Mojo bindings: Fix data race in Connector

Fixes a data race on the boolean |error_| field. This is almost
certainly innocuous in practice but is technically UB, and TSan
complains.

Bug: 850091
Change-Id: Icc6bfbc0a2e2bda6a87c455856ff5f3153568a02
Reviewed-on: https://chromium-review.googlesource.com/1105340Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568283}

mojo/public/cpp/bindings/connector.h

@@ -5,6 +5,7 @@
 #ifndef MOJO_PUBLIC_CPP_BINDINGS_CONNECTOR_H_
 #define MOJO_PUBLIC_CPP_BINDINGS_CONNECTOR_H_
 
+#include <atomic>
 #include <memory>
 #include <utility>
 
@@ -234,7 +235,7 @@ class MOJO_CPP_BINDINGS_EXPORT Connector : public MessageReceiver {
   std::unique_ptr<SimpleWatcher> handle_watcher_;
   base::Optional<HandleSignalTracker> peer_remoteness_tracker_;
 
-  bool error_ = false;
+  std::atomic<bool> error_;
   bool drop_writes_ = false;
   bool enforce_errors_from_incoming_receiver_ = true;
 

mojo/public/cpp/bindings/lib/connector.cc

@@ -140,6 +140,7 @@ Connector::Connector(ScopedMessagePipeHandle message_pipe,
                      scoped_refptr<base::SequencedTaskRunner> runner)
     : message_pipe_(std::move(message_pipe)),
       task_runner_(std::move(runner)),
+      error_(false),
       outgoing_serialization_mode_(g_default_outgoing_serialization_mode),
       incoming_serialization_mode_(g_default_incoming_serialization_mode),
       nesting_observer_(RunLoopNestingObserver::GetForThread()),
@@ -263,9 +264,6 @@ bool Connector::Accept(Message* message) {
   if (!lock_)
     DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
 
-  // It shouldn't hurt even if |error_| may be changed by a different sequence
-  // at the same time. The outcome is that we may write into |message_pipe_|
-  // after encountering an error, which should be fine.
   if (error_)
     return false;