Uses an open thread handle as the ThreadJoinId on Windows.

runtime/vm/os_thread_win.cc

Index: runtime/vm/os_thread_win.cc
diff --git a/runtime/vm/os_thread_win.cc b/runtime/vm/os_thread_win.cc
index e4083dca9c36c55e516fb48b911cc2c00e6224a1..d6bb2429b0306b4e6f9c3fe34265ad38e3dea2d1 100644
--- a/runtime/vm/os_thread_win.cc
+++ b/runtime/vm/os_thread_win.cc
@@ -91,7 +91,7 @@ int OSThread::Start(const char* name,
 
 
 const ThreadId OSThread::kInvalidThreadId = 0;
-const ThreadJoinId OSThread::kInvalidThreadJoinId = 0;
+const ThreadJoinId OSThread::kInvalidThreadJoinId = NULL;
 
 
 ThreadLocalKey OSThread::CreateThreadLocal(ThreadDestructor destructor) {
@@ -131,26 +131,15 @@ ThreadId OSThread::GetCurrentThreadTraceId() {
 
 
 ThreadJoinId OSThread::GetCurrentThreadJoinId() {
-  // TODO(zra): Use the thread handle as the join id in order to have a more
-  // reliable join on windows.
-  return ::GetCurrentThreadId();
+  HANDLE handle = OpenThread(SYNCHRONIZE, false, ::GetCurrentThreadId());
+  ASSERT(handle != NULL);
+  return handle;
 }
 
 
 void OSThread::Join(ThreadJoinId id) {
-  HANDLE handle = OpenThread(SYNCHRONIZE, false, id);
-
-  // TODO(zra): OSThread::Start() closes the handle to the thread. Thus, by the
-  // time we try to join the thread, its resources may have already been
-  // reclaimed, and joining will fail. This can be avoided in a couple of ways.
-  // First, GetCurrentThreadJoinId could call OpenThread and return a handle.
-  // This is bad, because each of those handles would have to be closed.
-  // Second OSThread could be refactored to no longer be AllStatic. Then the
-  // handle could be cached in the object by the Start method.
-  if (handle == NULL) {
-    return;
-  }
-
+  HANDLE handle = static_cast<HANDLE>(id);
+  ASSERT(handle != NULL);
   DWORD res = WaitForSingleObject(handle, INFINITE);
   CloseHandle(handle);
   ASSERT(res == WAIT_OBJECT_0);

[siva, 2016/05/16 17:59:27]

Are all thread guaranteed to call OSThread::Join() at some point? I only see
calls to Join from the thread pool threads, The main thread which is created by
the embedder and calls Dart_EnterIsolate will also end up with one of these
handles which never seems to be closed (leaks one handle per isolate).

[zra, 2016/05/16 20:32:54]

Changed to call GetCurrentThreadJoinId only for threads that will be joined.