GN threading refactor

tools/gn/input_file_manager.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 "tools/gn/input_file_manager.h"
 
 #include "base/bind.h"
 #include "base/stl_util.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/parser.h"
@@ skipping 134 matching lines @@
     }
 
     if (!data->loaded) {
       // Wait for the already-pending sync load to complete.
       if (!data->completion_event)
         data->completion_event.reset(new base::WaitableEvent(false, false));
       {
         base::AutoUnlock unlock(lock_);
         data->completion_event->Wait();
       }
+      // If there were multiple waiters on the same event, we now need to wake
+      // up the next one.
+      data->completion_event->Signal();
     }
   }
 
   // The other load could have failed. In this case that error will be printed
   // to the console, but we need to return something here, so make up a
   // dummy error.
   if (!data->parsed_root)
     *err = Err(origin, "File parse failed");
   return data->parsed_root.get();
 }
@@ skipping 102 matching lines @@
     callbacks.swap(data->scheduled_callbacks);
   }
 
   // Run pending invocations. Theoretically we could schedule each of these
   // separately to get some parallelism. But normally there will only be one
   // item in the list, so that's extra overhead and complexity for no gain.
   for (size_t i = 0; i < callbacks.size(); i++)
     callbacks[i].Run(unowned_root);
   return true;
 }