Uses an open thread handle as the ThreadJoinId on Windows.

runtime/vm/thread_pool.cc

Index: runtime/vm/thread_pool.cc
diff --git a/runtime/vm/thread_pool.cc b/runtime/vm/thread_pool.cc
index fb0f7d7c69b2fac5d7d1af0843eaa2e08c87500f..cd39a59afe26177f14dedc7f923d1b49bc434ef0 100644
--- a/runtime/vm/thread_pool.cc
+++ b/runtime/vm/thread_pool.cc
@@ -212,6 +212,15 @@ bool ThreadPool::RemoveWorkerFromAllList(Worker* worker) {
 }
 
 
+void ThreadPool::SetIdleLocked(Worker* worker) {

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

ASSERT(mutex_.IsOwnedByCurrentThread());

[zra, 2016/05/16 20:32:55]

Done.

+  ASSERT(worker->owned_ && !IsIdle(worker));
+  worker->idle_next_ = idle_workers_;
+  idle_workers_ = worker;
+  count_idle_++;
+  count_running_--;
+}
+
+
 void ThreadPool::SetIdleAndReapExited(Worker* worker) {
   JoinList* list = NULL;
   {
@@ -219,17 +228,25 @@ void ThreadPool::SetIdleAndReapExited(Worker* worker) {
     if (shutting_down_) {
       return;
     }
-    ASSERT(worker->owned_ && !IsIdle(worker));
-    worker->idle_next_ = idle_workers_;
-    idle_workers_ = worker;
-    count_idle_++;
-    count_running_--;
-
-    // While we have the lock, opportunistically grab and clear the join_list_.
+    if (join_list_ == NULL) {
+      // Nothing to join, add to the idle list and return.
+      SetIdleLocked(worker);
+      return;
+    }
+    // There is something to join. Grab the join list, drop the lock, do the
+    // join, then grab the lock again and add to the idle list.
     list = join_list_;
     join_list_ = NULL;
   }
   JoinList::Join(&list);
+
+  {
+    MutexLocker ml(&mutex_);
+    if (shutting_down_) {
+      return;
+    }
+    SetIdleLocked(worker);
+  }
 }