holding_bridge: ensure moh participants get frames

Currently, if a blank musiconhold.conf is used, musiconhold will fail
to start for a channel going into a holding bridge with an anticipation
of getting music on hold. That being the case, no frames will be written
to the channel and that can pose a problem for blind transfers in PJSIP
which may rely on frames being written to get past the REFER framehook.
This patch makes holding bridges start a silence generator if starting
music on hold fails and makes it so that if no music on hold functions
are installed that the ast_moh_start function will report a failure so
that consumers of that function will be able to respond appropriately.

ASTERISK-25271 #close

Change-Id: I06f066728604943cba0bb0b39fa7cf658a21cd99
(cherry picked from commit 8458b8d441)

bridges/bridge_holding.c

@@ -205,7 +205,11 @@ static void participant_entertainment_start(struct ast_bridge_channel *bridge_ch
 	switch(hc->idle_mode) {
 	case IDLE_MODE_MOH:
 		moh_class = ast_bridge_channel_get_role_option(bridge_channel, "holding_participant", "moh_class");
-		ast_moh_start(bridge_channel->chan, moh_class, NULL);
+		if (ast_moh_start(bridge_channel->chan, moh_class, NULL)) {
+			ast_log(LOG_WARNING, "Failed to start moh, starting silence generator instead\n");
+			hc->idle_mode = IDLE_MODE_SILENCE;
+			hc->silence_generator = ast_channel_start_silence_generator(bridge_channel->chan);
+		}
 		break;
 	case IDLE_MODE_RINGING:
 		ast_indicate(bridge_channel->chan, AST_CONTROL_RINGING);

main/channel.c

@@ -7403,7 +7403,7 @@ int ast_moh_start(struct ast_channel *chan, const char *mclass, const char *inte
 
 	ast_verb(3, "Music class %s requested but no musiconhold loaded.\n", mclass ? mclass : (interpclass ? interpclass : "default"));
 
-	return 0;
+	return -1;
 }
 
 void ast_moh_stop(struct ast_channel *chan)