sync: use Event for waitgroup on windows

vlib/builtin/cfns.c.v

@@ -379,20 +379,18 @@ fn C.GetLastError() u32
 
 fn C.CreateDirectory(byteptr, int) bool
 
-
+// win crypto
 fn C.BCryptGenRandom(int, voidptr, int, int) int
 
-
+// win synchronization
 fn C.CreateMutex(int, bool, byteptr) voidptr
-
-
 fn C.WaitForSingleObject(voidptr, int) int
-
-
 fn C.ReleaseMutex(voidptr) bool
+fn C.CreateEvent(int, bool, bool, byteptr) voidptr
+fn C.SetEvent(voidptr) int
+
 
 // pthread.h
-
 fn C.pthread_mutex_init(voidptr, voidptr) int
 fn C.pthread_mutex_lock(voidptr) int
 fn C.pthread_mutex_unlock(voidptr) int

vlib/sync/sync_windows.c.v

@@ -1,9 +1,11 @@
 // Copyright (c) 2019-2020 Alexander Medvednikov. All rights reserved.
 // Use of this source code is governed by an MIT license
 // that can be found in the LICENSE file.
-
 module sync
 
+// TODO: The suggestion of using CriticalSection instead of mutex
+// was discussed. Needs consideration.
+
 // Mutex HANDLE
 type MHANDLE voidptr
 

vlib/sync/waiter_nix.c.v

@@ -0,0 +1,22 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+module sync
+
+[ref_only]
+struct Waiter{
+mut:
+	mx &Mutex
+}
+
+pub fn (mut w Waiter) wait() {
+	w.mx.lock()
+}
+
+pub fn (mut w Waiter) stop() {
+	w.mx.unlock()
+}
+pub fn new_waiter() &Waiter {
+	w := &Waiter{mx: new_mutex()}
+	return w
+}

vlib/sync/waiter_windows.c.v

@@ -0,0 +1,35 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+module sync
+
+// We cannot simply use Mutex.lock() in the waitgroup.wait() method
+// as it will not block since it's used in the same thread.
+// docs:
+// Any thread with a handle to a mutex object can use one of the
+// wait functions to request ownership of the mutex object.
+// If the mutex object is owned by another thread, the wait
+// function blocks the requesting thread until the owning thread
+// releases the mutex object using the ReleaseMutex function.
+// Source:
+// https://docs.microsoft.com/en-us/windows/win32/sync/mutex-objects
+
+[ref_only]
+struct Waiter{
+mut:
+	event MHANDLE
+}
+
+pub fn (mut w Waiter) wait() {
+	C.WaitForSingleObject(w.event, C.INFINITE) // infinite wait
+}
+
+pub fn (mut w Waiter) stop() {
+	C.SetEvent(w.event)
+}
+pub fn new_waiter() &Waiter {
+	unsafe {
+		sm := &Waiter{event: MHANDLE(C.CreateEvent(0, false, true, 0))}
+		return sm
+	}
+}

vlib/sync/waitgroup.v

@@ -15,13 +15,13 @@ struct WaitGroup {
 mut:
 	task_count       int // current task count
 	task_count_mutex &Mutex = &Mutex(0) // This mutex protects the task_count count in add()
-	wait_blocker     &Mutex = &Mutex(0) // This mutex blocks the wait() until released by add()
+	wait_blocker     &Waiter = &Waiter(0) // This blocks the wait() until released by add()
 }
 
 pub fn new_waitgroup() &WaitGroup {
 	return &WaitGroup{
 		task_count_mutex: new_mutex()
-		wait_blocker: new_mutex()
+		wait_blocker: new_waiter()
 	}
 }
 
@@ -36,14 +36,14 @@ pub fn (mut wg WaitGroup) add(delta int) {
 	}
 	// If task_count likely to leave zero, set wait() to block
 	if wg.task_count == 0 {
-		wg.wait_blocker.lock()
+		wg.wait_blocker.wait()
 	}
 	wg.task_count += delta
 	if wg.task_count < 0 {
 		panic('Negative number of jobs in waitgroup')
 	}
 	if wg.task_count == 0 { // if no more task_count tasks
-		wg.wait_blocker.unlock() // unblock wait()
+		wg.wait_blocker.stop() // unblock wait()
 	}
 }
 
@@ -54,6 +54,6 @@ pub fn (mut wg WaitGroup) done() {
 
 // wait blocks until all tasks are done (task count becomes zero)
 pub fn (mut wg WaitGroup) wait() {
-	wg.wait_blocker.lock() // blocks until task_count becomes 0
+	wg.wait_blocker.wait() // blocks until task_count becomes 0
-	wg.wait_blocker.unlock() // allow other wait()s to unblock or reuse wait group
+	wg.wait_blocker.stop() // allow other wait()s to unblock or reuse wait group
 }