Cycles: Fix rare dead-locks on TaskScheduler::exit()

When the Moon is full it was possible to have a dead-lock in task
scheduler's  exit() method.

Similar problem was fixed in Blender's task scheduler 3 years ago
in bae2a2c.

intern/cycles/test/CMakeLists.txt

@@ -28,3 +28,4 @@ set(CMAKE_EXE_LINKER_FLAGS_DEBUG "${CMAKE_EXE_LINKER_FLAGS_DEBUG} ${PLATFORM_LIN
 CYCLES_TEST(util_aligned_malloc "cycles_util")
 CYCLES_TEST(util_path "cycles_util;${BOOST_LIBRARIES};${OPENIMAGEIO_LIBRARIES}")
 CYCLES_TEST(util_string "cycles_util;${BOOST_LIBRARIES}")
+CYCLES_TEST(util_task "cycles_util;${BOOST_LIBRARIES}")

intern/cycles/test/util_task_test.cpp

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2011-2016 Blender Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "testing/testing.h"
+
+#include "util/util_task.h"
+
+CCL_NAMESPACE_BEGIN
+
+namespace {
+
+void task_run() {
+}
+
+}  // namespace
+
+TEST(util_task, basic) {
+	TaskScheduler::init(0);
+	TaskPool pool;
+	for (int i = 0; i < 100; ++i) {
+		pool.push(function_bind(task_run));
+	}
+	TaskPool::Summary summary;
+	pool.wait_work(&summary);
+	TaskScheduler::exit();
+	EXPECT_EQ(summary.num_tasks_handled, 100);
+}
+
+TEST(util_task, multiple_times) {
+	for (int N = 0; N < 1000; ++N) {
+		TaskScheduler::init(0);
+		TaskPool pool;
+		for (int i = 0; i < 100; ++i) {
+			pool.push(function_bind(task_run));
+		}
+		TaskPool::Summary summary;
+		pool.wait_work(&summary);
+		TaskScheduler::exit();
+		EXPECT_EQ(summary.num_tasks_handled, 100);
+	}
+}
+
+CCL_NAMESPACE_END

intern/cycles/util/util_task.cpp

@@ -217,8 +217,10 @@ void TaskScheduler::exit()
 
 	if(users == 0) {
 		/* stop all waiting threads */
+		TaskScheduler::queue_mutex.lock();
 		do_exit = true;
 		TaskScheduler::queue_cond.notify_all();
+		TaskScheduler::queue_mutex.unlock();
 
 		/* delete threads */
 		foreach(thread *t, threads) {