workers: add worker_pool_wait_cb() and extend worker_pool_status()

src/knot/worker/pool.c

-/*  Copyright (C) 2019 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+/*  Copyright (C) 2021 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -191,7 +191,7 @@ void worker_pool_join(worker_pool_t *pool)
 	dt_join(pool->threads);
 }
 
-void worker_pool_wait(worker_pool_t *pool)
+void worker_pool_wait_cb(worker_pool_t *pool, wait_callback_t cb)
 {
 	if (!pool) {
 		return;
@@ -199,11 +199,19 @@ void worker_pool_wait(worker_pool_t *pool)
 
 	pthread_mutex_lock(&pool->lock);
 	while (!EMPTY_LIST(pool->tasks.list) || pool->running > 0) {
+		if (cb != NULL) {
+			cb(pool);
+		}
 		pthread_cond_wait(&pool->wake, &pool->lock);
 	}
 	pthread_mutex_unlock(&pool->lock);
 }
 
+void worker_pool_wait(worker_pool_t *pool)
+{
+	worker_pool_wait_cb(pool, NULL);
+}
+
 void worker_pool_assign(worker_pool_t *pool, struct task *task)
 {
 	if (!pool || !task) {
@@ -228,15 +236,19 @@ void worker_pool_clear(worker_pool_t *pool)
 	pthread_mutex_unlock(&pool->lock);
 }
 
-void worker_pool_status(worker_pool_t *pool, int *running, int *queued)
+void worker_pool_status(worker_pool_t *pool, bool locked, int *running, int *queued)
 {
 	if (!pool) {
 		*running = *queued = 0;
 		return;
 	}
 
-	pthread_mutex_lock(&pool->lock);
+	if (!locked) {
+		pthread_mutex_lock(&pool->lock);
+	}
 	*running = pool->running;
 	*queued = worker_queue_length(&pool->tasks);
-	pthread_mutex_unlock(&pool->lock);
+	if (!locked) {
+		pthread_mutex_unlock(&pool->lock);
+	}
 }

src/knot/worker/pool.h

-/*  Copyright (C) 2019 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+/*  Copyright (C) 2021 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -16,11 +16,15 @@
 
 #pragma once
 
+#include <stdbool.h>
+
 #include "knot/worker/queue.h"
 
 struct worker_pool;
 typedef struct worker_pool worker_pool_t;
 
+typedef void(*wait_callback_t)(worker_pool_t *);
+
 /*!
  * \brief Initialize worker pool.
  *
@@ -62,10 +66,15 @@ void worker_pool_join(worker_pool_t *pool);
 
 /*!
  * \brief Wait till the number of pending tasks is zero.
- *
  */
 void worker_pool_wait(worker_pool_t *pool);
 
+/*!
+ * \brief Wait till the number of pending tasks is zero. Callback emitted on
+ *  thread wakeup can be specified.
+ */
+void worker_pool_wait_cb(worker_pool_t *pool, wait_callback_t cb);
+
 /*!
  * \brief Assign a task to be performed by a worker in the pool.
  */
@@ -78,5 +87,7 @@ void worker_pool_clear(worker_pool_t *pool);
 
 /*!
  * \brief Obtain info regarding how the pool is busy.
+ *
+ * \note Locked means if the mutex `pool->lock` is locked.
  */
-void worker_pool_status(worker_pool_t *pool, int *running, int *queued);
+void worker_pool_status(worker_pool_t *pool, bool locked, int *running, int *queued);