Add initial prototype for the GN meta-buildsystem.

tools/gn/import_manager.h

+// 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.
+
+#ifndef TOOLS_GN_IMPORT_MANAGER_H_
+#define TOOLS_GN_IMPORT_MANAGER_H_
+
+#include <map>
+
+#include "base/synchronization/lock.h"
+
+class Err;
+class ParseNode;
+class Scope;
+class SourceFile;
+
+// Provides a cache of the results of importing scopes so the results can
+// be re-used rather than running the imported files multiple times.
+class ImportManager {
+ public:
+  ImportManager();
+  ~ImportManager();
+
+  // Does an import of the given file into the given scope. On error, sets the
+  // error and returns false.
+  bool DoImport(const SourceFile& file,
+                const ParseNode* node_for_err,
+                Scope* scope,
+                Err* err);
+
+ private:
+  base::Lock lock_;
+
+  // Owning pointers to the scopes.
+  typedef std::map<SourceFile, const Scope*> ImportMap;
+  ImportMap imports_;
+
+  DISALLOW_COPY_AND_ASSIGN(ImportManager);
+};
+
+#endif  // TOOLS_GN_IMPORT_MANAGER_H_