Prevent a race condition with MidiHost IPC sending.

content/browser/media/midi_host.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/media/midi_host.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/process/process.h"
 #include "base/trace_event/trace_event.h"
@@ skipping 41 matching lines @@
       renderer_process_id_(renderer_process_id),
       has_sys_ex_permission_(false),
       is_session_requested_(false),
       midi_manager_(midi_manager),
       sent_bytes_in_flight_(0),
       bytes_sent_since_last_acknowledgement_(0),
       output_port_count_(0) {
   DCHECK(midi_manager_);
 }
 
-MidiHost::~MidiHost() {
-  // Close an open session, or abort opening a session.
-  if (is_session_requested_ && midi_manager_)
+MidiHost::~MidiHost() {}

[Adam Goode, 2016/01/12 17:36:45]

Suggest (C++11 style):

MidiHost::~MidiHost() = default;

[Oliver Chang, 2016/01/12 17:48:55]

Done.

+
+void MidiHost::OnChannelClosing() {
+  // If we get here the MidiHost is going to be destroyed soon. Prevent any
+  // subsequent calls from MidiManager by closing our session.
+  // If Send() is called from a different thread (e.g. a separate thread owned
+  // by the MidiManager implementation), it will get posted to the IO thread.
+  // There is a race condition here if our refcount is 0 and we're about to or
+  // have already entered OnDestruct().
+  if (is_session_requested_ && midi_manager_) {
     midi_manager_->EndSession(this);
+    is_session_requested_ = false;
+  }
 }
 
 void MidiHost::OnDestruct() const {
   BrowserThread::DeleteOnIOThread::Destruct(this);
 }
 
 // IPC Messages handler
 bool MidiHost::OnMessageReceived(const IPC::Message& message) {
   bool handled = true;
   IPC_BEGIN_MESSAGE_MAP(MidiHost, message)
@@ skipping 173 matching lines @@
     }
     waiting_data_length = media::midi::GetMidiMessageLength(current);
     if (waiting_data_length == 0)
       return false;  // Error: |current| should have been a valid status byte.
     --waiting_data_length;  // Found status byte
   }
   return waiting_data_length == 0 && !in_sysex;
 }
 
 }  // namespace content