DNS: Retain stale entries in HostCache and return when requested

net/dns/host_cache.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 NET_DNS_HOST_CACHE_H_
 #define NET_DNS_HOST_CACHE_H_
 
 #include <stddef.h>
 
 #include <functional>
@@ skipping 46 matching lines @@
       return std::tie(address_family, host_resolver_flags, hostname) <
              std::tie(other.address_family, other.host_resolver_flags,
                       other.hostname);
     }
 
     std::string hostname;
     AddressFamily address_family;
     HostResolverFlags host_resolver_flags;
   };
 
-  struct EvictionHandler {
-    void Handle(const Key& key,
-                const Entry& entry,
-                const base::TimeTicks& expiration,
-                const base::TimeTicks& now,
-                bool onGet) const;
+  struct NET_EXPORT EntryStaleness {
+    // Time since the entry's TTL has expired. Negative if not expired.
+    base::TimeDelta expired_by;
+    // Number of network changes since this result was cached.
+    int network_changes;
+    // Number of hits to the cache entry while stale (expired or past-network).
+    int stale_hits;
+
+    bool is_stale() const {
+      return network_changes > 0 || expired_by >= base::TimeDelta();
+    }
   };
 
-  typedef ExpiringCache<Key, Entry, base::TimeTicks,
-                        std::less<base::TimeTicks>,
-                        EvictionHandler> EntryMap;
+  struct EntryInternal : public Entry {

[Ryan Sleevi, 2016/04/29 23:09:38]

Well, it's not really Internal now. Perhaps a comment update?

As it stands, there's now no way to publicly get access to an EntryInternal, nor
to the EntryMap. So shouldn't this be made private?

(IIRC - and I may be wrong - the value type for a map can be forward declared
until the point it's instantiated, but the key type must be visible. Since the
ctor/dtor is private, you SHOULD be fine making this type opaque)

[Julia Tuttle, 2016/05/03 20:36:19]

Done. This is *much* better, thank you.

+    EntryInternal(const Entry& entry,
+                  base::TimeTicks now,
+                  base::TimeDelta ttl,
+                  int network_changes);
+
+    base::TimeTicks expires;
+    // Copied from HostCache::network_changes when this entry is set; can later
+    // be compared to it to see if the entry was received on the current
+    // network.
+    int network_changes;
+    int total_hits;
+    int stale_hits;
+
+    bool IsStale(base::TimeTicks now, int network_changes) const;
+    void CountHit(bool hit_is_stale);
+    void GetStaleness(base::TimeTicks now,
+                      int network_changes,
+                      EntryStaleness* out) const;
+  };
+
+  // Ditto.

[Ryan Sleevi, 2016/04/29 23:09:38]

Ditto what? This doesn't seem to need to be public anymore, now that EntryMap
isn't exposed (line 105)

[Julia Tuttle, 2016/05/03 20:36:19]

Done.

+  typedef std::map<Key, EntryInternal> EntryMap;
 
   // Constructs a HostCache that stores up to |max_entries|.
   explicit HostCache(size_t max_entries);
 
   ~HostCache();
 
   // Returns a pointer to the entry for |key|, which is valid at time
   // |now|. If there is no such entry, returns NULL.
   const Entry* Lookup(const Key& key, base::TimeTicks now);
 
+  // Returns a pointer to the entry for |key|, whether it is valid or stale at
+  // time |now|. Fills in |stale_out| with information about how stale it is.
+  // If there is no entry for |key| at all, returns NULL.
+  const Entry* LookupStale(const Key& key,
+                           base::TimeTicks now,
+                           EntryStaleness* stale_out);
+
   // Overwrites or creates an entry for |key|.
   // |entry| is the value to set, |now| is the current time
   // |ttl| is the "time to live".
   void Set(const Key& key,
            const Entry& entry,
            base::TimeTicks now,
            base::TimeDelta ttl);
 
+  // Marks all entries as stale on account of a network change.
+  void OnNetworkChange();
+
   // Empties the cache
   void clear();
 
   // Returns the number of entries in the cache.
   size_t size() const;
 
   // Following are used by net_internals UI.
   size_t max_entries() const;
 
-  const EntryMap& entries() const;
+  std::unique_ptr<base::Value> GetEntriesAsValue() const;
 
   // Creates a default cache.
   static std::unique_ptr<HostCache> CreateDefaultCache();
 
  private:
   FRIEND_TEST_ALL_PREFIXES(HostCacheTest, NoCache);
 
+  EntryInternal* LookupInternal(const Key& key);
+
   // Returns true if this HostCache can contain no entries.
-  bool caching_is_disabled() const {
-    return entries_.max_entries() == 0;
-  }
+  bool caching_is_disabled() const { return max_entries_ == 0; }
+
+  void EvictOneEntry(base::TimeTicks now);
 
   // Map from hostname (presumably in lowercase canonicalized format) to
   // a resolved result entry.
   EntryMap entries_;
+  size_t max_entries_;
+  int network_changes_;
 
   DISALLOW_COPY_AND_ASSIGN(HostCache);
 };
 
 }  // namespace net
 
 #endif  // NET_DNS_HOST_CACHE_H_