res_pjsip_rfc3326.c: Fix crash when channel goes away.

If an authenticated incoming caller does not respond to our 200 OK INVITE
response with an ACK then PJSIP will hangup the call.  Unfortunately,
there is a chance that the session's channel will go away between one use
of the channel pointer and another when building the BYE request because
the BYE is being built by the monitor thread and not the call's serializer
thread.

* Added a check to ensure that the thread trying to add the Reason header
is the call's serializer thread.  This ensures that the channel will not
go away on us.

Change-Id: I866388d2b97ea2032eaae3f3ab3f1ca6cbd2df89

res/res_pjsip_rfc3326.c

@@ -32,6 +32,7 @@
 #include "asterisk/res_pjsip_session.h"
 #include "asterisk/module.h"
 #include "asterisk/causes.h"
+#include "asterisk/threadpool.h"
 
 static void rfc3326_use_reason_header(struct ast_sip_session *session, struct pjsip_rx_data *rdata)
 {
@@ -101,9 +102,15 @@ static void rfc3326_add_reason_header(struct ast_sip_session *session, struct pj
 
 static void rfc3326_outgoing_request(struct ast_sip_session *session, struct pjsip_tx_data *tdata)
 {
-	if ((pjsip_method_cmp(&tdata->msg->line.req.method, &pjsip_bye_method) &&
-	     pjsip_method_cmp(&tdata->msg->line.req.method, &pjsip_cancel_method)) ||
-	    !session->channel) {
+	if ((pjsip_method_cmp(&tdata->msg->line.req.method, &pjsip_bye_method)
+			&& pjsip_method_cmp(&tdata->msg->line.req.method, &pjsip_cancel_method))
+		|| !session->channel
+		/*
+		 * The session->channel has been seen to go away on us between
+		 * checks so we must also be running under the call's serializer
+		 * thread.
+		 */
+		|| session->serializer != ast_threadpool_serializer_get_current()) {
 		return;
 	}
 
@@ -114,7 +121,9 @@ static void rfc3326_outgoing_response(struct ast_sip_session *session, struct pj
 {
 	struct pjsip_status_line status = tdata->msg->line.status;
 
-	if ((status.code < 300) || !session->channel) {
+	if (status.code < 300
+		|| !session->channel
+		|| session->serializer != ast_threadpool_serializer_get_current()) {
 		return;
 	}