res_pjsip_pubsub: Prevent removing subscriptions.

pjproject does not provide any mechanism of removing
event packages, which means that once a subscription
handler is registered, it is effectively permanent.

pjproject will assert if the same event package is
ever registered again, so currently unloading and
loading any Asterisk modules that use subscriptions
will cause a crash that is beyond our control.

For that reason, we now prevent users from being
able to unload these modules, to prevent them
from ever being loaded twice.

ASTERISK-30264 #close

Change-Id: I7fdcb1a5e44d38b7ba10c44259fe98f0ae9bc12c

res/res_pjsip_exten_state.c

@@ -973,6 +973,7 @@ static int publisher_stop(struct ast_sip_outbound_publish_client *client)
 
 static int unload_module(void)
 {
+#if 0
 	ast_sip_unregister_event_publisher_handler(&dialog_publisher);
 	ast_sip_unregister_subscription_handler(&dialog_handler);
 	ast_sip_unregister_event_publisher_handler(&presence_publisher);
@@ -987,6 +988,18 @@ static int unload_module(void)
 	publishers = NULL;
 
 	return 0;
+#else
+	/* If we were allowed to unload, the above is what we would do.
+	 * pjsip_evsub_register_pkg is called by ast_sip_register_subscription_handler
+	 * but there is no corresponding unregister function, so unloading
+	 * a module does not remove the event package. If this module is ever
+	 * loaded again, then pjproject will assert and cause a crash.
+	 * For that reason, we must not be allowed to unload, but if
+	 * a pjsip_evsub_unregister_pkg API is added in the future
+	 * then we should go back to unloading the module as intended.
+	 */
+	return -1;
+#endif
 }
 
 static int load_module(void)

res/res_pjsip_mwi.c

@@ -1524,6 +1524,7 @@ static int reload(void)
 
 static int unload_module(void)
 {
+#if 0
 	struct ao2_container *unsolicited_mwi;
 
 	ast_sorcery_observer_remove(ast_sip_get_sorcery(), "global", &global_observer);
@@ -1548,6 +1549,18 @@ static int unload_module(void)
 	ast_free(default_voicemail_extension);
 	default_voicemail_extension = NULL;
 	return 0;
+#else
+	/* If we were allowed to unload, the above is what we would do.
+	 * pjsip_evsub_register_pkg is called by ast_sip_register_subscription_handler
+	 * but there is no corresponding unregister function, so unloading
+	 * a module does not remove the event package. If this module is ever
+	 * loaded again, then pjproject will assert and cause a crash.
+	 * For that reason, we must not be allowed to unload, but if
+	 * a pjsip_evsub_unregister_pkg API is added in the future
+	 * then we should go back to unloading the module as intended.
+	 */
+	return -1;
+#endif
 }
 
 static int load_module(void)