Make ui gn check pass. Misc GN build improvements.

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 197 matching lines @@
       {
         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
+  // The other load could have failed. In this case that error was probably
+  // 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");
+  //
+  // There is a race condition. The other load could have failed, but if the
+  // other thread is delayed for some reason, this thread could end up
+  // reporting the error to the scheduler first (since first error report
+  // wins). The user will see this one and the "real" one will be discarded.
+  if (!data->parsed_root) {
+    *err = Err(origin, "File parse failed.",
+        "If you see this, I'm really sorry, but a race condition has caused\n"
+        "me to eat your error message. It was crunchy. If the parse error\n"
+        "in your imported file isn't obvious, try re-running GN.");
+  }
   return data->parsed_root.get();
 }
 
 void InputFileManager::AddDynamicInput(const SourceFile& name,
                                        InputFile** file,
                                        std::vector<Token>** tokens,
                                        scoped_ptr<ParseNode>** parse_root) {
   InputFileData* data = new InputFileData(name);
   {
     base::AutoLock lock(lock_);
@@ skipping 76 matching lines @@
 
   // 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.
   if (success) {
     for (size_t i = 0; i < callbacks.size(); i++)
       callbacks[i].Run(unowned_root);
   }
   return success;
 }