cc: Make sure a CompletionEvent is always signaled.

It looks like there is a CompletionEvent that is never getting signaled
from the compositor thread, and the main-thread stays blocked on it.
This causes a fair number of hangs in the renderers (~30% of all hangs
or crashes in cc/ code in stable channel on Windows). Try using a
ScopedCompletionEvent to make sure all CompletionEvents are always
signaled, and see if that helps.

BUG=622080

Change-Id: Ic25089f0e4f671182b18c1b44e278196eeb040c2
Reviewed-on: https://chromium-review.googlesource.com/c/1332726Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607868}

cc/trees/proxy_impl.cc

@@ -40,13 +40,22 @@ unsigned int nextBeginFrameId = 0;
 
 }  // namespace
 
+// Ensures that a CompletionEvent is always signaled.
+class ScopedCompletionEvent {
+ public:
+  explicit ScopedCompletionEvent(CompletionEvent* event) : event_(event) {}
+  ~ScopedCompletionEvent() { event_->Signal(); }
+
+ private:
+  CompletionEvent* const event_;
+  DISALLOW_COPY_AND_ASSIGN(ScopedCompletionEvent);
+};
+
 ProxyImpl::ProxyImpl(base::WeakPtr<ProxyMain> proxy_main_weak_ptr,
                      LayerTreeHost* layer_tree_host,
                      TaskRunnerProvider* task_runner_provider)
     : layer_tree_host_id_(layer_tree_host->GetId()),
       commit_completion_waits_for_activation_(false),
-      commit_completion_event_(nullptr),
-      activation_completion_event_(nullptr),
       next_frame_is_newly_committed_frame_(false),
       inside_draw_(false),
       input_throttled_until_commit_(false),
@@ -261,7 +270,8 @@ void ProxyImpl::NotifyReadyToCommitOnImpl(
 
   host_impl_->ReadyToCommit();
 
-  commit_completion_event_ = completion;
+  commit_completion_event_ =
+      std::make_unique<ScopedCompletionEvent>(completion);
   commit_completion_waits_for_activation_ = hold_commit_for_activation;
 
   DCHECK(!blocked_main_commit().layer_tree_host);
@@ -440,7 +450,6 @@ void ProxyImpl::DidActivateSyncTree() {
   if (activation_completion_event_) {
     TRACE_EVENT_INSTANT0("cc", "ReleaseCommitbyActivation",
                          TRACE_EVENT_SCOPE_THREAD);
-    activation_completion_event_->Signal();
     activation_completion_event_ = nullptr;
   }
 }
@@ -578,9 +587,7 @@ void ProxyImpl::ScheduledActionCommit() {
     // already activated if there was no work to be done.
     TRACE_EVENT_INSTANT0("cc", "HoldCommit", TRACE_EVENT_SCOPE_THREAD);
     commit_completion_waits_for_activation_ = false;
-    activation_completion_event_ = commit_completion_event_;
-  } else {
-    commit_completion_event_->Signal();
+    activation_completion_event_ = std::move(commit_completion_event_);
   }
   commit_completion_event_ = nullptr;
 

cc/trees/proxy_impl.h

@@ -20,6 +20,8 @@ class LayerTreeHost;
 class ProxyMain;
 class RenderFrameMetadataObserver;
 
+class ScopedCompletionEvent;
+
 // This class aggregates all the interactions that the main side of the
 // compositor needs to have with the impl side.
 // The class is created and lives on the impl thread.
@@ -143,10 +145,10 @@ class CC_EXPORT ProxyImpl : public LayerTreeHostImplClient,
   bool commit_completion_waits_for_activation_;
 
   // Set when the main thread is waiting on a commit to complete.
-  CompletionEvent* commit_completion_event_;
+  std::unique_ptr<ScopedCompletionEvent> commit_completion_event_;
 
   // Set when the main thread is waiting for activation to complete.
-  CompletionEvent* activation_completion_event_;
+  std::unique_ptr<ScopedCompletionEvent> activation_completion_event_;
 
   // Set when the next draw should post DidCommitAndDrawFrame to the main
   // thread.