Fix race-condition in TaskMutex debug code.

m_owner could change between the two conditions checked in the assertion in lock(), causing a false assertion failure. Using atomic operations to avoid that.

source/vibe/core/sync.d

@@ -1934,7 +1934,7 @@ private struct TaskMutexImpl(bool INTERRUPTIBLE) {
 		shared(bool) m_locked = false;
 		shared(uint) m_waiters = 0;
 		shared(ManualEvent) m_signal;
-		debug Task m_owner;
+		debug shared Task m_owner;
 	}
 
 	shared:
@@ -1947,7 +1947,10 @@ private struct TaskMutexImpl(bool INTERRUPTIBLE) {
 	@trusted bool tryLock()
 	nothrow {
 		if (cas(&m_locked, false, true)) {
-			debug m_owner = Task.getThis();
+			debug {
+				auto swapped = cas(&m_owner, Task.init, Task.getThis());
+				assert(swapped);
+			}
 			debug(VibeMutexLog) logTrace("mutex %s lock %s", cast(void*)&this, atomicLoad(m_waiters));
 			return true;
 		}
@@ -1957,7 +1960,11 @@ private struct TaskMutexImpl(bool INTERRUPTIBLE) {
 	@trusted bool lock(Duration timeout = Duration.max)
 	{
 		if (tryLock()) return true;
-		debug assert(m_owner == Task() || m_owner != Task.getThis(), "Recursive mutex lock.");
+		debug {
+			auto thist = Task.getThis();
+			auto owner = atomicLoad(m_owner);
+			assert(owner == Task.init || owner != thist, "Recursive mutex lock.");
+		}
 		atomicOp!"+="(m_waiters, 1);
 		debug(VibeMutexLog) logTrace("mutex %s wait %s", cast(void*)&this, atomicLoad(m_waiters));
 		scope(exit) atomicOp!"-="(m_waiters, 1);
@@ -1979,8 +1986,9 @@ private struct TaskMutexImpl(bool INTERRUPTIBLE) {
 	{
 		assert(m_locked);
 		debug {
-			assert(m_owner == Task.getThis());
-			m_owner = Task();
+			auto thist = Task.getThis();
+			auto swapped = cas(&m_owner, thist, Task());
+			assert(swapped);
 		}
 		atomicStore!(MemoryOrder.rel)(m_locked, false);
 		debug(VibeMutexLog) logTrace("mutex %s unlock %s", cast(void*)&this, atomicLoad(m_waiters));