DNS: Retain stale entries in HostCache and return when requested

net/dns/host_cache.h

Index: net/dns/host_cache.h
diff --git a/net/dns/host_cache.h b/net/dns/host_cache.h
index 752ebeb6c0732f32e6677babe7b517d11ebf0354..1e8b9052cdd6a1ea71e52f81f3a9b9129041844f 100644
--- a/net/dns/host_cache.h
+++ b/net/dns/host_cache.h
@@ -64,18 +64,19 @@ class NET_EXPORT HostCache : NON_EXPORTED_BASE(public base::NonThreadSafe) {
     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;

[Randy Smith (Not in Mondays), 2016/05/04 18:15:25]

I don't have a sense as to the usage of ExpiringCache in the system as a whole;
does this removal imply that we should queue getting rid of it in other places
because it's more heavyweight than needed if the host cache isn't using it?

[Julia Tuttle, 2016/05/05 14:35:53]

The host cache isn't using it because the expiration/eviction semantics didn't
make sense anymore.

It still makes sense for the other two users, but whether the *abstraction* is
worth the hassle is another question.

I think we should leave it around for now.

-
   // Constructs a HostCache that stores up to |max_entries|.
   explicit HostCache(size_t max_entries);
 
@@ -85,6 +86,13 @@ class NET_EXPORT HostCache : NON_EXPORTED_BASE(public base::NonThreadSafe) {
   // |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".
@@ -93,6 +101,9 @@ class NET_EXPORT HostCache : NON_EXPORTED_BASE(public base::NonThreadSafe) {
            base::TimeTicks now,
            base::TimeDelta ttl);
 
+  // Marks all entries as stale on account of a network change.
+  void OnNetworkChange();
+
   // Empties the cache
   void clear();
 
@@ -102,7 +113,7 @@ class NET_EXPORT HostCache : NON_EXPORTED_BASE(public base::NonThreadSafe) {
   // 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();
@@ -110,14 +121,41 @@ class NET_EXPORT HostCache : NON_EXPORTED_BASE(public base::NonThreadSafe) {
  private:
   FRIEND_TEST_ALL_PREFIXES(HostCacheTest, NoCache);
 
+  struct EntryInternal;
+  typedef std::map<Key, EntryInternal> EntryMap;
+  enum SetOutcome : int;
+  enum LookupOutcome : int;
+  enum EraseReason : int;
+
+  EntryInternal* LookupInternal(const Key& key);
+
+  void RecordSet(SetOutcome outcome,
+                 base::TimeTicks now,
+                 const EntryInternal* old_entry,
+                 const Entry& new_entry);
+  void RecordUpdateStale(const EntryInternal& old_entry,
+                         const Entry& new_entry,
+                         base::TimeDelta expired_by,
+                         int network_changes,
+                         int stale_hits);
+  void RecordLookup(LookupOutcome outcome,
+                    base::TimeTicks now,
+                    const EntryInternal* entry);
+  void RecordErase(EraseReason reason,
+                   base::TimeTicks now,
+                   const EntryInternal& entry);
+  void RecordEraseAll(EraseReason reason, base::TimeTicks now);
+
   // 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);
 };