Add a set of text to the file astcanary uses to communicate back the main

Asterisk process, which explains the purpose for the file being there.  This
should assist people who find the file and wonder why it exists.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@116948 65c4cc65-6c06-0410-ace0-fbb531ad65f3

utils/astcanary.c

@@ -24,6 +24,7 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <stdlib.h>
+#include <string.h>
 
 /*!\brief
  * At one time, canaries were carried along with coal miners down
@@ -58,6 +59,19 @@
  * Asterisk itself.
  */
 
+const char explanation[] =
+"This file is created when Asterisk is run with a realtime priority (-p).  It\n"
+"must continue to exist, and the astcanary process must be allowed to continue\n"
+"running, or else the Asterisk process will, within a short period of time,\n"
+"slow itself down to regular priority.\n\n"
+"The technical explanation for this file is to provide an assurance to Asterisk\n"
+"that there are no threads that have gone into runaway mode, thus hogging the\n"
+"CPU, and making the Asterisk machine seem to be unresponsive.  When that\n"
+"happens, the astcanary process will be unable to update the timestamp on this\n"
+"file, and Asterisk will notice within 120 seconds and react.  Slowing the\n"
+"Asterisk process down to regular priority will permit an administrator to\n"
+"intervene, thus avoiding a need to reboot the entire machine.\n";
+
 int main(int argc, char *argv[])
 {
 	int fd;
@@ -67,10 +81,12 @@ int main(int argc, char *argv[])
 		/* Update the modification times (checked from Asterisk) */
 		if (utime(argv[1], NULL)) {
 			/* Recreate the file if it doesn't exist */
-			if ((fd = open(argv[1], O_RDWR | O_TRUNC | O_CREAT, 0777)) > -1)
+			if ((fd = open(argv[1], O_RDWR | O_TRUNC | O_CREAT, 0777)) > -1) {
+				write(fd, explanation, strlen(explanation));
 				close(fd);
-			else
+			} else {
 				exit(1);
+			}
 			continue;
 		}