changelog: Document -t option

[chiark-utils.git] / cprogs / with-lock-ex.c

diff --git a/cprogs/with-lock-ex.c b/cprogs/with-lock-ex.c
index f10e68208b72f0ea799a5632cef219f94ea59e1a..f69949e563f55f4e7ccbfec6f684c120c14ba478 100644 (file)
--- a/cprogs/with-lock-ex.c
+++ b/cprogs/with-lock-ex.c
@@ -2,7 +2,7 @@
  * File locker
  *
  * Usage:
- *  with-lock-ex -<mode> <lockfile> <command> <args>...
+ *  with-lock-ex -<mode> [-t <secs>] <lockfile> <command> <args>...
  *  with-lock-ex -l      <lockfile>
  *
  * modes are
@@ -13,6 +13,11 @@
  *         or "write <pid>"; lockfile opened for reading;
  *         no command may be specified)
  *
+ * If -t is specified, then with-lock-ex will wait for up to <secs>
+ * seconds to acquire the lock, and then fail or silently do nothing
+ * (depending on whether -f or -q is specified). You cannot specify
+ * a timeout for modes l or w.
+ *
  * with-lock-ex will open and lock the lockfile for writing and
  * then feed the remainder of its arguments to exec(2); when
  * that process terminates the fd will be closed and the file
@@ -26,6 +31,7 @@
  *
  * Copyright 1993-2016 Ian Jackson in some jurisdictions
  * Copyright 2017      Ian Jackson in all jurisdictions
+ * Copyright 2017      Genome Research Ltd
  *
  * (MIT licence:)
  *
@@ -55,6 +61,9 @@
 #include <unistd.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <limits.h>
+#include <signal.h>
+#include <sys/time.h>
 
 static const char *cmd;
 
@@ -65,28 +74,87 @@ static void fail(const char *why) {
   exit(255);
 }
 
-int main(int argc, char **argv) {
-  int fd, mode, um;
-  struct stat stab, fstab;
-  long cloexec;
-  struct flock fl;
-  const char *p;
-
-  if (argc >= 3 && !strcmp((p= strrchr(argv[0],'/')) ? ++p : argv[0], "with-lock")) {
-    mode= 'f';
-  } else if (argc < 3 || argv[1][0] != '-' || argv[1][2] ||
-            ((mode= argv[1][1]) != 'w' && mode != 'q' && mode != 'f'
-             && mode != 'l') ||
-            (mode != 'l' && argc < 4) ||
-            (mode == 'l' && argc != 3)) {
-    fputs("usage: with-lock-ex -w|-q|-f <lockfile> <command> <args>...\n"
+static void badusage(void) __attribute__((noreturn));
+
+static void badusage(void) {
+    fputs("usage: with-lock-ex -w|-q|-f [-t <secs>] <lockfile> <command> <args>...\n"
          "       with-lock-ex -l       <lockfile>\n"
-         "       with-lock             <lockfile> <command> <args>...\n",
+         "       with-lock-ex          <lockfile> <command> <args>...\n",
          stderr);
     exit(255);
+}
+
+static int mode;
+
+/* This signal handler uses unsafe functions, so MUST NOT be callable
+ * during an unsafe function, as that is Undefined Behaviour
+ */
+static void alrm_handler(int signum) {
+  if (mode=='q') {
+    exit(0);
   } else {
-    argv++; argc--;
+    fprintf(stderr,
+           "with-lock-ex %s: timer expired while trying to acquire lock\n",
+           cmd);
+    exit(255);
   }
+}
+
+int main(int argc, char **argv) {
+  int fd, um, c, r;
+  struct stat stab, fstab;
+  long cloexec, secs=0;
+  struct flock fl;
+  char *endptr;
+  sigset_t sigs, oldsigs;
+  struct sigaction siga;
+  struct itimerval itv;
+
+  mode= 'x';
+  while ((c= getopt(argc,argv,"+wfqlt:")) != -1) {
+    switch(c) {
+    case 'l':
+    case 'w':
+    case 'f':
+    case 'q':
+      if (mode != 'x') badusage();
+      mode= c;
+      break;
+    case 't':
+      errno = 0;
+      secs = strtol(optarg, &endptr, 0);
+      if (*endptr || endptr==optarg || errno==ERANGE)
+       fail("parsing timeout value");
+      if (secs < 0) {
+       fprintf(stderr,"timeout value must be >=0\n");
+       exit(255);
+      }
+      break;
+    default:
+      badusage();
+    }
+  }
+
+  if (secs && (mode=='l' || mode=='w')) {
+    fputs("-t only allowed with -q or -f.\n", stderr);
+    exit(255);
+  }
+
+  argv += optind-1; argc -= optind-1;
+  if (argc < 2) badusage();
+
+  if (secs) {
+    if (sigemptyset(&sigs)) fail("Initialising signal set");
+    if (sigaddset(&sigs,SIGALRM)) fail("Adding SIGALRM to signal set");
+    if (sigprocmask(SIG_BLOCK,&sigs,&oldsigs)) fail("Blocking SIGALRM");
+    memset(&siga,0,sizeof(siga));
+    siga.sa_handler=alrm_handler;
+    if (sigaction(SIGALRM,&siga,NULL)) fail("Installing SIGALRM handler");
+    memset(&itv,0,sizeof(itv));
+    itv.it_value.tv_sec=secs;
+    if (setitimer(ITIMER_REAL,&itv,NULL)) fail("Setting timer");
+  }
+
   cmd= argv[2];
   um= umask(0777); if (um==-1) fail("find umask");
   if (umask(um)==-1) fail("reset umask");
@@ -103,11 +171,16 @@ int main(int argc, char **argv) {
       fl.l_whence= SEEK_SET;
       fl.l_start= 0;
       fl.l_len= mode=='l' ? 0 : 1;
-      if (fcntl(fd,
+      if (secs) sigprocmask(SIG_UNBLOCK,&sigs,NULL);
+      r = fcntl(fd,
                mode=='l' ? F_GETLK :
-               mode=='w' ? F_SETLKW :
+               mode=='w' || secs > 0 ? F_SETLKW :
                F_SETLK,
-               &fl) != -1) break;
+               &fl);
+      if (secs) sigprocmask(SIG_BLOCK,&sigs,NULL);
+      if (!r) {
+       break;
+      }
       if (mode=='q' &&
          (errno == EAGAIN || errno == EWOULDBLOCK || errno == EBUSY))
        exit(0);
@@ -136,6 +209,14 @@ int main(int argc, char **argv) {
     close(fd);
   }
 
+  if (secs) {
+    itv.it_value.tv_sec=0;
+    if (setitimer(ITIMER_REAL,&itv,NULL)) fail("Clearing timer");
+    sigprocmask(SIG_SETMASK,&oldsigs,NULL);
+    siga.sa_handler=SIG_DFL;
+    sigaction(SIGALRM,&siga,NULL);
+  }
+
   cloexec= fcntl(fd, F_GETFD); if (cloexec==-1) fail("fcntl F_GETFD");
   cloexec &= ~1;
   if (fcntl(fd, F_SETFD, cloexec)==-1) fail("fcntl F_SETFD");