res_pjsip_mwi.c: Fix MWI subscription memory corruption crash.

MWI subscriptions can crash or corrupt memory when using the subscription
datastore to access the MWI subscription object because the datastore is
not holding a reference to the object.

* Give the subscription datastore a ref to the MWI subscription object.
It is unfortunate that the ref causes a circular ref chain that must be
explicitly broken to allow the memory to get released.  The loop is broken
when the subscription is shutdown and if the subscription setup fails.

ASTERISK-25168 #close
Reported by: Carl Fortin

Change-Id: Ice4fa823f138ff10a6c74d280699c41a82836d4f

res/res_pjsip_mwi.c

@@ -138,9 +138,17 @@ static struct mwi_stasis_subscription *mwi_stasis_subscription_alloc(const char
 
 	/* Safe strcpy */
 	strcpy(mwi_stasis_sub->mailbox, mailbox);
+
+	ast_debug(3, "Creating stasis MWI subscription to mailbox %s for endpoint %s\n",
+		mailbox, mwi_sub->id);
 	ao2_ref(mwi_sub, +1);
-	ast_debug(3, "Creating stasis MWI subscription to mailbox %s for endpoint %s\n", mailbox, mwi_sub->id);
 	mwi_stasis_sub->stasis_sub = stasis_subscribe_pool(topic, mwi_stasis_cb, mwi_sub);
+	if (!mwi_stasis_sub->stasis_sub) {
+		/* Failed to subscribe. */
+		ao2_ref(mwi_stasis_sub, -1);
+		ao2_ref(mwi_sub, -1);
+		mwi_stasis_sub = NULL;
+	}
 	return mwi_stasis_sub;
 }
 
@@ -491,25 +499,41 @@ static void mwi_subscription_shutdown(struct ast_sip_subscription *sub)
 
 	mwi_sub = mwi_datastore->data;
 	ao2_callback(mwi_sub->stasis_subs, OBJ_UNLINK | OBJ_NODATA | OBJ_MULTIPLE, unsubscribe_stasis, NULL);
+	ast_sip_subscription_remove_datastore(sub, MWI_DATASTORE);
 
 	ao2_ref(mwi_datastore, -1);
 }
 
-static struct ast_datastore_info mwi_ds_info = { };
+static void mwi_ds_destroy(void *data)
+{
+	struct mwi_subscription *sub = data;
+
+	ao2_ref(sub, -1);
+}
+
+static struct ast_datastore_info mwi_ds_info = {
+	.destroy = mwi_ds_destroy,
+};
 
 static int add_mwi_datastore(struct mwi_subscription *sub)
 {
 	struct ast_datastore *mwi_datastore;
+	int res;
 
 	mwi_datastore = ast_sip_subscription_alloc_datastore(&mwi_ds_info, MWI_DATASTORE);
 	if (!mwi_datastore) {
 		return -1;
 	}
+	ao2_ref(sub, +1);
 	mwi_datastore->data = sub;
 
-	ast_sip_subscription_add_datastore(sub->sip_sub, mwi_datastore);
+	/*
+	 * NOTE:  Adding the datastore to the subscription creates a ref loop
+	 * that must be manually broken.
+	 */
+	res = ast_sip_subscription_add_datastore(sub->sip_sub, mwi_datastore);
 	ao2_ref(mwi_datastore, -1);
-	return 0;
+	return res;
 }
 
 /*!
@@ -621,8 +645,8 @@ static struct mwi_subscription *mwi_create_subscription(
 	}
 
 	if (add_mwi_datastore(sub)) {
-		ast_log(LOG_WARNING, "Unable to allocate datastore on MWI "
-			"subscription from %s\n", sub->id);
+		ast_log(LOG_WARNING, "Unable to add datastore for MWI subscription to %s\n",
+			sub->id);
 		ao2_ref(sub, -1);
 		return NULL;
 	}
@@ -715,12 +739,19 @@ static int mwi_subscription_established(struct ast_sip_subscription *sip_sub)
 	} else {
 		sub = mwi_subscribe_single(endpoint, sip_sub, resource);
 	}
-
 	if (!sub) {
 		ao2_cleanup(endpoint);
 		return -1;
 	}
 
+	if (!ao2_container_count(sub->stasis_subs)) {
+		/*
+		 * We setup no MWI subscriptions so remove the MWI datastore
+		 * to break the ref loop.
+		 */
+		ast_sip_subscription_remove_datastore(sip_sub, MWI_DATASTORE);
+	}
+
 	ao2_cleanup(sub);
 	ao2_cleanup(endpoint);
 	return 0;