Show the callstack that a nontransferable message pipe was first used if it's erroneously sent later

mojo/message_pump/handle_watcher.cc

 // Copyright 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 "mojo/message_pump/handle_watcher.h"
 
 #include <map>
 
 #include "base/atomic_sequence_num.h"
 #include "base/bind.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/singleton.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/lock.h"
 #include "base/thread_task_runner_handle.h"
 #include "base/threading/thread.h"
 #include "base/time/time.h"
 #include "mojo/message_pump/message_pump_mojo.h"
 #include "mojo/message_pump/message_pump_mojo_handler.h"
 #include "mojo/message_pump/time_helper.h"
+#include "mojo/public/c/system/message_pipe.h"
 
 namespace mojo {
 namespace common {
 
 typedef int WatcherID;
 
 namespace {
 
 const char kWatcherThreadName[] = "handle-watcher-thread";
 
@@ skipping 414 matching lines @@
                           const base::Callback<void(MojoResult)>& callback) {
   DCHECK(handle.is_valid());
   DCHECK_NE(MOJO_HANDLE_SIGNAL_NONE, handle_signals);
 
   // Need to clear the state before creating a new one.
   state_.reset();
   if (MessagePumpMojo::IsCurrent()) {
     state_.reset(new SameThreadWatchingState(
         this, handle, handle_signals, deadline, callback));
   } else {
+#if !defined(OFFICIAL_BUILD)
+    // Just for making debugging non-transferable message pipes easier. Since
+    // they can't be sent after they're read/written/listened to,
+    // MessagePipeDispatcher saves the callstack of when it's "bound" to a
+    // pipe id. Triggering a read here, instead of later in the PostTask, means
+    // we have a callstack that is useful to check if the pipe is erronously
+    // attempted to be sent.
+    uint32_t temp = 0;
+    MojoReadMessage(handle.value(), nullptr, &temp, nullptr, nullptr,
+                    MOJO_READ_MESSAGE_FLAG_NONE);
+#endif
     state_.reset(new SecondaryThreadWatchingState(
         this, handle, handle_signals, deadline, callback));
   }
 }
 
 void HandleWatcher::Stop() {
   state_.reset();
 }
 
 }  // namespace common
 }  // namespace mojo