Add |joinable| to Thread::Options

base/threading/thread.h

Index: base/threading/thread.h
diff --git a/base/threading/thread.h b/base/threading/thread.h
index 45064240d76f8436ceb7ab51ff205be4646a8485..221fe4e032e4229c1fe9adf58104e25db4883c5d 100644
--- a/base/threading/thread.h
+++ b/base/threading/thread.h
@@ -74,6 +74,12 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
 
     // Specifies the initial thread priority.
     ThreadPriority priority = ThreadPriority::NORMAL;
+
+    // If false, the thread will not be joined on destruction. This is intended
+    // for threads that want TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN
+    // semantics. Stop() will not be synchronous and will instead merely have
+    // StopSoon() semantics on such threads.
+    bool joinable = true;
   };
 
   // Constructor.
@@ -131,12 +137,14 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // carefully for production code.
   bool WaitUntilThreadStarted() const;
 
-  // Signals the thread to exit and returns once the thread has exited.  After
-  // this method returns, the Thread object is completely reset and may be used
-  // as if it were newly constructed (i.e., Start may be called again).
+  // Signals the thread to exit and returns once the thread has exited (or right
+  // away if the thread is non-joinable).  For joinable threads only: after this
+  // method returns, the Thread object is completely reset and may be used as if
+  // it were newly constructed (i.e., Start may be called again) -- non-joinable
+  // threads are not re-usable.
   //
   // Stop may be called multiple times and is simply ignored if the thread is
-  // already stopped.
+  // already stopped or currently stopping.
   //
   // NOTE: If you are a consumer of Thread, it is not necessary to call this
   // before deleting your Thread objects, as the destructor will do it.
@@ -228,10 +236,8 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   static void SetThreadWasQuitProperly(bool flag);
   static bool GetThreadWasQuitProperly();
 
-  void set_message_loop(MessageLoop* message_loop) {
-    DCHECK(owning_sequence_checker_.CalledOnValidSequencedThread());
-    message_loop_ = message_loop;
-  }
+  // Bind this Thread to an existing MessageLoop instead of starting a new one.
+  void SetMessageLoop(MessageLoop* message_loop);
 
  private:
 #if defined(OS_WIN)
@@ -275,6 +281,11 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   MessageLoop* message_loop_ = nullptr;
   RunLoop* run_loop_ = nullptr;
 
+  // True only if |message_loop_| was externally provided by |SetMessageLoop()|
+  // in which case this Thread has no underlying |thread_| and should merely
+  // drop |message_loop_| on Stop().
+  bool using_external_message_loop_ = false;
+
   // Stores Options::timer_slack_ until the message loop has been bound to
   // a thread.
   TimerSlack message_loop_timer_slack_ = TIMER_SLACK_NONE;