Make nesting/running states a RunLoop rather than a MessageLoop concept.

base/run_loop.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_RUN_LOOP_H_
 #define BASE_RUN_LOOP_H_
 
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/macros.h"
@@ skipping 25 matching lines @@
   // calling Run, be sure to grab the QuitClosure in order to stop the
   // MessageLoop asynchronously. MessageLoop::QuitWhenIdle and QuitNow will also
   // trigger a return from Run, but those are deprecated.
   void Run();
 
   // Run the current MessageLoop until it doesn't find any tasks or messages in
   // the queue (it goes idle). WARNING: This may never return! Only use this
   // when repeating tasks such as animated web pages have been shut down.
   void RunUntilIdle();
 
-  bool running() const { return running_; }
+  bool running() const {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    return running_;
+  }
 
   // Quit() quits an earlier call to Run() immediately. QuitWhenIdle() quits an
   // earlier call to Run() when there aren't any tasks or messages in the queue.
   //
   // There can be other nested RunLoops servicing the same task queue
   // (MessageLoop); Quitting one RunLoop has no bearing on the others. Quit()
   // and QuitWhenIdle() can be called before, during or after Run(). If called
   // before Run(), Run() will return immediately when called. Calling Quit() or
   // QuitWhenIdle() after the RunLoop has already finished running has no
   // effect.
   //
   // WARNING: You must NEVER assume that a call to Quit() or QuitWhenIdle() will
   // terminate the targetted message loop. If a nested message loop continues
   // running, the target may NEVER terminate. It is very easy to livelock (run
   // forever) in such a case.
   void Quit();
   void QuitWhenIdle();
 
   // Convenience methods to get a closure that safely calls Quit() or
   // QuitWhenIdle() (has no effect if the RunLoop instance is gone).
   //
   // Example:
   //   RunLoop run_loop;
   //   PostTask(run_loop.QuitClosure());
   //   run_loop.Run();
   base::Closure QuitClosure();
   base::Closure QuitWhenIdleClosure();
 
+  // Returns true if there is an active RunLoop on this thread.
+  static bool IsRunningOnCurrentThread();
+
+  // Returns true if there is an active RunLoop on this thread and it's nested
+  // within another active RunLoop.
+  static bool IsNestedOnCurrentThread();
+
+  // A NestingObserver is notified when a nested run loop begins. The observers
+  // are notified before the current thread's RunLoop::Delegate::Run() is
+  // invoked and nested work begins.
+  class BASE_EXPORT NestingObserver {
+   public:
+    virtual void OnBeginNestedMessageLoop() = 0;

[Sami, 2017/04/20 10:13:22]

nit: TODO to rename this to OnBeginNestedRunLoop (or just do it now?)

[gab, 2017/04/20 14:21:18]

Ah yes, missed that one, definitely doing it now, thanks!

Done.

[danakj, 2017/04/20 14:57:32]

+1

[gab, 2017/04/20 16:22:02]

Done.

+
+   protected:

[danakj, 2017/04/20 14:57:32]

why protected

[gab, 2017/04/20 16:22:02]

Copied as-is from MessageLoop::NestingObserver.

I assume this was done to make it clear that the owner of the full-type keeps
ownership (it's impossible for someone getting a plain NestingObserver* to
delete).

+    virtual ~NestingObserver() = default;
+  };
+
+  static void AddNestingObserverOnCurrentThread(NestingObserver* observer);
+  static void RemoveNestingObserverOnCurrentThread(NestingObserver* observer);
+
+  // Returns true if nesting is allowed on this thread.
+  static bool IsNestingAllowedOnCurrentThread();
+
+  // Disallow nesting. After this is called, running a nested RunLoop or calling
+  // Add/RemoveNestingObserverOnCurrentThread() on this thread will crash.
+  static void DisallowNestingOnCurrentThread();
+
  private:
   friend class MessageLoop;
 #if defined(OS_ANDROID)
   // Android doesn't support the blocking MessageLoop::Run, so it calls
   // BeforeRun and AfterRun directly.
   friend class base::MessagePumpForUI;
 #endif
 
 #if defined(OS_IOS)
   // iOS doesn't support the blocking MessageLoop::Run, so it calls
   // BeforeRun directly.
   friend class base::MessagePumpUIApplication;
 #endif
 
   // Return false to abort the Run.
   bool BeforeRun();
   void AfterRun();
 
   MessageLoop* loop_;
 
-  // Parent RunLoop or NULL if this is the top-most RunLoop.
-  RunLoop* previous_run_loop_;
-
-  // Used to count how many nested Run() invocations are on the stack.
-  int run_depth_;
-
-  bool run_called_;
-  bool quit_called_;
-  bool running_;
+  bool run_called_ = false;
+  bool quit_called_ = false;
+  bool running_ = false;
 
   // Used to record that QuitWhenIdle() was called on the MessageLoop, meaning
   // that we should quit Run once it becomes idle.
-  bool quit_when_idle_received_;
+  bool quit_when_idle_received_ = false;
 
+  // RunLoop's non-static methods are affine to the thread it's running on.
   base::ThreadChecker thread_checker_;
 
   // WeakPtrFactory for QuitClosure safety.
   base::WeakPtrFactory<RunLoop> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(RunLoop);
 };
 
 }  // namespace base
 
 #endif  // BASE_RUN_LOOP_H_