res_pjsip_exten_state: Fix race condition between sending NOTIFY and termination

The res_pjsip_exten_state module currently has a race condition between
processing the extension state callback from the PBX core and processing
the subscription shutdown callback from res_pjsip_pubsub. There is currently
no synchronization between the two. This can present a problem as while
the SIP subscription will remain valid the tree it points to may not.
This is in particular a problem as a task to send a NOTIFY may get queued
which will try to use the tree that may no longer be valid.

This change does the following to fix this problem:

1. All access to the subscription tree is done within the task that
sends the NOTIFY to ensure that no other thread is modifying or
destroying the tree. This task executes on the serializer for the
subscriptions.

2. A reference to the subscription serializer is kept to ensure it
remains valid for the lifetime of the extension state subscription.

3. The NOTIFY task has been changed so it will no longer attempt
to send a NOTIFY if the subscription has already been terminated.

ASTERISK-25057 #close
Reported by: Matt Jordan

Change-Id: I0b3cd2fac5be8d9b3dc5e693aaa79846eeaf5643

include/asterisk/res_pjsip_pubsub.h

@@ -405,6 +405,16 @@ void ast_sip_subscription_get_remote_uri(struct ast_sip_subscription *sub, char
  */
 const char *ast_sip_subscription_get_resource_name(struct ast_sip_subscription *sub);
 
+/*!
+ * \brief Get whether the subscription has been terminated or not.
+ *
+ * \param sub The subscription.
+ * \retval 0 not terminated.
+ * \retval 1 terminated.
+ * \since 13.4.0
+ */
+int ast_sip_subscription_is_terminated(const struct ast_sip_subscription *sub);
+
 /*!
  * \brief Get a header value for a subscription.
  *

res/res_pjsip_exten_state.c

@@ -37,6 +37,7 @@
 #include "asterisk/astobj2.h"
 #include "asterisk/sorcery.h"
 #include "asterisk/app.h"
+#include "asterisk/taskprocessor.h"
 
 #define BODY_SIZE 1024
 #define EVENT_TYPE_SIZE 50
@@ -53,6 +54,8 @@ struct exten_state_subscription {
 	int id;
 	/*! The SIP subscription */
 	struct ast_sip_subscription *sip_sub;
+	/*! The serializer to use for notifications */
+	struct ast_taskprocessor *serializer;
 	/*! Context in which subscription looks for updates */
 	char context[AST_MAX_CONTEXT];
 	/*! Extension within the context to receive updates from */
@@ -113,6 +116,7 @@ static void exten_state_subscription_destructor(void *obj)
 
 	ast_free(sub->user_agent);
 	ao2_cleanup(sub->sip_sub);
+	ast_taskprocessor_unreference(sub->serializer);
 }
 
 static char *get_user_agent(const struct ast_sip_subscription *sip_sub)
@@ -157,6 +161,13 @@ static struct exten_state_subscription *exten_state_subscription_alloc(
 	}
 
 	exten_state_sub->sip_sub = ao2_bump(sip_sub);
+
+	/* We keep our own reference to the serializer as there is no guarantee in state_changed
+	 * that the subscription tree is still valid when it is called. This can occur when
+	 * the subscription is terminated at around the same time as the state_changed
+	 * callback is invoked.
+	 */
+	exten_state_sub->serializer = ao2_bump(ast_sip_subscription_get_serializer(sip_sub));
 	exten_state_sub->last_exten_state = INITIAL_LAST_EXTEN_STATE;
 	exten_state_sub->last_presence_state = AST_PRESENCE_NOT_SET;
 	exten_state_sub->user_agent = get_user_agent(sip_sub);
@@ -205,11 +216,6 @@ static struct notify_task_data *alloc_notify_task_data(char *exten, struct exten
 	task_data->exten_state_data.device_state_info = ao2_bump(info->device_state_info);
 	task_data->exten_state_data.sub = exten_state_sub->sip_sub;
 
-	ast_sip_subscription_get_local_uri(exten_state_sub->sip_sub,
-			task_data->exten_state_data.local, sizeof(task_data->exten_state_data.local));
-	ast_sip_subscription_get_remote_uri(exten_state_sub->sip_sub,
-			task_data->exten_state_data.remote, sizeof(task_data->exten_state_data.remote));
-
 	if ((info->exten_state == AST_EXTENSION_DEACTIVATED) ||
 	    (info->exten_state == AST_EXTENSION_REMOVED)) {
 		ast_verb(2, "Watcher for hint %s %s\n", exten, info->exten_state
@@ -228,6 +234,19 @@ static int notify_task(void *obj)
 		.body_data = &task_data->exten_state_data,
 	};
 
+	/* Terminated subscriptions are no longer associated with a valid tree, and sending
+	 * NOTIFY messages on a subscription which has already been terminated won't work.
+	 */
+	if (ast_sip_subscription_is_terminated(task_data->exten_state_sub->sip_sub)) {
+		return 0;
+	}
+
+	/* All access to the subscription must occur within a task executed within its serializer */
+	ast_sip_subscription_get_local_uri(task_data->exten_state_sub->sip_sub,
+			task_data->exten_state_data.local, sizeof(task_data->exten_state_data.local));
+	ast_sip_subscription_get_remote_uri(task_data->exten_state_sub->sip_sub,
+			task_data->exten_state_data.remote, sizeof(task_data->exten_state_data.remote));
+
 	/* Pool allocation has to happen here so that we allocate within a PJLIB thread */
 	task_data->exten_state_data.pool = pjsip_endpt_create_pool(ast_sip_get_pjsip_endpoint(),
 			"exten_state", 1024, 1024);
@@ -263,8 +282,8 @@ static int state_changed(char *context, char *exten,
 
 	/* safe to push this async since we copy the data from info and
 	   add a ref for the device state info */
-	if (ast_sip_push_task(ast_sip_subscription_get_serializer(task_data->exten_state_sub->sip_sub),
+	if (ast_sip_push_task(task_data->exten_state_sub->serializer, notify_task,
-			      notify_task, task_data)) {
+		task_data)) {
 		ao2_cleanup(task_data);
 		return -1;
 	}

res/res_pjsip_pubsub.c

@@ -2185,6 +2185,11 @@ const char *ast_sip_subscription_get_resource_name(struct ast_sip_subscription *
 	return sub->resource;
 }
 
+int ast_sip_subscription_is_terminated(const struct ast_sip_subscription *sub)
+{
+	return sub->subscription_state == PJSIP_EVSUB_STATE_TERMINATED ? 1 : 0;
+}
+
 static int sip_subscription_accept(struct sip_subscription_tree *sub_tree, pjsip_rx_data *rdata, int response)
 {
 	pjsip_hdr res_hdr;

res/res_pjsip_pubsub.exports.in

@@ -37,6 +37,7 @@
 		LINKER_SYMBOL_PREFIXast_sip_subscription_get_local_uri;
 		LINKER_SYMBOL_PREFIXast_sip_subscription_get_remote_uri;
 		LINKER_SYMBOL_PREFIXast_sip_subscription_get_header;
+		LINKER_SYMBOL_PREFIXast_sip_subscription_is_terminated;
 	local:
 		*;
 };