Automatic thread cleanup on non-Windows platforms.

runtime/vm/thread.h

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_THREAD_H_
 #define VM_THREAD_H_
 
 #include "vm/base_isolate.h"
 #include "vm/globals.h"
 #include "vm/os_thread.h"
 
 namespace dart {
 
 class CHA;
 class Isolate;
 
 // A VM thread; may be executing Dart code or performing helper tasks like
 // garbage collection or compilation. The Thread structure associated with
-// a thread is allocated when the thread enters an isolate, and destroyed
-// upon an explicit call to CleanUp.
-//
-// NOTE: When the underlying OS thread is started by the embedder, i.e., not
-// part of a ThreadPool managed by the VM, we currently leak the Thread
-// structure after its last isolate exit. We could add an explicit cleanup API,
-// or, on platforms that support it, register a cleanup callback.
+// a thread is allocated by EnsureInit before entering an isolate, and destroyed
+// automatically when the underlying OS thread exits. NOTE: On Windows, CleanUp
+// must currently be called manually (issue 23474).
 class Thread {
  public:
   // The currently executing thread, or NULL if not yet initialized.
   static Thread* Current() {
     return reinterpret_cast<Thread*>(OSThread::GetThreadLocal(thread_key_));
   }
 
-  // Makes the current thread enter 'isolate'. Also calls EnsureInit.
+  // Initializes the current thread as a VM thread, if not already done.
+  static void EnsureInit();
+
+  // Makes the current thread enter 'isolate'.
   static void EnterIsolate(Isolate* isolate);
   // Makes the current thread exit its isolate.
   static void ExitIsolate();
 
   // A VM thread other than the main mutator thread can enter an isolate as a
   // "helper" to gain limited concurrent access to the isolate. One example is
   // SweeperTask (which uses the class table, which is copy-on-write).
   // TODO(koda): Properly synchronize heap access to expand allowed operations.
   static void EnterIsolateAsHelper(Isolate* isolate);
   static void ExitIsolateAsHelper();
 
-  // Initializes the current thread as a VM thread, if not already done.
-  static void EnsureInit();
-  // Clears the state of the current thread.
+#if defined(TARGET_OS_WINDOWS)
+  // Clears the state of the current thread and frees the allocation.
   static void CleanUp();
+#endif
 
   // Called at VM startup.
   static void InitOnce();
 
   // The topmost zone used for allocation in this thread.
   Zone* zone() {
     return reinterpret_cast<BaseIsolate*>(isolate())->current_zone();
   }
 
   // The isolate that this thread is operating on, or NULL if none.
@@ skipping 13 matching lines @@
       : isolate_(NULL) {}
 
   static void SetCurrent(Thread* current);
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_H_