Enforce the 'requirements' field in manifests.

chrome/browser/extensions/requirements_checker.h

+// Copyright (c) 2012 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_REQUIREMENTS_CHECKER_H_
+#define CHROME_BROWSER_EXTENSIONS_REQUIREMENTS_CHECKER_H_
+
+#include <vector>
+
+#include "base/callback.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
+#include "chrome/browser/extensions/extension_service.h"
+
+class GPUFeatureChecker;
+
+namespace extensions {
+class Extension;
+
+class RequirementsChecker : public base::SupportsWeakPtr<RequirementsChecker> {
+ public:
+  RequirementsChecker();
+  ~RequirementsChecker();
+
+  // The vector passed to the callback are any localized errors describing
+  // requirement violations. If this vector is non-empty, requirements checking
+  // failed. This should only be called once. |callback| will be invoked on
+  // the calling thread.
+  void Check(scoped_refptr<const Extension> extension,
+      base::Callback<void(std::vector<std::string>)> callback,
+      content::BrowserThread::ID callback_thread);

[Aaron Boodman, 2012/08/01 03:58:54]

Yeah, so I think you can just let this callback on the UI thread, which will be
the same thread it is created on.

[eaugusti, 2012/08/03 01:06:26]

Done.

+
+  // Because of the GPUFeatureChecker, any WeakPtr to RequirementsChecker must
+  // be bound to the UI thread. Therefore, there is a pitfall if someone does
+  // something seemingly innocent like this:
+  // BrowserThread::PostTask(BrowserThread::UI, FROM_HERE,
+  //     base::Bind(&RequirementsChecker::Check,
+  //                my_requirements_checker_->AsWeakPtr()),
+  //                ...);
+  // If the caller is not on the UI thread (hence the PostTask()) and this is
+  // the first call to AsWeakPtr(), then the subsequent WeakPtrs will be bound
+  // to a thread that is not the UI thread. This will cause problems for the
+  // GPUFeatureChecker which calls back (and therefore dereferences the
+  // WeakPtr) from the UI thread. To help avoid this pitfall, AsWeakPtr() is
+  // overridden and the UI thread is asserted.
+  virtual base::WeakPtr<RequirementsChecker> AsWeakPtr() OVERRIDE;

[Aaron Boodman, 2012/08/01 03:58:54]

Nice catch, but do we have a need to call this from a non-UI thread? I think it
is probably sufficient to just say that it is UI-thread-only in the comments.

[Yoyo Zhou, 2012/08/02 22:23:30]

There is also WeakPtrFactory which gives a bit more control over who can get a
weak pointer. Something to look into, not necessarily better for this case.

[eaugusti, 2012/08/03 01:06:26]

Done.

+
+ private:
+  friend class GPUFeatureChecker;

[Aaron Boodman, 2012/08/01 03:58:54]

Curiosity: Is this needed so GPUFeatureChecker can call Is*Available() ?

[eaugusti, 2012/08/03 01:06:26]

Yes, but it is totally unnecessary. I must be going crazy.

+
+  // Callbacks for the GPUFeatureChecker.
+  void IsWebGLAvailable(bool available);
+  void IsCSS3DAvailable(bool available);
+
+  void MaybeRunCallback();
+
+  std::vector<std::string> errors_;
+
+  // Every requirments that needs to be resolved asynchroniously will add to

[Yoyo Zhou, 2012/08/02 22:23:30]

typo: requirement, asynchronously

[eaugusti, 2012/08/03 01:06:26]

Done.

+  // this counter. When the counter is depleted, the callback will be run.
+  int async_requirement_checks_;
+
+  scoped_refptr<GPUFeatureChecker> webgl_checker_;
+  scoped_refptr<GPUFeatureChecker> css3d_checker_;
+
+  base::Callback<void(std::vector<std::string>)> callback_;
+  content::BrowserThread::ID callback_thread_;
+};
+
+}  // namespace extensions
+
+#endif  // CHROME_BROWSER_EXTENSIONS_REQUIREMENTS_CHECKER_H_