scheduler: Use queue for allocating sched IDs.

It has been observed that on long-running busy systems, a scheduler
context can eventually hit INT_MAX for its assigned IDs and end up
overflowing into a very low negative number. When this occurs, this can
result in odd behaviors, because a negative return is interpreted by
callers as being a failure. However, the item actually was successfully
scheduled. The result may be that a freed item remains in the scheduler,
resulting in a crash at some point in the future.

The scheduler can overflow because every time that an item is added to
the scheduler, a counter is bumped and that counter's current value is
assigned as the new item's ID.

This patch introduces a new method for assigning scheduler IDs. Instead
of assigning from a counter, a queue of available IDs is maintained.
When assigning a new ID, an ID is pulled from the queue. When a
scheduler item is released, its ID is pushed back onto the queue. This
way, IDs may be reused when they become available, and the growth of ID
numbers is directly related to concurrent activity within a scheduler
context rather than the uptime of the system.

Change-Id: I532708eef8f669d823457d7fefdad9a6078b99b2

main/sched.c

@@ -62,9 +62,26 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 
 AST_THREADSTORAGE(last_del_id);
 
+/*!
+ * \brief Scheduler ID holder
+ *
+ * These form a queue on a scheduler context. When a new
+ * scheduled item is created, a sched_id is popped off the
+ * queue and its id is assigned to the new scheduled item.
+ * When the scheduled task is complete, the sched_id on that
+ * task is then pushed to the back of the queue to be re-used
+ * on some future scheduled item.
+ */
+struct sched_id {
+	/*! Immutable ID number that is copied onto the scheduled task */
+	int id;
+	AST_LIST_ENTRY(sched_id) list;
+};
+
 struct sched {
 	AST_LIST_ENTRY(sched) list;
-	int id;                       /*!< ID number of event */
+	/*! The ID that has been popped off the scheduler context's queue */
+	struct sched_id *sched_id;
 	struct timeval when;          /*!< Absolute time event should take place */
 	int resched;                  /*!< When to reschedule */
 	int variable;                 /*!< Use return value from callback to reschedule */
@@ -99,6 +116,10 @@ struct ast_sched_context {
 	AST_LIST_HEAD_NOLOCK(, sched) schedc;   /*!< Cache of unused schedule structures and how many */
 	unsigned int schedccnt;
 #endif
+	/*! Queue of scheduler task IDs to assign */
+	AST_LIST_HEAD_NOLOCK(, sched_id) id_queue;
+	/*! The number of IDs in the id_queue */
+	int id_queue_size;
 };
 
 static void *sched_run(void *data)
@@ -208,6 +229,8 @@ struct ast_sched_context *ast_sched_context_create(void)
 	ast_mutex_init(&tmp->lock);
 	tmp->eventcnt = 1;
 
+	AST_LIST_HEAD_INIT_NOLOCK(&tmp->id_queue);
+
 	if (!(tmp->sched_heap = ast_heap_create(8, sched_time_cmp,
 			offsetof(struct sched, __heap_index)))) {
 		ast_sched_context_destroy(tmp);
@@ -219,6 +242,11 @@ struct ast_sched_context *ast_sched_context_create(void)
 
 static void sched_free(struct sched *task)
 {
+	/* task->sched_id will be NULL most of the time, but when the
+	 * scheduler context shuts down, it will free all scheduled
+	 * tasks, and in that case, the task->sched_id will be non-NULL
+	 */
+	ast_free(task->sched_id);
 	ast_cond_destroy(&task->cond);
 	ast_free(task);
 }
@@ -226,6 +254,7 @@ static void sched_free(struct sched *task)
 void ast_sched_context_destroy(struct ast_sched_context *con)
 {
 	struct sched *s;
+	struct sched_id *sid;
 
 	sched_thread_destroy(con);
 	con->sched_thread = NULL;
@@ -246,40 +275,75 @@ void ast_sched_context_destroy(struct ast_sched_context *con)
 		con->sched_heap = NULL;
 	}
 
+	while ((sid = AST_LIST_REMOVE_HEAD(&con->id_queue, list))) {
+		ast_free(sid);
+	}
+
 	ast_mutex_unlock(&con->lock);
 	ast_mutex_destroy(&con->lock);
 
 	ast_free(con);
 }
 
-static struct sched *sched_alloc(struct ast_sched_context *con)
-{
-	struct sched *tmp;
+#define ID_QUEUE_INCREMENT 16
 
-	/*
-	 * We keep a small cache of schedule entries
-	 * to minimize the number of necessary malloc()'s
+/*!
+ * \brief Add new scheduler IDs to the queue.
+ *
+ * \retval The number of IDs added to the queue
+ */
+static int add_ids(struct ast_sched_context *con)
+{
+	int new_size;
+	int original_size;
+	int i;
+
+	original_size = con->id_queue_size;
+	/* So we don't go overboard with the mallocs here, we'll just up
+	 * the size of the list by a fixed amount each time instead of
+	 * multiplying the size by any particular factor
 	 */
-#ifdef SCHED_MAX_CACHE
-	if ((tmp = AST_LIST_REMOVE_HEAD(&con->schedc, list))) {
-		con->schedccnt--;
-	} else 
-#endif
-	{
-		tmp = ast_calloc(1, sizeof(*tmp));
-		ast_cond_init(&tmp->cond, NULL);
+	new_size = original_size + ID_QUEUE_INCREMENT;
+	if (new_size < 0) {
+		/* Overflow. Cap it at INT_MAX. */
+		new_size = INT_MAX;
 	}
+	for (i = original_size; i < new_size; ++i) {
+		struct sched_id *new_id;
 
-	return tmp;
+		new_id = ast_calloc(1, sizeof(*new_id));
+		if (!new_id) {
+			break;
+		}
+		new_id->id = i;
+		AST_LIST_INSERT_TAIL(&con->id_queue, new_id, list);
+		++con->id_queue_size;
+	}
+
+	return con->id_queue_size - original_size;
+}
+
+static int set_sched_id(struct ast_sched_context *con, struct sched *new_sched)
+{
+	if (AST_LIST_EMPTY(&con->id_queue) && (add_ids(con) == 0)) {
+		return -1;
+	}
+
+	new_sched->sched_id = AST_LIST_REMOVE_HEAD(&con->id_queue, list);
+	return 0;
 }
 
 static void sched_release(struct ast_sched_context *con, struct sched *tmp)
 {
+	if (tmp->sched_id) {
+		AST_LIST_INSERT_TAIL(&con->id_queue, tmp->sched_id, list);
+		tmp->sched_id = NULL;
+	}
+
 	/*
 	 * Add to the cache, or just free() if we
 	 * already have too many cache entries
 	 */
-
 #ifdef SCHED_MAX_CACHE
 	if (con->schedccnt < SCHED_MAX_CACHE) {
 		AST_LIST_INSERT_HEAD(&con->schedc, tmp, list);
@@ -289,6 +353,35 @@ static void sched_release(struct ast_sched_context *con, struct sched *tmp)
 		sched_free(tmp);
 }
 
+static struct sched *sched_alloc(struct ast_sched_context *con)
+{
+	struct sched *tmp;
+
+	/*
+	 * We keep a small cache of schedule entries
+	 * to minimize the number of necessary malloc()'s
+	 */
+#ifdef SCHED_MAX_CACHE
+	if ((tmp = AST_LIST_REMOVE_HEAD(&con->schedc, list))) {
+		con->schedccnt--;
+	} else
+#endif
+	{
+		tmp = ast_calloc(1, sizeof(*tmp));
+		if (!tmp) {
+			return NULL;
+		}
+		ast_cond_init(&tmp->cond, NULL);
+	}
+
+	if (set_sched_id(con, tmp)) {
+		sched_release(con, tmp);
+		return NULL;
+	}
+
+	return tmp;
+}
+
 void ast_sched_clean_by_callback(struct ast_sched_context *con, ast_sched_cb match, ast_sched_cb cleanup_cb)
 {
 	int i = 1;
@@ -388,7 +481,7 @@ int ast_sched_add_variable(struct ast_sched_context *con, int when, ast_sched_cb
 
 	ast_mutex_lock(&con->lock);
 	if ((tmp = sched_alloc(con))) {
-		tmp->id = con->eventcnt++;
+		con->eventcnt++;
 		tmp->callback = callback;
 		tmp->data = data;
 		tmp->resched = when;
@@ -399,7 +492,7 @@ int ast_sched_add_variable(struct ast_sched_context *con, int when, ast_sched_cb
 			sched_release(con, tmp);
 		} else {
 			schedule(con, tmp);
-			res = tmp->id;
+			res = tmp->sched_id->id;
 		}
 	}
 #ifdef DUMP_SCHEDULER
@@ -437,7 +530,7 @@ static struct sched *sched_find(struct ast_sched_context *con, int id)
 	for (x = 1; x <= heap_size; x++) {
 		struct sched *cur = ast_heap_peek(con->sched_heap, x);
 
-		if (cur->id == id) {
+		if (cur->sched_id->id == id) {
 			return cur;
 		}
 	}
@@ -488,16 +581,16 @@ int _ast_sched_del(struct ast_sched_context *con, int id, const char *file, int
 	s = sched_find(con, id);
 	if (s) {
 		if (!ast_heap_remove(con->sched_heap, s)) {
-			ast_log(LOG_WARNING,"sched entry %d not in the sched heap?\n", s->id);
+			ast_log(LOG_WARNING,"sched entry %d not in the sched heap?\n", s->sched_id->id);
 		}
 		sched_release(con, s);
-	} else if (con->currently_executing && (id == con->currently_executing->id)) {
+	} else if (con->currently_executing && (id == con->currently_executing->sched_id->id)) {
 		s = con->currently_executing;
 		s->deleted = 1;
 		/* Wait for executing task to complete so that caller of ast_sched_del() does not
 		 * free memory out from under the task.
 		 */
-		while (con->currently_executing && (id == con->currently_executing->id)) {
+		while (con->currently_executing && (id == con->currently_executing->sched_id->id)) {
 			ast_cond_wait(&s->cond, &con->lock);
 		}
 		/* Do not sched_release() here because ast_sched_runq() will do it */
@@ -586,7 +679,7 @@ void ast_sched_dump(struct ast_sched_context *con)
 		q = ast_heap_peek(con->sched_heap, x);
 		delta = ast_tvsub(q->when, when);
 		ast_debug(1, "|%.4d | %-15p | %-15p | %.6ld : %.6ld |\n",
-			q->id,
+			q->sched_id->id,
 			q->callback,
 			q->data,
 			(long)delta.tv_sec,