Add the AsyncPolicyProvider and AsyncPolicyLoader

chrome/browser/policy/async_policy_loader.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_POLICY_ASYNC_POLICY_LOADER_H_
+#define CHROME_BROWSER_POLICY_ASYNC_POLICY_LOADER_H_
+#pragma once
+
+#include "base/callback.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/time.h"
+
+namespace policy {
+
+class PolicyBundle;
+
+// Base implementation for platform-specific policy loaders. Together with the
+// AsyncPolicyProvider, this base implementation takes care of the initial load,
+// periodic reloads, watching file changes, refreshing policies and object
+// lifetime.
+//
+// All methods are invoked on the FILE thread, including the destructor.
+// The only exceptions are the constructor (which may be called on any thread),
+// and the initial Load() which is called on the thread that owns the provider.
+// LastModificationTime() is also invoked once on that thread at startup.
+class AsyncPolicyLoader {
+ public:
+  AsyncPolicyLoader();
+  virtual ~AsyncPolicyLoader();
+
+  // Returns the currently configured policies. Load() is always invoked on
+  // the FILE thread, except for the initial Load() at startup which is invoked
+  // from the thread that owns the provider.
+  virtual scoped_ptr<PolicyBundle> Load() = 0;
+
+  // Allows implementations to finalize their initialization on the FILE
+  // thread (e.g. setup file watchers).
+  virtual void InitOnFile() = 0;
+
+  // Implementations should return the time of the last modification detected,
+  // or base::Time() if it doesn't apply, which is the default.
+  virtual base::Time LastModificationTime();
+
+  // Implementations should invoke Reload() when a change is detected. This
+  // must be invoked from the FILE thread and will trigger a Load(), and pass
+  // the returned bundle to the provider.
+  // The load is immediate when |force| is true. Otherwise, the loader
+  // reschedules the reload until the LastModificationTime() is a couple of
+  // seconds in the past. This mitigates the problem of reading files that are
+  // currently being written to, and whose contents are incomplete.
+  // A reload is posted periodically, if it hasn't been triggered recently. This
+  // makes sure the policies are reloaded if the update events aren't triggered.
+  void Reload(bool force);
+
+ private:
+  // Allow AsyncPolicyProvider to call Init().
+  friend class AsyncPolicyProvider;
+
+  typedef base::Callback<void(scoped_ptr<PolicyBundle>)> UpdateCallback;
+
+  // Used by the AsyncPolicyProvider to do the initial Load(). The first load
+  // is also used to initialize |last_modification_time_|.
+  scoped_ptr<PolicyBundle> InitialLoad();
+
+  // Used by the AsyncPolicyProvider to install the |update_callback_|.
+  // Invoked on the FILE thread.
+  void Init(const UpdateCallback& update_callback);
+
+  // Cancels any pending periodic reload and posts one |delay| time units from
+  // now.
+  void ScheduleNextReload(base::TimeDelta delay);
+
+  // Checks if the underlying files haven't changed recently, by checking the
+  // LastModificationTime(). |delay| is updated with a suggested time to wait
+  // before retrying when this returns false.
+  bool IsSafeToReload(const base::Time& now, base::TimeDelta* delay);
+
+  // Callback for updates, passed in Init().
+  UpdateCallback update_callback_;
+
+  // Used to get WeakPtrs for the periodic reload task.
+  base::WeakPtrFactory<AsyncPolicyLoader> weak_factory_;
+
+  // Records last known modification timestamp.
+  base::Time last_modification_time_;
+
+  // The wall clock time at which the last modification timestamp was
+  // recorded.  It's better to not assume the file notification time and the
+  // wall clock times come from the same source, just in case there is some
+  // non-local filesystem involved.
+  base::Time last_modification_clock_;
+
+  DISALLOW_COPY_AND_ASSIGN(AsyncPolicyLoader);
+};
+
+}  // namespace policy
+
+#endif  // CHROME_BROWSER_POLICY_ASYNC_POLICY_LOADER_H_