res_pjsip_pubsub.c: Use distributor serializer for incoming subscriptions.

We must continue using the serializer that the original SUBSCRIBE came in
on for the dialog.  There may be retransmissions already enqueued in the
original serializer that can result in reentrancy and message sequencing
problems.  The "sip_transaction Unable to register SUBSCRIBE transaction
(key exists)" message is a notable symptom of this issue.

Outgoing subscriptions still create the pjsip/pubsub/<endpoint>
serializers for their dialogs.

ASTERISK-26088
Reported by:  Richard Mudgett

Change-Id: I18b00bb74a56747b2c8c29543a82440b110bf0b0

res/res_pjsip_pubsub.c

@@ -1229,10 +1229,9 @@ static void subscription_setup_dialog(struct sip_subscription_tree *sub_tree, pj
 	pjsip_dlg_inc_session(dlg, &pubsub_module);
 }
 
-static struct sip_subscription_tree *allocate_subscription_tree(struct ast_sip_endpoint *endpoint)
+static struct sip_subscription_tree *allocate_subscription_tree(struct ast_sip_endpoint *endpoint, pjsip_rx_data *rdata)
 {
 	struct sip_subscription_tree *sub_tree;
-	char tps_name[AST_TASKPROCESSOR_MAX_NAME + 1];
 
 	sub_tree = ao2_alloc(sizeof *sub_tree, subscription_tree_destructor);
 	if (!sub_tree) {
@@ -1241,11 +1240,24 @@ static struct sip_subscription_tree *allocate_subscription_tree(struct ast_sip_e
 
 	ast_module_ref(ast_module_info->self);
 
+	if (rdata) {
+		/*
+		 * We must continue using the serializer that the original
+		 * SUBSCRIBE came in on for the dialog.  There may be
+		 * retransmissions already enqueued in the original
+		 * serializer that can result in reentrancy and message
+		 * sequencing problems.
+		 */
+		sub_tree->serializer = ast_sip_get_distributor_serializer(rdata);
+	} else {
+		char tps_name[AST_TASKPROCESSOR_MAX_NAME + 1];
+
 		/* Create name with seq number appended. */
 		ast_taskprocessor_build_name(tps_name, sizeof(tps_name), "pjsip/pubsub/%s",
 			ast_sorcery_object_get_id(endpoint));
 
 		sub_tree->serializer = ast_sip_create_serializer_named(tps_name);
+	}
 	if (!sub_tree->serializer) {
 		ao2_ref(sub_tree, -1);
 		return NULL;
@@ -1286,7 +1298,7 @@ static struct sip_subscription_tree *create_subscription_tree(const struct ast_s
 	pjsip_dialog *dlg;
 	struct subscription_persistence *persistence;
 
-	sub_tree = allocate_subscription_tree(endpoint);
+	sub_tree = allocate_subscription_tree(endpoint, rdata);
 	if (!sub_tree) {
 		*dlg_status = PJ_ENOMEM;
 		return NULL;
@@ -1593,7 +1605,7 @@ struct ast_sip_subscription *ast_sip_create_subscription(const struct ast_sip_su
 	pjsip_evsub *evsub;
 	struct sip_subscription_tree *sub_tree = NULL;
 
-	sub_tree = allocate_subscription_tree(endpoint);
+	sub_tree = allocate_subscription_tree(endpoint, NULL);
 	if (!sub_tree) {
 		return NULL;
 	}