From da6f4aa5d906c32cf13f393e3cc0f91cc5dcf9c1 Mon Sep 17 00:00:00 2001
From: Matthew Vernon <mv3@sanger.ac.uk>
Date: Wed, 3 May 2017 14:30:02 +0100
Subject: [PATCH 1/1] with-lock-ex: Provide -t (timeout) option

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>
---
v4: Initial capital and final full-stop on sentences
    Remove code that attempted to reduce post-lock timeout race
    Move timer clearing to outside the retry loop

v3: split off from conversion to getopt
---
 cprogs/with-lock-ex.c | 78 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 72 insertions(+), 6 deletions(-)

diff --git a/cprogs/with-lock-ex.c b/cprogs/with-lock-ex.c
index d1b23db..f69949e 100644
--- 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
@@ -56,6 +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;
 
@@ -78,11 +86,29 @@ static void badusage(void) {
 
 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 {
+    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;
+  int fd, um, c, r;
   struct stat stab, fstab;
-  long cloexec;
+  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) {
@@ -94,14 +120,41 @@ int main(int argc, char **argv) {
       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");
@@ -118,11 +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);
@@ -151,6 +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");
-- 
2.30.2