bridge_native_rtp.c: Fix native rtp bridge data race.

native_rtp_bridge_compatible() didn't lock the bridge channels before
checking the channels for native bridging ability.  As a result, one of
the channel's native format capabilities structure got replaced out from
under the native bridge check.  Use of a stale pointer to freed memory
causes bad things to happen.

MALLOC_DEBUG, DO_CRASH, and the
tests/channels/pjsip/transfers/blind_transfer/caller_direct_media
testsuite test caught this.

* Add missing channel locking in native_rtp_bridge_compatible().

Change-Id: If25fdb3ac8e85563c4857fb8216b3d9dc3d0fa53

bridges/bridge_native_rtp.c

@@ -310,10 +310,8 @@ static int native_rtp_bridge_capable(struct ast_channel *chan)
 	return !ast_channel_has_hook_requiring_audio(chan);
 }
 
-static int native_rtp_bridge_compatible(struct ast_bridge *bridge)
+static int native_rtp_bridge_compatible_check(struct ast_bridge *bridge, struct ast_bridge_channel *bc0, struct ast_bridge_channel *bc1)
 {
-	struct ast_bridge_channel *bc0 = AST_LIST_FIRST(&bridge->channels);
-	struct ast_bridge_channel *bc1 = AST_LIST_LAST(&bridge->channels);
 	enum ast_rtp_glue_result native_type;
 	struct ast_rtp_glue *glue0, *glue1;
 	RAII_VAR(struct ast_rtp_instance *, instance0, NULL, ao2_cleanup);
@@ -324,13 +322,6 @@ static int native_rtp_bridge_compatible(struct ast_bridge *bridge)
 	RAII_VAR(struct ast_format_cap *, cap1, NULL, ao2_cleanup);
 	int read_ptime0, read_ptime1, write_ptime0, write_ptime1;
 
-	/* We require two channels before even considering native bridging */
-	if (bridge->num_channels != 2) {
-		ast_debug(1, "Bridge '%s' can not use native RTP bridge as two channels are required\n",
-			bridge->uniqueid);
-		return 0;
-	}
-
 	if (!native_rtp_bridge_capable(bc0->chan)) {
 		ast_debug(1, "Bridge '%s' can not use native RTP bridge as channel '%s' has features which prevent it\n",
 			bridge->uniqueid, ast_channel_name(bc0->chan));
@@ -406,6 +397,30 @@ static int native_rtp_bridge_compatible(struct ast_bridge *bridge)
 	return 1;
 }
 
+static int native_rtp_bridge_compatible(struct ast_bridge *bridge)
+{
+	struct ast_bridge_channel *bc0;
+	struct ast_bridge_channel *bc1;
+	int is_compatible;
+
+	/* We require two channels before even considering native bridging */
+	if (bridge->num_channels != 2) {
+		ast_debug(1, "Bridge '%s' can not use native RTP bridge as two channels are required\n",
+			bridge->uniqueid);
+		return 0;
+	}
+
+	bc0 = AST_LIST_FIRST(&bridge->channels);
+	bc1 = AST_LIST_LAST(&bridge->channels);
+
+	ast_channel_lock_both(bc0->chan, bc1->chan);
+	is_compatible = native_rtp_bridge_compatible_check(bridge, bc0, bc1);
+	ast_channel_unlock(bc0->chan);
+	ast_channel_unlock(bc1->chan);
+
+	return is_compatible;
+}
+
 /*! \brief Helper function which adds frame hook to bridge channel */
 static int native_rtp_bridge_framehook_attach(struct ast_bridge_channel *bridge_channel)
 {