Prevent potential race condition in multiparty basic bridges.

For more details about the race condition see the linked review
at the bottom of this commit

(closes issue ASTERISK-21882)
Reported by Matt Jordan

Review: https://reviewboard.asterisk.org/r/2663



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394232 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/bridging_basic.c

@@ -99,9 +99,16 @@ struct ast_flags *ast_bridge_features_ds_get(struct ast_channel *chan)
  */
 static int basic_hangup_hook(struct ast_bridge *bridge, struct ast_bridge_channel *bridge_channel, void *hook_pvt)
 {
-/* BUGBUG Race condition.  If all parties but one hangup at the same time, the bridge may not be dissolved on the remaining party. */
+	int bridge_count = 0;
+	struct ast_bridge_channel *iter;
+
 	ast_bridge_channel_lock_bridge(bridge_channel);
-	if (2 < bridge_channel->bridge->num_channels) {
+	AST_LIST_TRAVERSE(&bridge->channels, iter, entry) {
+		if (iter != bridge_channel && iter->state == AST_BRIDGE_CHANNEL_STATE_WAIT) {
+			++bridge_count;
+		}
+	}
+	if (2 <= bridge_count) {
 		/* Just allow this channel to leave the multi-party bridge. */
 		ast_bridge_change_state(bridge_channel, AST_BRIDGE_CHANNEL_STATE_HANGUP);
 	}