Refactor Thread lifecycle interface, add Thread::Enter/ExitIsolate.

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.
+// garbage collection or compilation. The Thread structure associated with
+// a thread is allocated when the thread enters an isolate, and destroyed
+// upon exiting an isolate or an explicit call to CleanUp.
 class Thread {
  public:
-  explicit Thread(Isolate* isolate)
-      : isolate_(isolate),
-        cha_(NULL) {}
-
-  static void InitOnce();
-
+  // The currently executing thread, or NULL if not yet initialized.
   static Thread* Current() {
     return reinterpret_cast<Thread*>(OSThread::GetThreadLocal(thread_key_));
   }
-  static void SetCurrent(Thread* current);
+
+  // Makes the current thread enter 'isolate'. Also calls EnsureInit.
+  static void EnterIsolate(Isolate* isolate);
+  // Makes the current thread exit its isolate. Also calls CleanUp.
+  static void ExitIsolate();
+
+  // Initializes the current thread as a VM thread, if not already done.
+  static void EnsureInit();
+  // Clears the state of the current thread.
+  static void CleanUp();
+
+  // 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.
   Isolate* isolate() const { return isolate_; }
 
   // The (topmost) CHA for the compilation in this thread.
   CHA* cha() const { return cha_; }
   void set_cha(CHA* value) { cha_ = value; }
 
  private:
   static ThreadLocalKey thread_key_;
 
   Isolate* isolate_;
   CHA* cha_;
 
+  Thread()
+      : isolate_(NULL),
+        cha_(NULL) {}
+
+  static void SetCurrent(Thread* current);
+
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_H_