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

base/run_loop.h

Index: base/run_loop.h
diff --git a/base/run_loop.h b/base/run_loop.h
index 077d097ba9a2a35c6ec51d017f588b9f195e48c0..398ddc46406aa5f9e247abe9fd97b19e91554b3d 100644
--- a/base/run_loop.h
+++ b/base/run_loop.h
@@ -43,7 +43,10 @@ class BASE_EXPORT RunLoop {
   // 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.
@@ -72,6 +75,34 @@ class BASE_EXPORT RunLoop {
   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)
@@ -92,20 +123,15 @@ class BASE_EXPORT RunLoop {
 
   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.