event: attempt to support workqueue monitoring on Windows

This adds a poor approximation for the thread counting support.  This
hopefully enables some level of overcommit support for Dispatch root
queues.  Enabling proper support for overcommit should resolve the
recent hangs with package resolution.

Author: compnerd

src/event/workqueue.c

@@ -180,6 +180,34 @@ _dispatch_workq_count_runnable_workers(dispatch_workq_monitor_t mon)
 
 	_dispatch_unfair_lock_unlock(&mon->registered_tid_lock);
 }
+#elif defined(_WIN32)
+static void
+_dispatch_workq_count_runnable_workers(dispatch_workq_monitor_t mon)
+{
+	int running_count = 0;
+
+	_dispatch_unfair_lock_lock(&mon->registered_tid_lock);
+
+	for (int i = 0; i < mon->num_registered_tids; ++i) {
+		dispatch_tid tid = mon->registered_tids[i];
+		HANDLE hThread = OpenThread(THREAD_QUERY_INFORMATION, FALSE, tid);
+		if (hThread == NULL) {
+			_dispatch_debug("workq: unable to open thread %u: %u", tid, GetLastError());
+			continue;
+		}
+
+		BOOL IOPending = TRUE;
+		if (GetThreadIOPendingFlag(hThread, &IOPending))
+			if (!IOPending)
+				++running_count;
+
+		CloseHandle(hThread);
+	}
+
+	mon->num_runnable = running_count;
+
+	_dispatch_unfair_lock_unlock(&mon->registered_tid_lock);
+}
 #else
 #error must define _dispatch_workq_count_runnable_workers
 #endif

src/event/workqueue_internal.h

@@ -30,7 +30,7 @@
 void _dispatch_workq_worker_register(dispatch_queue_global_t root_q);
 void _dispatch_workq_worker_unregister(dispatch_queue_global_t root_q);
 
-#if defined(__linux__)
+#if defined(__linux__) || defined(_WIN32)
 #define HAVE_DISPATCH_WORKQ_MONITORING 1
 #else
 #define HAVE_DISPATCH_WORKQ_MONITORING 0