MPRIS: Prevent race condition with emitting signals before MPRIS starts

This CL adds a check to delay emitting signals until the service has
started. This prevents a race condition where an MprisService user
(e.g. MediaKeysListenerManager) would cause signals to be emitted
before the service was ready.

Bug: 952113
Change-Id: I2fecfbb5507e9ab25700263e1972bc80a606a3a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566625Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650549}

ui/base/mpris/mpris_service_impl.cc

@@ -227,8 +227,12 @@ void MprisServiceImpl::InitializeDbusInterface() {
 void MprisServiceImpl::OnExported(const std::string& interface_name,
                                   const std::string& method_name,
                                   bool success) {
-  if (success)
-    num_methods_exported_++;
+  if (!success) {
+    service_failed_to_start_ = true;
+    return;
+  }
+
+  num_methods_exported_++;
 
   // Still waiting for more methods to finish exporting.
   if (num_methods_exported_ < kNumMethodsToExport)
@@ -242,8 +246,10 @@ void MprisServiceImpl::OnExported(const std::string& interface_name,
 
 void MprisServiceImpl::OnOwnership(const std::string& service_name,
                                    bool success) {
-  if (!success)
+  if (!success) {
+    service_failed_to_start_ = true;
     return;
+  }
 
   service_ready_ = true;
 
@@ -441,6 +447,12 @@ void MprisServiceImpl::EmitPropertiesChangedSignalDebounced() {
 }
 
 void MprisServiceImpl::EmitPropertiesChangedSignal() {
+  // If we're still trying to start the service, delay emitting.
+  if (!service_ready_ && !service_failed_to_start_) {
+    EmitPropertiesChangedSignalDebounced();
+    return;
+  }
+
   if (!bus_ || !exported_object_ || !service_ready_)
     return;
 

ui/base/mpris/mpris_service_impl.h

@@ -132,6 +132,9 @@ class COMPONENT_EXPORT(MPRIS) MprisServiceImpl : public MprisService {
   // True if we have finished creating the DBus service and received ownership.
   bool service_ready_ = false;
 
+  // True if we failed to start the MPRIS DBus service.
+  bool service_failed_to_start_ = false;
+
   // Used to only send 1 PropertiesChanged signal when many properties are
   // changed at once.
   base::OneShotTimer properties_changed_debounce_timer_;