Add yieldLock(), which enables enforcing no task switches within a scope.

runTask() was altered to delay the task switch in case it gets called within an active yieldLock(), so that running tasks is still possible.

source/vibe/core/core.d

@@ -27,7 +27,7 @@ import std.functional;
 import std.range : empty, front, popFront;
 import std.string;
 import std.traits : isFunctionPointer;
-import std.typecons : Typedef, Tuple, tuple;
+import std.typecons : Flag, Yes, Typedef, Tuple, tuple;
 import std.variant;
 import core.atomic;
 import core.sync.condition;
@@ -373,12 +373,11 @@ package Task runTask_internal(alias TFI_SETUP)()
 	f.bumpTaskCounter();
 	auto handle = f.task();
 
-	debug Task self = Task.getThis();
 	debug if (TaskFiber.ms_taskEventCallback) {
 		() @trusted { TaskFiber.ms_taskEventCallback(TaskEvent.preStart, handle); } ();
 	}
 
-	s_scheduler.switchTo(handle);
+	s_scheduler.switchTo(handle, TaskFiber.getThis().m_yieldLockCount > 0 ? Flag!"defer".yes : Flag!"defer".no);
 
 	debug if (TaskFiber.ms_taskEventCallback) {
 		() @trusted { TaskFiber.ms_taskEventCallback(TaskEvent.postStart, handle); } ();
@@ -648,6 +647,7 @@ void yield()
 		tf.handleInterrupt();
 	} else {
 		// Let yielded tasks execute
+		assert(TaskFiber.getThis().m_yieldLockCount == 0, "May not yield within an active yieldLock()!");
 		() @safe nothrow { performIdleProcessing(); } ();
 	}
 }
@@ -669,6 +669,7 @@ void hibernate(scope void delegate() @safe nothrow on_interrupt = null)
 @safe nothrow {
 	auto t = Task.getThis();
 	if (t == Task.init) {
+		assert(TaskFiber.getThis().m_yieldLockCount == 0, "May not yield within an active yieldLock!");
 		runEventLoopOnce();
 	} else {
 		auto tf = () @trusted { return t.taskFiber; } ();
@@ -1062,6 +1063,52 @@ struct Timer {
 }
 
 
+/** Returns an object that ensures that no task switches happen during its life time.
+
+	Any attempt to run the event loop or switching to another task will cause
+	an assertion to be thrown within the scope that defines the lifetime of the
+	returned object.
+
+	Multiple yield locks can appear in nested scopes.
+*/
+auto yieldLock()
+{
+	static struct YieldLock {
+		private this(bool) { inc(); }
+		@disable this();
+		@disable this(this);
+		~this() { dec(); }
+
+		private void inc()
+		{
+			TaskFiber.getThis().m_yieldLockCount++;
+		}
+
+		private void dec()
+		{
+			TaskFiber.getThis().m_yieldLockCount--;
+		}
+	}
+
+	return YieldLock(true);
+}
+
+unittest {
+	auto tf = TaskFiber.getThis();
+	assert(tf.m_yieldLockCount == 0);
+	{
+		auto lock = yieldLock();
+		assert(tf.m_yieldLockCount == 1);
+		{
+			auto lock2 = yieldLock();
+			assert(tf.m_yieldLockCount == 2);
+		}
+		assert(tf.m_yieldLockCount == 1);
+	}
+	assert(tf.m_yieldLockCount == 0);
+}
+
+
 /**************************************************************************************************/
 /* private types                                                                                  */
 /**************************************************************************************************/

source/vibe/core/task.d

@@ -302,6 +302,7 @@ final package class TaskFiber : Fiber {
 		void[] m_fls;
 
 		bool m_interrupt; // Task.interrupt() is progress
+		package int m_yieldLockCount;
 
 		static TaskFiber ms_globalDummyFiber;
 		static FLSInfo[] ms_flsInfo;
@@ -325,7 +326,7 @@ final package class TaskFiber : Fiber {
 		auto f = () @trusted nothrow {
 			return Fiber.getThis();
 		} ();
-		if (f) return cast(TaskFiber)f;
+		if (auto tf = cast(TaskFiber)f) return tf;
 		if (!ms_globalDummyFiber) ms_globalDummyFiber = new TaskFiber;
 		return ms_globalDummyFiber;
 	}
@@ -637,6 +638,8 @@ package struct TaskScheduler {
 	*/
 	ExitReason process()
 	{
+		assert(TaskFiber.getThis().m_yieldLockCount == 0, "May not process events within an active yieldLock()!");
+
 		bool any_events = false;
 		while (true) {
 			// process pending tasks
@@ -742,7 +745,7 @@ package struct TaskScheduler {
 		This forces immediate execution of the specified task. After the tasks finishes or yields,
 		the calling task will continue execution.
 	*/
-	void switchTo(Task t)
+	void switchTo(Task t, Flag!"defer" defer = No.defer)
 	{
 		auto thist = Task.getThis();
 
@@ -750,14 +753,15 @@ package struct TaskScheduler {
 
 		auto thisthr = thist ? thist.thread : () @trusted { return Thread.getThis(); } ();
 		assert(t.thread is thisthr, "Cannot switch to a task that lives in a different thread.");
-		if (thist == Task.init) {
+		if (thist == Task.init && defer == No.defer) {
+			assert(TaskFiber.getThis().m_yieldLockCount == 0, "Cannot yield within an active yieldLock()!");
 			logTrace("switch to task from global context");
 			resumeTask(t);
 			logTrace("task yielded control back to global context");
 		} else {
 			auto tf = () @trusted { return t.taskFiber; } ();
 			auto thistf = () @trusted { return thist.taskFiber; } ();
-			assert(!thistf.m_queue, "Calling task is running, but scheduled to be resumed!?");
+			assert(!thistf || !thistf.m_queue, "Calling task is running, but scheduled to be resumed!?");
 			if (tf.m_queue) {
 				logTrace("Task to switch to is already scheduled. Moving to front of queue.");
 				assert(tf.m_queue is &m_taskQueue, "Task is already enqueued, but not in the main task queue.");
@@ -766,9 +770,13 @@ package struct TaskScheduler {
 			}
 
 			logDebugV("Switching tasks (%s already in queue)", m_taskQueue.length);
-			m_taskQueue.insertFront(thistf);
-			m_taskQueue.insertFront(tf);
-			doYield(thist);
+			if (defer) {
+				m_taskQueue.insertFront(tf);
+			} else {
+				m_taskQueue.insertFront(thistf);
+				m_taskQueue.insertFront(tf);
+				doYield(thist);
+			}
 		}
 	}
 
@@ -840,6 +848,7 @@ package struct TaskScheduler {
 
 	private void doYield(Task task)
 	{
+		assert(() @trusted { return task.taskFiber; } ().m_yieldLockCount == 0, "May not yield while in an active yieldLock()!");
 		debug if (TaskFiber.ms_taskEventCallback) () @trusted { TaskFiber.ms_taskEventCallback(TaskEvent.yield, task); } ();
 		() @trusted { Fiber.yield(); } ();
 		debug if (TaskFiber.ms_taskEventCallback) () @trusted { TaskFiber.ms_taskEventCallback(TaskEvent.resume, task); } ();