Mojo: In Debug builds, log when a race condition in user code is likely observed.

mojo/system/handle_table.cc

 // Copyright 2014 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/system/handle_table.h"
 
 #include "base/basictypes.h"
 #include "base/logging.h"
 #include "mojo/system/constants.h"
 #include "mojo/system/dispatcher.h"
@@ skipping 128 matching lines @@
     }
     // Note: By marking the handle as busy here, we're also preventing the
     // same handle from being sent multiple times in the same message.
     entries[i]->busy = true;
 
     // Try to start the transport.
     DispatcherTransport transport =
         Dispatcher::HandleTableAccess::TryStartTransport(
             entries[i]->dispatcher.get());
     if (!transport.is_valid()) {
+      // Only log for Debug builds, since this is not a problem with the system
+      // code, but with user code.
+      DLOG(WARNING) << "Likely race condition in user code detected: attempt "
+                       "to transfer handle "
+                    << handles[i]
+                    << " while it is in use on a different thread";
+
       // Unset the busy flag (since it won't be unset below).
       entries[i]->busy = false;
       error_result = MOJO_RESULT_BUSY;
       break;
     }
 
     // Check if the dispatcher is busy (e.g., in a two-phase read/write).
     // (Note that this must be done after the dispatcher's lock is acquired.)
     if (transport.IsBusy()) {
       // Unset the busy flag and end the transport (since it won't be done
@@ skipping 69 matching lines @@
   for (uint32_t i = 0; i < num_handles; i++) {
     HandleToEntryMap::iterator it = handle_to_entry_map_.find(handles[i]);
     DCHECK(it != handle_to_entry_map_.end());
     DCHECK(it->second.busy);
     it->second.busy = false;
   }
 }
 
 }  // namespace system
 }  // namespace mojo