Adds custom ttl argument to HostCache::Set.

net/base/host_cache.h

 // Copyright (c) 2011 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_
 #pragma once
 
 #include <map>
 #include <string>
@@ skipping 70 matching lines @@
             base::TimeDelta failure_entry_ttl);
 
   ~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) const;
 
   // Overwrites or creates an entry for |key|. Returns the pointer to the
   // entry, or NULL on failure (fails if caching is disabled).
-  // (|error|, |addrlist|) is the value to set, and |now| is the current
-  // timestamp.
+  // (|error|, |addrlist|) is the value to set, |ttl| is the "time to live",

[cbentzel, 2012/01/13 01:06:14]

How much work would it be to only allow this version of Set and not have the ttl
== 0 be equivalent to "use the default"?

I wonder because DNS TTLs of 0 are potentially allowed. It would also make the
API easier - could remove the defaults in the constructor, and the description
of what |ttl| represents becomes clearer.

[szym, 2012/01/14 16:30:54]

(Excluding unittests) There are three calls to HostCache::Set:
1) HostResolverImpl::OnJobComplete
2) AsyncHostResolver::OnDnsRequestComplete
3) (ProxyResolverJSBindings) DefaultJSBindings::DnsResolveHelper

There are two calls to HostCache::HostCache where the default TTL values are
set:
1) HostCache::CreateDefaultCache // success: 1 min, failure: 0
2) ProxyResolverV8::GetProxyURL  // success: 5 min, failure: 5 min

So we could easily move the default TTL values from HostCache to the sites
calling HostCache::Set. 

Regarding the other issue, entries with TTLs of 0 will never be returned by
HostCache::Lookup, so the only benefit of storing such entries afaik is
net-internals, even though they won't survive for long if the cache is at
capacity.

+  // and |now| is the current timestamp. If |ttl| is zero, the default,
+  // |success_entry_ttl| or |failure_entry_ttl| is used.
   Entry* Set(const Key& key,
              int error,
              const AddressList& addrlist,
+             base::TimeDelta ttl,
+             base::TimeTicks now);
+
+  // Same as |Set(key, error, addrlist, TimeDelta(), now)|.
+  Entry* Set(const Key& key,
+             int error,
+             const AddressList& addrlist,
              base::TimeTicks now);
 
   // 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;
@@ skipping 34 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_