mojo: Allow basic RunLoop to be quit from a posted task.

mojo/public/cpp/utility/lib/run_loop.cc

Index: mojo/public/cpp/utility/lib/run_loop.cc
diff --git a/mojo/public/cpp/utility/lib/run_loop.cc b/mojo/public/cpp/utility/lib/run_loop.cc
index b06880dc9d6e0a1dbd02aaeb1cb105a9a39bad54..d545ec5fba7da51603113f7ae28f0fb0c6fbdc86 100644
--- a/mojo/public/cpp/utility/lib/run_loop.cc
+++ b/mojo/public/cpp/utility/lib/run_loop.cc
@@ -99,7 +99,8 @@ void RunLoop::Run() {
   run_state_ = &run_state;
   while (!run_state.should_quit) {
     DoDelayedWork();
-    Wait(false);
+    if (!run_state.should_quit)
+      Wait(false);

[jamesr, 2014/09/22 22:30:18]

hmm, i think this would be cleaner if it was structured more like the base/
version, but this seems ok

   }
   run_state_ = old_state;
 }
@@ -111,7 +112,7 @@ void RunLoop::RunUntilIdle() {
   run_state_ = &run_state;
   while (!run_state.should_quit) {
     DoDelayedWork();
-    if (!Wait(true) && delayed_tasks_.empty())
+    if (!run_state.should_quit && !Wait(true) && delayed_tasks_.empty())

[jamesr, 2014/09/22 22:30:18]

this is getting too hard to follow IMO, especially since the thing that actually
does work here is buried in the middle of the if. why would we want to keep
looping here if we should quit? 

how about:

bool keep_running = true;

do {
  // Try to do one unit of work without blocking and if we did, keep running.
  keep_running = Wait(true);
  if (run_state.should_quit)
    break;
  // Try to do a unit of delayed work.
  keep_running |= DoDelayedWork();
} while (keep_running && !run_state.should_quit);

and have ::DoDelayedWork() return true if it actually ran a task?  then this
loop would continue until nothing happened in Wait(true) and DoDelayedWork()

[qsr, 2014/09/23 10:40:15]

 That would be incorrect though. This must continue until we do not do anything
on Wait and the queue is empty. By the way, I just realized that the current
implementation is doing a busy loop. Let me try to refactor all of this.

       break;
   }
   run_state_ = old_state;