Cause astcanary to exit if Asterisk exits abnormally and doesn't kill astcanary.

Also, add some documentation supporting the use of astcanary.
(closes issue #14538)
 Reported by: KNK
 Patches: 
       asterisk-1.6.x-astcanary.diff uploaded by KNK (license 545)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@178303 65c4cc65-6c06-0410-ace0-fbb531ad65f3

utils/astcanary.c

@@ -30,7 +30,7 @@
  * At one time, canaries were carried along with coal miners down
  * into a mine.  Their purpose was to alert the miners when they
  * had drilled into a pocket of methane gas or another noxious
- * substance.  The canary, being the most sensitive animal would
+ * substance.  The canary, being the most sensitive animal, would
  * immediately fall over.  Seeing this, the miners could take
  * action to escape the mine, seeing an imminent danger.
  *
@@ -57,6 +57,18 @@
  * the same time.  This is also why this canary must exist as a
  * completely separate process and not simply as a thread within
  * Asterisk itself.
+ *
+ * Quote:
+ * "The nice value set with setpriority() shall be applied to the
+ * process. If the process is multi-threaded, the nice value shall
+ * affect all system scope threads in the process."
+ *
+ * Source:
+ * http://www.opengroup.org/onlinepubs/000095399/functions/setpriority.html
+ *
+ * In answer to the question, what aren't system scope threads, the
+ * answer is, in Asterisk, nothing.  Process scope threads are the
+ * alternative, but they aren't supported in Linux.
  */
 
 static const char explanation[] =
@@ -77,7 +89,7 @@ int main(int argc, char *argv[])
 	int fd;
 	/* Run at normal priority */
 	setpriority(PRIO_PROCESS, 0, 0);
-	for (;;) {
+	for (; getppid() != 1;) {
 		/* Update the modification times (checked from Asterisk) */
 		if (utime(argv[1], NULL)) {
 			/* Recreate the file if it doesn't exist */
@@ -96,7 +108,7 @@ int main(int argc, char *argv[])
 		sleep(5);
 	}
 
-	/* Never reached */
+	/* Reached if asterisk (our parent process) dies - its chldren are inherited by the init process (pid is 1). */
 	return 0;
 }