main/utils: Don't emit an ERROR message if the read end of a pipe closes

An ERROR or WARNING message should generally indicate that something has gone
wrong in Asterisk. In the case of writing to a file descriptor, Asterisk is not
in control of when the far end closes its reading on a file descriptor. If the
far end does close the file descriptor in an unclean fashion, this isn't a bug
or error in Asterisk, particularly when the situation can be gracefully
handled in Asterisk.

Currently, when this happens, a user would see the following somewhat cryptic
ERROR message:

  "utils.c: write() returned error: Broken pipe"

There's a few problems with this:
(1) It doesn't provide any context, other than 'something broke a pipe'
(2) As noted, it isn't actually an error in Asterisk
(3) It can get rather spammy if the thing breaking the pipe occurs often, such
    as a FastAGI server
(4) Spammy ERROR messages make Asterisk appear to be having issues, or can even
    mask legitimate issues

This patch changes ast_carefulwrite to only log an ERROR if we actually had one
that was reasonably under our control. For debugging purposes, we still emit
a debug message if we detect that the far side has stopped reading.

Change-Id: Ia503bb1efcec685fa6f3017bedf98061f8e1b566

main/utils.c

@@ -1399,7 +1399,13 @@ int ast_carefulwrite(int fd, char *s, int len, int timeoutms)
 
 		if (res < 0 && errno != EAGAIN && errno != EINTR) {
 			/* fatal error from write() */
-			ast_log(LOG_ERROR, "write() returned error: %s\n", strerror(errno));
+			if (errno == EPIPE) {
+#ifndef STANDALONE
+				ast_debug(1, "write() failed due to reading end being closed: %s\n", strerror(errno));
+#endif
+			} else {
+				ast_log(LOG_ERROR, "write() returned error: %s\n", strerror(errno));
+			}
 			return -1;
 		}