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>
 #include <memory>
 #include <string>
 #include <tuple>
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/threading/non_thread_safe.h"
 #include "base/time/time.h"
 #include "net/base/address_family.h"
 #include "net/base/address_list.h"
 #include "net/base/expiring_cache.h"
 #include "net/base/net_export.h"
 
 namespace net {
 
 // Cache used by HostResolver to map hostnames to their resolved result.
 class NET_EXPORT HostCache : NON_EXPORTED_BASE(public base::NonThreadSafe) {
  public:
+  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();
+    }
+  };
+
   // Stores the latest address list that was looked up for a hostname.
   struct NET_EXPORT Entry {
+   public:
     Entry(int error, const AddressList& addrlist, base::TimeDelta ttl);
     // Use when |ttl| is unknown.
     Entry(int error, const AddressList& addrlist);
     ~Entry();
 
     bool has_ttl() const { return ttl >= base::TimeDelta(); }
 
     // The resolve results for this entry.
     int error;
     AddressList addrlist;
     // TTL obtained from the nameserver. Negative if unknown.
     base::TimeDelta ttl;

[Randy Smith (Not in Mondays), 2016/05/05 20:41:51]

How would you feel about turning Entry into a class and just exposing these
through accessors?  I think the style guide encourages that:
https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes

[Julia Tuttle, 2016/05/06 18:32:43]

Done.

+
+   private:
+    friend class HostCache;
+
+    Entry(const Entry& entry,
+          base::TimeTicks now,
+          base::TimeDelta ttl,
+          int network_changes);
+
+    base::TimeTicks expires;
+    // Copied from the cache's network_changes_ when the 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;

[Randy Smith (Not in Mondays), 2016/05/05 20:41:51]

I believe the style guide says that within each section, methods should be
before data members.

[Julia Tuttle, 2016/05/06 18:32:43]

Done.

   };
 
   struct Key {
     Key(const std::string& hostname, AddressFamily address_family,
         HostResolverFlags host_resolver_flags)
         : hostname(hostname),
           address_family(address_family),
           host_resolver_flags(host_resolver_flags) {}
 
     bool operator<(const Key& other) const {
       // The order of comparisons of |Key| fields is arbitrary, thus
       // |address_family| and |host_resolver_flags| are compared before
       // |hostname| under assumption that integer comparisons are faster than
       // string comparisons.
       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;
-  };
-
-  typedef ExpiringCache<Key, Entry, base::TimeTicks,
-                        std::less<base::TimeTicks>,
-                        EvictionHandler> 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);
 
+  typedef std::map<Key, Entry> EntryMap;

[Randy Smith (Not in Mondays), 2016/05/05 20:41:51]

I believe that "using EntryMap = std::map<Key, Entry>;" is the new, preferred
hotness; see http://chromium-cpp.appspot.com/.

[Julia Tuttle, 2016/05/06 18:32:43]

Done.

+  enum SetOutcome : int;
+  enum LookupOutcome : int;
+  enum EraseReason : int;
+
+  Entry* LookupInternal(const Key& key);
+
+  void RecordSet(SetOutcome outcome,
+                 base::TimeTicks now,
+                 const Entry* old_entry,
+                 const Entry& new_entry);
+  void RecordUpdateStale(const Entry& old_entry,
+                         const Entry& new_entry,
+                         base::TimeDelta expired_by,
+                         int network_changes,
+                         int stale_hits);
+  void RecordLookup(LookupOutcome outcome,
+                    base::TimeTicks now,
+                    const Entry* entry);
+  void RecordErase(EraseReason reason, base::TimeTicks now, const Entry& 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);
 };
 
 }  // namespace net
 
 #endif  // NET_DNS_HOST_CACHE_H_