stasis.c: Fix deadlock in stasis_topic_pool_get_topic during module load

stasis.c: Fix deadlock in stasis_topic_pool_get_topic during module load.

Deadlock occurs when res_manager_devicestate loads concurrently with
device state operations due to lock ordering violation:

Thread 1: Holds pool lock → needs topic lock (in stasis_forward_all)
Thread 2: Holds topic lock → needs pool lock (in stasis_topic_pool_get_topic)

Fix: Release pool lock before calling stasis_topic_create() and
stasis_forward_all(). Re-acquire only for insertion with race check.

Preserves borrowed reference semantics while breaking the deadlock cycle.

Fixes: #1611

main/stasis.c

@@ -1776,6 +1776,22 @@ static void send_subscription_unsubscribe(struct stasis_topic *topic,
 struct topic_pool_entry {
 	struct stasis_forward *forward;
 	struct stasis_topic *topic;
+	/*
+	 * Per-entry initialization state. This lets us serialize creation of a
+	 * given topic name without holding the pool container lock while doing
+	 * the heavy lifting (topic creation, forwarding setup, etc).
+	 *
+	 * A topic_pool_entry starts life in an "in-flight" state where neither
+	 * initialized nor failed are set.  The first thread to link the entry
+	 * into the pool becomes the creator and is responsible for completing
+	 * initialization, setting one of the flags, and broadcasting init_cond.
+	 * Other threads that find the same entry simply wait for initialization
+	 * to complete and then reuse the created topic.
+	 */
+	ast_mutex_t init_lock;
+	ast_cond_t init_cond;
+	unsigned int initialized; /* terminal success state */
+	unsigned int failed; /* terminal failure state */
 	char name[0];
 };
 
@@ -1786,6 +1802,8 @@ static void topic_pool_entry_dtor(void *obj)
 	entry->forward = stasis_forward_cancel(entry->forward);
 	ao2_cleanup(entry->topic);
 	entry->topic = NULL;
+	ast_cond_destroy(&entry->init_cond);
+	ast_mutex_destroy(&entry->init_lock);
 }
 
 static struct topic_pool_entry *topic_pool_entry_alloc(const char *topic_name)
@@ -1797,9 +1815,9 @@ static struct topic_pool_entry *topic_pool_entry_alloc(const char *topic_name)
 	if (!topic_pool_entry) {
 		return NULL;
 	}
-
+	ast_mutex_init(&topic_pool_entry->init_lock);
+	ast_cond_init(&topic_pool_entry->init_cond, NULL);
 	strcpy(topic_pool_entry->name, topic_name); /* Safe */
-
 	return topic_pool_entry;
 }
 
@@ -1947,48 +1965,164 @@ void stasis_topic_pool_delete_topic(struct stasis_topic_pool *pool, const char *
 
 	ao2_find(pool->pool_container, search_topic_name, OBJ_SEARCH_KEY | OBJ_NODATA | OBJ_UNLINK);
 }
+/*!
+ * \brief Get a topic from the pool for the given name.
+ *
+ * This returns a **borrowed** reference: the pool container owns the topic
+ * and callers MUST NOT ao2_cleanup() the returned pointer.
+ *
+ * To avoid both deadlocks and wasted work we use a per-name "in-flight"
+ * topic_pool_entry while a topic is being created:
+ *
+ *  - The pool container lock is held only while looking up or inserting
+ *    the topic_pool_entry for a name.
+ *  - Exactly one thread becomes the creator for a given name and is
+ *    responsible for allocating the topic and wiring up forwarding.
+ *  - Other threads that race for the same name find the in-flight entry
+ *    and wait on its condition variable until initialization completes.
+ */
 
 struct stasis_topic *stasis_topic_pool_get_topic(struct stasis_topic_pool *pool, const char *topic_name)
 {
-	RAII_VAR(struct topic_pool_entry *, topic_pool_entry, NULL, ao2_cleanup);
-	SCOPED_AO2LOCK(topic_container_lock, pool->pool_container);
-	char *new_topic_name;
+	/*
+	* Lock ordering:
+	*
+	*   pool->pool_container (AO2 lock)
+	*       → entry->init_lock
+	*           → topic locks (inside stasis_topic_create() /
+	*                            stasis_forward_all())
+	*
+	* We intentionally do NOT hold the pool container lock while calling
+	* stasis_topic_create() or stasis_forward_all() to avoid deadlocks with
+	* other code that may take topic locks first and then need the pool lock.
+	*/
+	RAII_VAR(struct topic_pool_entry *, entry, NULL, ao2_cleanup);
+	char *fq = NULL;
+	int creator = 0;
 	int ret;
 
-	topic_pool_entry = ao2_find(pool->pool_container, topic_name, OBJ_SEARCH_KEY | OBJ_NOLOCK);
-	if (topic_pool_entry) {
-		return topic_pool_entry->topic;
-	}
-
-	topic_pool_entry = topic_pool_entry_alloc(topic_name);
-	if (!topic_pool_entry) {
+	if (!pool || ast_strlen_zero(topic_name)) {
 		return NULL;
 	}
 
-	/* To provide further detail and to ensure that the topic is unique within the scope of the
-	 * system we prefix it with the pooling topic name, which should itself already be unique.
+	/* Creator / waiter split:
+	 *
+	 * - The first thread to create/link an entry for topic_name becomes the
+	 *   "creator" and is responsible for creating the underlying stasis
+	 *   topic and wiring up forwarding.
+	 *
+	 * - Other threads that find the entry become "waiters"; they block on
+	 *   entry->init_cond until either initialization succeeds or fails.
 	 */
-	ret = ast_asprintf(&new_topic_name, "%s/%s", stasis_topic_name(pool->pool_topic), topic_name);
-	if (ret < 0) {
-		return NULL;
+
+	/* --- Creator selection under pool container lock --- */
+	ao2_lock(pool->pool_container);
+
+	entry = ao2_find(pool->pool_container, topic_name, OBJ_SEARCH_KEY | OBJ_NOLOCK);
+	if (!entry) {
+		entry = topic_pool_entry_alloc(topic_name);
+		if (!entry) {
+			ao2_unlock(pool->pool_container);
+			return NULL;
+		}
+
+		if (!ao2_link_flags(pool->pool_container, entry, OBJ_NOLOCK)) {
+			struct topic_pool_entry *other;
+
+			other = ao2_find(pool->pool_container, topic_name, OBJ_SEARCH_KEY | OBJ_NOLOCK);
+			if (other) {
+				struct topic_pool_entry *tmp = entry;
+
+				entry = other;
+				creator = 0;
+				ao2_unlock(pool->pool_container);
+				ao2_ref(tmp, -1);
+				goto waiter_path;
+			}
+			
+			ao2_unlock(pool->pool_container);
+			return NULL;
+		}
+
+		creator = 1;
 	}
 
-	topic_pool_entry->topic = stasis_topic_create(new_topic_name);
-	ast_free(new_topic_name);
-	if (!topic_pool_entry->topic) {
+	ao2_unlock(pool->pool_container);
+
+/* --- Waiter path: wait for creator to finish --- */
+waiter_path:
+	if (!creator) {
+		ast_mutex_lock(&entry->init_lock);
+		while (!entry->initialized && !entry->failed) {
+			ast_cond_wait(&entry->init_cond, &entry->init_lock);
+		}
+
+		if (entry->initialized && !entry->failed) {
+			struct stasis_topic *topic = entry->topic;
+			
+			if (!topic) {
+				ast_debug(1, "Pooled topic '%s' marked initialized but topic is NULL\n", entry->name);
+				ast_mutex_unlock(&entry->init_lock);
+				return NULL;
+			}
+			ast_mutex_unlock(&entry->init_lock);
+			/* Borrowed reference: container owns the topic */
+			return topic;
+		}
+
+		ast_mutex_unlock(&entry->init_lock);
 		return NULL;
 	}
 
-	topic_pool_entry->forward = stasis_forward_all(topic_pool_entry->topic, pool->pool_topic);
-	if (!topic_pool_entry->forward) {
-		return NULL;
+	/* --- Creator path: perform topic creation without pool lock --- */
+	ast_mutex_lock(&entry->init_lock);
+	/* Defensive: entry may have been initialized/failed before we acquired init_lock. */
+	if (entry->initialized || entry->failed) {
+		struct stasis_topic *topic = entry->initialized ? entry->topic : NULL;
+		ast_mutex_unlock(&entry->init_lock);
+		return topic;
 	}
 
-	if (!ao2_link_flags(pool->pool_container, topic_pool_entry, OBJ_NOLOCK)) {
-		return NULL;
+	ret = ast_asprintf(&fq, "%s/%s", stasis_topic_name(pool->pool_topic), topic_name);
+	if (ret < 0) {
+		entry->failed = 1;
+		goto creator_fail;
+	}
+
+	entry->topic = stasis_topic_create(fq);
+	ast_free(fq);
+	fq = NULL;
+
+	if (!entry->topic) {
+		entry->failed = 1;
+		goto creator_fail;
+	}
+
+	entry->forward = stasis_forward_all(entry->topic, pool->pool_topic);
+	if (!entry->forward) {
+		ao2_cleanup(entry->topic);
+		entry->topic = NULL;
+		entry->failed = 1;
+		goto creator_fail;
 	}
 
-	return topic_pool_entry->topic;
+	entry->initialized = 1;
+	ast_cond_broadcast(&entry->init_cond);
+	ast_mutex_unlock(&entry->init_lock);
+
+	return entry->topic; /* borrowed ref */
+
+creator_fail:
+	ast_debug(1, "Failed to create pooled stasis topic '%s/%s'\n", stasis_topic_name(pool->pool_topic), entry->name);
+	ast_cond_broadcast(&entry->init_cond);
+	ast_mutex_unlock(&entry->init_lock);
+
+	/* Remove failed entry so future callers can retry */
+	ao2_lock(pool->pool_container);
+	ao2_unlink(pool->pool_container, entry);
+	ao2_unlock(pool->pool_container);
+
+	return NULL;
 }
 
 int stasis_topic_pool_topic_exists(const struct stasis_topic_pool *pool, const char *topic_name)