mojo: Reset ReceiverSet's dispatch context after dispatching a message.

This should make it easier to catch code trying to access the dispatch
context outside of the context of dispatching a message.

I confirmed by manual testing that this does in fact make the (manual)
font access API web platform tests hit a DCHECK for its incorrect usage
of ReceiverSet::current_context.

Bug: 1228510, 1229481
Change-Id: Ibeb498a297de2f709bbffa561ea9458958fedfe9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3025524
Reviewed-by: Thanh Nguyen <thanhdng@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Thanh Nguyen <thanhdng@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Auto-Submit: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#901880}

chrome/browser/ash/power/auto_screen_brightness/light_samples_observer_unittest.cc

@@ -60,6 +60,8 @@ class LightSamplesObserverTest : public testing::Test {
   }
 
   void DisableIlluminanceChannel() {
+    // TODO(https://crbug.com/1229481): SetChannelsEnabled assumes it is only
+    // called via a mojo pipe, as such this call is not valid and will DCHECK.
     sensor_device_->SetChannelsEnabled(
         {0}, false,
         base::BindOnce(&LightSamplesObserverTest::SetChannelsEnabledCallback,
@@ -121,7 +123,9 @@ TEST_F(LightSamplesObserverTest, StartReadingTwiceError) {
   EXPECT_EQ(fake_observer_.num_received_ambient_lights(), 0);
 }
 
-TEST_F(LightSamplesObserverTest, GetSamplesWithoutColorChannels) {
+// TODO(https://crbug.com/1229481): This test hits a DCHECK in the
+// DisableIlluminanceChannel call.
+TEST_F(LightSamplesObserverTest, DISABLED_GetSamplesWithoutColorChannels) {
   SetChannels(false);
 
   mojo::Remote<chromeos::sensors::mojom::SensorDevice> light;

mojo/public/cpp/bindings/receiver_set.cc

@@ -29,7 +29,9 @@ class ReceiverSetState::Entry::DispatchFilter : public MessageFilter {
     return true;
   }
 
-  void DidDispatchOrReject(Message* message, bool accepted) override {}
+  void DidDispatchOrReject(Message* message, bool accepted) override {
+    entry_.DidDispatchOrReject();
+  }
 
   Entry& entry_;
 };
@@ -50,6 +52,10 @@ void ReceiverSetState::Entry::WillDispatch() {
   state_.SetDispatchContext(receiver_->GetContext(), id_);
 }
 
+void ReceiverSetState::Entry::DidDispatchOrReject() {
+  state_.SetDispatchContext(nullptr, 0);
+}
+
 void ReceiverSetState::Entry::OnDisconnect(uint32_t custom_reason_code,
                                            const std::string& description) {
   WillDispatch();

mojo/public/cpp/bindings/receiver_set.h

@@ -80,6 +80,7 @@ class COMPONENT_EXPORT(MOJO_CPP_BINDINGS) ReceiverSetState {
     class DispatchFilter;
 
     void WillDispatch();
+    void DidDispatchOrReject();
     void OnDisconnect(uint32_t custom_reason_code,
                       const std::string& description);