[net] Add AsyncDNS.TTL histogram.

net/base/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_BASE_HOST_CACHE_H_
 #define NET_BASE_HOST_CACHE_H_
 
 #include <functional>
 #include <string>
 
 #include "base/gtest_prod_util.h"
 #include "base/threading/non_thread_safe.h"
 #include "base/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:
   // Stores the latest address list that was looked up for a hostname.
   struct Entry {
+    Entry(int error, const AddressList& addrlist, base::TimeDelta ttl);
+    // As above, but sets |ttl| to negative.

[cbentzel, 2012/10/13 10:50:45]

This comment actually confuses things a bit - it's clearer if it says that the
entry has no actual TTL, and then comment the ttl field to specify that negative
values mean no ttl.

     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;
+    base::TimeDelta ttl;
   };
 
   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 {
@@ skipping 18 matching lines @@
   // 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);
 
   // Overwrites or creates an entry for |key|.
-  // (|error|, |addrlist|) is the value to set, |now| is the current time
+  // |entry| is the value to set, |now| is the current time
   // |ttl| is the "time to live".
   void Set(const Key& key,
-           int error,
-           const AddressList& addrlist,
+           const Entry& entry,
            base::TimeTicks now,
            base::TimeDelta ttl);
 
   // Empties the cache
   void clear();
 
   // Returns the number of entries in the cache.
   size_t size() const;
 
   // Following are used by net_internals UI.
@@ skipping 15 matching lines @@
   // Map from hostname (presumably in lowercase canonicalized format) to
   // a resolved result entry.
   EntryMap entries_;
 
   DISALLOW_COPY_AND_ASSIGN(HostCache);
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_HOST_CACHE_H_