Send DBus signals synchronously if called from the DBusTaskRunner

The Chrome OS power manager (powerd) does not use a dedicated task
runner for handling DBus messages and defaults to using the same
MessageLoop as the one on which the dbus::Bus object was created.

An unfortunate effect of this is that if powerd calls
ExportedObject::SendSignal and then ObjectProxy::CallMethodAndBlock
without returning to the top level of the MessageLoop, the method call
will actually go out before the signal since SendSignal is
asynchronous.  To deal with this, make ExportedObject::SendSignal
synchronous when it is called from the same thread that hosts the
DBusTaskRunner.

BUG=472361

Review URL: https://codereview.chromium.org/1066193002

Cr-Commit-Position: refs/heads/master@{#324485}

2 files changed, 17 insertions, 7 deletions

diff --git a/dbus/exported_object.cc b/dbus/exported_object.cc
index 107d2e5..669b871 100644
--- a/dbus/exported_object.cc
+++ b/dbus/exported_object.cc
@@ -91,12 +91,21 @@ void ExportedObject::SendSignal(Signal* signal) {
   dbus_message_ref(signal_message);
 
   const base::TimeTicks start_time = base::TimeTicks::Now();
-  bus_->GetDBusTaskRunner()->PostTask(
-      FROM_HERE,
-      base::Bind(&ExportedObject::SendSignalInternal,
-                 this,
-                 start_time,
-                 signal_message));
+  if (bus_->GetDBusTaskRunner()->RunsTasksOnCurrentThread()) {
+    // The Chrome OS power manager doesn't use a dedicated TaskRunner for
+    // sending DBus messages.  Sending signals asynchronously can cause an
+    // inversion in the message order if the power manager calls
+    // ObjectProxy::CallMethodAndBlock() before going back to the top level of
+    // the MessageLoop: crbug.com/472361.
+    SendSignalInternal(start_time, signal_message);
+  } else {
+    bus_->GetDBusTaskRunner()->PostTask(
+        FROM_HERE,
+        base::Bind(&ExportedObject::SendSignalInternal,
+                   this,
+                   start_time,
+                   signal_message));
+  }
 }
 
 void ExportedObject::Unregister() {
diff --git a/dbus/exported_object.h b/dbus/exported_object.h
index 5f74dc2..89de096 100644
--- a/dbus/exported_object.h
+++ b/dbus/exported_object.h
@@ -92,7 +92,8 @@ class CHROME_DBUS_EXPORT ExportedObject
                             OnExportedCallback on_exported_callback);
 
   // Requests to send the signal from this object. The signal will be sent
-  // asynchronously from the message loop in the D-Bus thread.
+  // synchronously if this method is called from the message loop in the D-Bus
+  // thread and asynchronously otherwise.
   virtual void SendSignal(Signal* signal);
 
   // Unregisters the object from the bus. The Bus object will take care of