locked\n");
    return EX_TEMPFAIL;
  }
#else
  memset(&fl, 0, sizeof(fl));
  fl.l_type = shared ? F_RDLCK : F_WRLCK;
  c = fcntl(fd, test ? F_GETLK : waittime ? F_SETLKW : F_SETLK, &fl);
  if (test && c == 0) {
    if (fl.l_type == F_UNLCK) {
      if (!quiet)
        printf("lockfile `%s' is not locked\n", lckfile);
      return 0;
    }
    if (!quiet)
      printf("lockfile `%s' is locked by process %d\n", lckfile, fl.l_pid);
    else
      printf("%d\n", fl.l_pid);
    return EX_TEMPFAIL;
  }
#endif
  if (waittime > 0)
    alarm(0);
  if (c < 0) {
    if (errno != EWOULDBLOCK && errno != EAGAIN && errno != EACCES)
      error(errno, EX_OSERR, "unable to lock `%s'", lckfile);
    else if (quiet)
      return EX_TEMPFAIL;
    else
      error(0, EX_TEMPFAIL, "lockfile `%s' is already locked", lckfile);
  }

  if (dofork) {
    pid_t pid;
    fcntl(fd, F_SETFD, FD_CLOEXEC);
    pid = fork();
    if (pid < 0)
      error(errno, EX_OSERR, "unable to fork");
    else if (pid) {
      if (wait(&c) < 0)
        error(errno, EX_OSERR, "wait() failed");
      if (WIFSIGNALED(c))
        error(0, EX_SOFTWARE, "%s: %s", *argv, strsignal(WTERMSIG(c)));
      return WEXITSTATUS(c);
    }
  }
  execvp(*argv, argv);
  error(errno, EX_OSERR, "unable to execute %s", *argv);
}