Add initial prototype for the GN meta-buildsystem.

tools/gn/import_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/import_manager.h"
+
+#include "base/memory/scoped_ptr.h"
+#include "base/stl_util.h"
+#include "tools/gn/parse_tree.h"
+#include "tools/gn/scheduler.h"
+
+namespace {
+
+// Returns a newly-allocated scope on success, null on failure.
+Scope* UncachedImport(const Settings* settings,
+                      const SourceFile& file,
+                      const ParseNode* node_for_err,
+                      Err* err) {
+  const ParseNode* node = g_scheduler->input_file_manager()->SyncLoadFile(
+      node_for_err->GetRange(), settings->build_settings(), file, err);
+  if (!node)
+    return NULL;
+  const BlockNode* block = node->AsBlock();
+  CHECK(block);
+
+  scoped_ptr<Scope> scope(new Scope(settings->base_config()));
+  scope->SetProcessingImport();
+  block->ExecuteBlockInScope(scope.get(), err);
+  if (err->has_error())
+    return NULL;
+  scope->ClearProcessingImport();
+
+  return scope.release();
+}
+
+}  // namesapce
+
+ImportManager::ImportManager() {
+}
+
+ImportManager::~ImportManager() {
+  STLDeleteContainerPairSecondPointers(imports_.begin(), imports_.end());
+}
+
+bool ImportManager::DoImport(const SourceFile& file,
+                             const ParseNode* node_for_err,
+                             Scope* scope,
+                             Err* err) {
+  // See if we have a cached import, but be careful to actually do the scope
+  // copying outside of the lock.
+  const Scope* imported_scope = NULL;
+  {
+    base::AutoLock lock(lock_);
+    ImportMap::const_iterator found = imports_.find(file);
+    if (found != imports_.end())
+      imported_scope = found->second;
+  }
+
+  if (!imported_scope) {
+    // Do a new import of the file.
+    imported_scope = UncachedImport(scope->settings(), file,
+                                    node_for_err, err);
+    if (!imported_scope)
+      return false;
+
+    // We loaded the file outside the lock. This means that there could be a
+    // race and the file was already loaded on a background thread. Recover
+    // from this and use the existing one if that happens.
+    {
+      base::AutoLock lock(lock_);
+      ImportMap::const_iterator found = imports_.find(file);
+      if (found != imports_.end()) {
+        delete imported_scope;
+        imported_scope = found->second;
+      } else {
+        imports_[file] = imported_scope;
+      }
+    }
+  }
+
+  return imported_scope->NonRecursiveMergeTo(scope, node_for_err,
+                                             "import", err);
+}