Fix a GN race condition.

tools/gn/setup.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/setup.h"
 
 #include <stdlib.h>
 #include <algorithm>
 #include <sstream>
 #include <utility>
@@ skipping 119 matching lines @@
     return try_this_file;
 
   base::FilePath with_no_slash = current_dir.StripTrailingSeparators();
   base::FilePath up_one_dir = with_no_slash.DirName();
   if (up_one_dir == current_dir)
     return base::FilePath();  // Got to the top.
 
   return FindDotFile(up_one_dir);
 }
 
+void ForwardItemDefinedToBuilderInMainThread(
+    Builder* builder_call_on_main_thread_only,
+    std::unique_ptr<Item> item) {
+  builder_call_on_main_thread_only->ItemDefined(std::move(item));
+
+  // Pair to the Increment in ItemDefinedCallback.
+  g_scheduler->DecrementWorkCount();
+}
+
 // Called on any thread. Post the item to the builder on the main thread.
 void ItemDefinedCallback(
     scoped_refptr<base::SingleThreadTaskRunner> task_runner,
     Builder* builder_call_on_main_thread_only,
     std::unique_ptr<Item> item) {
   DCHECK(item);
+
+  // Increment the work count for the duration of defining the item with the
+  // builder. Otherwise finishing this callback will race finishing loading
+  // files. If there is no other pending work at any point in the middle of
+  // this call completing on the main thread, the 'Complete' function will
+  // be signaled and we'll stop running with an incomplete build.
+  g_scheduler->IncrementWorkCount();
   task_runner->PostTask(
       FROM_HERE,
-      base::Bind(&Builder::ItemDefined,
+      base::Bind(&ForwardItemDefinedToBuilderInMainThread,
                  base::Unretained(builder_call_on_main_thread_only),
                  base::Passed(&item)));
 }
 
 void DecrementWorkCount() {
   g_scheduler->DecrementWorkCount();
 }
 
 #if defined(OS_WIN)
 
@@ skipping 155 matching lines @@
   if (!scheduler_.Run())
     return false;
   return RunPostMessageLoop();
 }
 
 SourceFile Setup::GetBuildArgFile() const {
   return SourceFile(build_settings_.build_dir().value() + kBuildArgFileName);
 }
 
 void Setup::RunPreMessageLoop() {
+  // Will be decremented with the loader is drained.
+  g_scheduler->IncrementWorkCount();
+
   // Load the root build file.
   loader_->Load(root_build_file_, LocationRange(), Label());
-
-  // Will be decremented with the loader is drained.
-  g_scheduler->IncrementWorkCount();
 }
 
 bool Setup::RunPostMessageLoop() {
   Err err;
   if (!builder_.CheckForBadItems(&err)) {
     err.PrintToStdout();
     return false;
   }
 
   if (!build_settings_.build_args().VerifyAllOverridesUsed(&err)) {
@@ skipping 394 matching lines @@
       if (err.has_error()) {
         err.PrintToStdout();
         return false;
       }
     }
     build_settings_.set_exec_script_whitelist(std::move(whitelist));
   }
 
   return true;
 }