apps/app_chanspy: don't delay spyee Hangup when spy channel has no reverse media

channel_spy() waited indefinitely on the spy channel, so after the spyee
hung up the spy audiohook leaving RUNNING was not noticed until waitfor
timed out. That kept an autochan reference on the spyee and delayed its
destructor / Hangup.

Poll with a short timeout instead. On timeout, only re-check hook status;
call ast_read() only when the spy channel is readable.

Resolves: #2123

apps/app_chanspy.c

@@ -811,14 +811,23 @@ static int channel_spy(struct ast_channel *chan, struct ast_autochan *spyee_auto
 	   channel has gone away.
 	*/
 
-	/* Note: it is very important that the ast_waitfor() be the first
-	   condition in this expression, so that if we wait for some period
-	   of time before receiving a frame from our spying channel, we check
-	   for hangup on the spied-on channel _after_ knowing that a frame
-	   has arrived, since the spied-on channel could have gone away while
-	   we were waiting
+	/* Use a short waitfor timeout so we notice the spy audiohook leaving
+	   RUNNING promptly. An indefinite wait holds an autochan ref on the
+	   spyee until waitfor times out, delaying destructor / Hangup when
+	   the spy channel is not readable.
 	*/
-	while (ast_waitfor(chan, -1) > -1 && csth.spy_audiohook.status == AST_AUDIOHOOK_STATUS_RUNNING) {
+	while (csth.spy_audiohook.status == AST_AUDIOHOOK_STATUS_RUNNING) {
+		int waitres = ast_waitfor(chan, 100);
+
+		if (waitres < 0) {
+			running = -1;
+			break;
+		}
+		if (waitres == 0) {
+			/* Timeout: re-check hook status without reading. */
+			continue;
+		}
+
 		if (!(f = ast_read(chan)) || ast_check_hangup(chan)) {
 			running = -1;
 			if (f) {