During thread-pool shutdown, wait for worker threads to shutdown.

runtime/vm/thread_pool.h

Index: runtime/vm/thread_pool.h
===================================================================
--- runtime/vm/thread_pool.h	(revision 45833)
+++ runtime/vm/thread_pool.h	(working copy)
@@ -43,8 +43,6 @@
   uint64_t workers_stopped() const { return count_stopped_; }
 
  private:
-  friend class ThreadPoolTestPeer;
-
   class Worker {
    public:
     explicit Worker(ThreadPool* pool);
@@ -56,12 +54,16 @@
     // after a task has been set by the initial call to SetTask().
     void StartThread();
 
-    // Main loop for a worker.
-    void Loop();
+    // Main loop for a worker. Returns true if worker is removed from thread
+    // lists, false otherwise.
+    bool Loop();
 
     // Causes worker to terminate eventually.
-    void Shutdown();
+    bool Shutdown();
 
+    // Get the Worker's thread id.
+    ThreadId id() { return id_; }
+
    private:
     friend class ThreadPool;
 
@@ -74,6 +76,8 @@
     Monitor monitor_;
     ThreadPool* pool_;
     Task* task_;
+    ThreadId id_;
+    bool started_;
 
     // Fields owned by ThreadPool.  Workers should not look at these
     // directly.  It's like looking at the sun.
@@ -80,6 +84,7 @@
     bool owned_;         // Protected by ThreadPool::mutex_
     Worker* all_next_;   // Protected by ThreadPool::mutex_
     Worker* idle_next_;  // Protected by ThreadPool::mutex_
+    Worker* shutdown_next_;  // Protected by ThreadPool::exit_monitor
 
     DISALLOW_COPY_AND_ASSIGN(Worker);
   };
@@ -92,6 +97,9 @@
   bool RemoveWorkerFromIdleList(Worker* worker);
   bool RemoveWorkerFromAllList(Worker* worker);
 
+  static void AddWorkerToShutdownList(Worker* worker);
+  static bool RemoveWorkerFromShutdownList(Worker* worker);
+
   // Worker operations.
   void SetIdle(Worker* worker);
   bool ReleaseIdleWorker(Worker* worker);
@@ -105,8 +113,8 @@
   uint64_t count_running_;
   uint64_t count_idle_;
 
-  static Monitor* exit_monitor_;  // Used only in testing.
-  static int* exit_count_;        // Used only in testing.
+  static Monitor exit_monitor_;
+  static Worker* shutting_down_workers_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadPool);
 };