[Windows] Fix main dispatch queue processing in CFRunLoop

Dispatch uses SetEvent to notify Foundation about pending work in main queue.
CFRunLoop handles event, but not resets it, leaving event in "pending" state forever.
This makes all consequent calls to __CFRunLoopRun exit immediately without
waiting for real events.

Author: lxbndr

CoreFoundation/RunLoop.subproj/CFRunLoop.c

@@ -2874,6 +2874,16 @@ static int32_t __CFRunLoopRun(CFRunLoopRef rl, CFRunLoopModeRef rlm, CFTimeInter
 #if TARGET_OS_WIN32 || TARGET_OS_LINUX
             void *msg = 0;
 #endif
+
+#if TARGET_OS_WIN32
+            // Dispatch sets this event to notify us about
+            // pending work on main queue.
+            // We should consume it properly, or CFRunLoop will
+            // immediately poke empty main queue on next run(s)
+            // and exit without any actual work done.
+            ResetEvent(dispatchPort);
+#endif
+
             cf_trace(KDEBUG_EVENT_CFRL_IS_CALLING_DISPATCH | DBG_FUNC_START, rl, rlm, msg, livePort);
             __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__(msg);
             cf_trace(KDEBUG_EVENT_CFRL_IS_CALLING_DISPATCH | DBG_FUNC_END, rl, rlm, msg, livePort);

Tests/Foundation/Tests/TestRunLoop.swift

@@ -125,6 +125,41 @@ class TestRunLoop : XCTestCase {
         XCTAssertTrue(didDeallocate)
     }
 
+    func test_mainDispatchQueueCallout() {
+        let runLoop = RunLoop.current
+
+        var asyncExecuted = false
+        DispatchQueue.main.async {
+            asyncExecuted = true
+        }
+
+        // RunLoop should service main queue
+        _ = runLoop.run(mode: .default, before: Date(timeIntervalSinceNow: 2))
+        XCTAssertTrue(asyncExecuted, "Main queue async code should be executed")
+
+        asyncExecuted = false
+        DispatchQueue.main.async {
+            asyncExecuted = true
+        }
+
+        // Second run to be sure RunLoop will not stuck
+        _ = runLoop.run(mode: .default, before: Date(timeIntervalSinceNow: 2))
+        XCTAssertTrue(asyncExecuted, "Main queue async code should be executed")
+
+        var timerFired = false
+        let dummyTimer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: false) { _ in
+            timerFired = true
+        }
+        runLoop.add(dummyTimer, forMode: .default)
+
+        // At this moment RunLoop has no work to do except waiting for timer.
+        // But RunLoop will exit prematurely if event from previous async calls
+        // got stuck in wrong state.
+        _ = runLoop.run(mode: .default, before: Date(timeIntervalSinceNow: 2))
+        
+        XCTAssertTrue(timerFired, "Time should fire already")
+    }
+
     static var allTests : [(String, (TestRunLoop) -> () throws -> Void)] {
         return [
             ("test_constants", test_constants),
@@ -134,6 +169,7 @@ class TestRunLoop : XCTestCase {
             ("test_runLoopLimitDate", test_runLoopLimitDate),
             ("test_runLoopPoll", test_runLoopPoll),
             ("test_addingRemovingPorts", test_addingRemovingPorts),
+            ("test_mainDispatchQueueCallout", test_mainDispatchQueueCallout)
         ]
     }
 }