Implementing EvictedDomainCookieCounter to keep user metrics on cookie eviction and reinstatement.

chrome/browser/net/evicted_domain_cookie_counter.h

+// Copyright 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 CHROME_BROWSER_NET_EVICTED_DOMAIN_COOKIE_COUNTER_H_
+#define CHROME_BROWSER_NET_EVICTED_DOMAIN_COOKIE_COUNTER_H_
+
+#include <map>
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/time.h"
+#include "net/cookies/cookie_monster.h"
+
+namespace net {
+class CanonicalCookie;
+}  // namespace net
+
+namespace chrome_browser_net {
+
+// The Evicted Domain Cookie Counter generates statistics on "wrongly evicted"
+// cookies, i.e., cookies that were "evicted" (on reaching domain cookie limit)
+// but are then "reinstated" later because they were important. A specific
+// scenario is as follows: a long-lived login session cookie gets evicted owing
+// to its age, thereby forcing the user to lose session, and is reinstated when
+// the user re-authenticates.
+//
+// A solution to the above problem is the Cookie Priority Field, which enables
+// servers to protect important cookies, thereby decreasing the chances that
+// these cookies are wrongly evicted. To measure the effectiveness of this
+// solution, we will compare eviction user metrics before vs. after the fix.
+//
+// Specifically, we wish to record user metrics on "reinstatement delay", i.e.,
+// the duration between eviction and reinstatement of cookie. We expect that
+// after the fix, average reinstatement delays will increase, since low priority
+// cookies are less likely to be reinstated after eviction.
+//
+// Metrics for Google domains are tracked separately.
+//
+class EvictedDomainCookieCounter : public net::CookieMonster::Delegate {
+ public:
+  // Structure to store sanitized data from CanonicalCookie.
+  struct EvictedCookie {
+    EvictedCookie(base::Time eviction_time_in,
+                  base::Time expiry_time_in,
+                  bool is_google_in)
+        : eviction_time(eviction_time_in),
+          expiry_time(expiry_time_in),
+          is_google(is_google_in) {}
+
+    bool is_expired(const base::Time& current_time) const {
+      return !expiry_time.is_null() && current_time >= expiry_time;
+    }
+
+    base::Time eviction_time;
+    base::Time expiry_time;
+    bool is_google;
+  };
+
+  class Delegate {
+   public:
+    virtual ~Delegate() {}
+
+    // Called when a stored evicted cookie is reinstated.
+    virtual void Report(const EvictedCookie& evicted_cookie,
+                        const base::Time& reinstatement_time) = 0;
+
+    // Getter of time is placed here to enable mocks.
+    virtual base::Time CurrentTime() const = 0;
+  };
+
+  // |next_cookie_monster_delegate| can be NULL.
+  explicit EvictedDomainCookieCounter(
+      scoped_refptr<net::CookieMonster::Delegate> next_cookie_monster_delegate);
+
+  // Constructor exposed for testing only.
+  EvictedDomainCookieCounter(
+      scoped_refptr<net::CookieMonster::Delegate> next_cookie_monster_delegate,
+      scoped_ptr<Delegate> cookie_counter_delegate,
+      size_t max_size,
+      size_t purge_count);
+
+  // Returns the number of evicted cookies stored.
+  size_t GetStorageSize() const;
+
+  // CookieMonster::Delegate implementation.
+  virtual void OnCookieChanged(const net::CanonicalCookie& cookie,
+                               bool removed,
+                               ChangeCause cause) OVERRIDE;
+
+ private:
+  // Identifier of an evicted cookie.
+  typedef std::string EvictedCookieKey;
+
+  // Storage class of evicted cookie.
+  typedef std::map<EvictedCookieKey, EvictedCookie*> EvictedCookieMap;
+
+  virtual ~EvictedDomainCookieCounter();
+
+  // Computes key for |cookie| compatible with CanonicalCookie::IsEquivalent(),
+  // i.e., IsEquivalent(a, b) ==> GetKey(a) == GetKey(b).
+  static EvictedCookieKey GetKey(const net::CanonicalCookie& cookie);
+
+  static bool CompareEvictedCookie(
+      const EvictedCookieMap::iterator evicted_cookie1,
+      const EvictedCookieMap::iterator evicted_cookie2);
+
+  // If too many evicted cookies are stored, delete the expired ones, then
+  // delete cookies that were evicted the longest, until size limit reached.
+  void GarbageCollect(const base::Time& current_time);
+
+  // Called when a cookie is evicted. Adds the evicted cookie to storage,
+  // possibly replacing an existing equivalent cookie.
+  void StoreEvictedCookie(const EvictedCookieKey& key,
+                          const net::CanonicalCookie& cookie,
+                          const base::Time& current_time);
+
+  // Called when a new cookie is added. If reinstatement occurs, then notifies
+  // |cookie_counter_delegate_| and then removes the evicted cookie.
+  void ProcessNewCookie(const EvictedCookieKey& key,
+                        const net::CanonicalCookie& cookie,
+                        const base::Time& current_time);
+
+  // Another delegate to forward events to.
+  scoped_refptr<net::CookieMonster::Delegate> next_cookie_monster_delegate_;
+
+  scoped_ptr<Delegate> cookie_counter_delegate_;
+
+  EvictedCookieMap evicted_cookies_;
+
+  // Capacity of the evicted cookie storage, before garbage collection occurs.
+  const size_t max_size_;
+
+  // After garbage collection, size reduces to <= |max_size_| - |purge_count_|.
+  const size_t purge_count_;
+
+  DISALLOW_COPY_AND_ASSIGN(EvictedDomainCookieCounter);
+};
+
+}  // namespace chrome_browser_net
+
+#endif  // CHROME_BROWSER_NET_EVICTED_DOMAIN_COOKIE_COUNTER_H_