Clean up threading model of external extension providers

chrome/browser/extensions/external_extension_loader.h

Index: chrome/browser/extensions/external_extension_loader.h
diff --git a/chrome/browser/extensions/external_extension_loader.h b/chrome/browser/extensions/external_extension_loader.h
new file mode 100644
index 0000000000000000000000000000000000000000..594fb9e75c072867db449faf2334e9587a83b61b
--- /dev/null
+++ b/chrome/browser/extensions/external_extension_loader.h
@@ -0,0 +1,71 @@
+// Copyright (c) 2011 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 CHROME_BROWSER_EXTENSIONS_EXTERNAL_EXTENSION_LOADER_H_
+#define CHROME_BROWSER_EXTENSIONS_EXTERNAL_EXTENSION_LOADER_H_
+#pragma once
+
+#include "base/ref_counted.h"
+#include "base/scoped_ptr.h"
+
+class DictionaryValue;
+class ExternalExtensionProviderImpl;
+
+// Base class for gathering a list of external extensions. Subclasses
+// implement loading from registry, JSON file, policy.
+// Instances are owned by ExternalExtensionProviderImpl objects.
+// Instances are created on the UI thread and expect public method calls from
+// the UI thread. Some subclasses introduce new methods that are executed on the
+// FILE thread.
+// The sequence of loading the extension list:
+// 1.) StartLoading() - checks if a loading task is already running
+// 2.) Load() - implemented in subclasses
+// 3.) FinishLoading()
+// 4.) owner_->SetPrefs()
+class ExternalExtensionLoader
+    : public base::RefCountedThreadSafe<ExternalExtensionLoader> {
+ public:
+  explicit ExternalExtensionLoader() : running_(false) {}
+
+  // Specifies the provider that owns this object.
+  void Init(ExternalExtensionProviderImpl* owner);
+
+  // Called by the owner before it gets deleted.
+  void OwnerShutdown();
+
+  // Initiates the possibly asynchronous loading of extension list.
+  // It is the responsibility of the caller to ensure that calls to
+  // this method do not overlap with each other.
+  // Implementations of this method should save the loaded results
+  // in prefs_ and then call LoadFinished.
+  virtual void StartLoading() = 0;
+
+ protected:
+  virtual ~ExternalExtensionLoader() {}
+
+  // Notifies the provider that the list of extensions has been loaded.
+  void LoadFinished();
+
+  // Used for passing the list of extensions from the method that loads them
+  // to |LoadFinished|. To ensure thread safety, the rules are the following:
+  // if this value is written on another thread than the UI, then it should
+  // only be written in a task that was posted from |StartLoading|. After that,
+  // this task should invoke |LoadFinished| with a PostTask. This scheme of
+  // posting tasks will avoid concurrent access and imply the necessary memory
+  // barriers.
+  scoped_ptr<DictionaryValue> prefs_;
+
+ private:
+  friend class base::RefCountedThreadSafe<ExternalExtensionLoader>;
+
+  ExternalExtensionProviderImpl* owner_;  // weak
+
+  // Set to true if loading the extensions is already running. New requests
+  // are ignored while this is set true.
+  bool running_;
+
+  DISALLOW_COPY_AND_ASSIGN(ExternalExtensionLoader);
+};
+
+#endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_EXTENSION_LOADER_H_