Persist info about subresources on pages for DNS pre-resolution

The DNS pre-resolution system already "learns" what domains are commonly
needed when rendering sub-resources of a page at a given domain.
This patch saves (some of) the information learned into a persistent
pref, and restores it on startup.

For now, I put in a wimpy pruning of the list each time I save, so that
the list will not grow endlessly from session to session.  I probably need
a better pruning algorithm, such as one that prunes after a given amount
of time, rather than only during shutdown.  For now, this should get
a lot of nice results, and provide slightly larger than needed lists to
users that have long lived sessions, which is similar to the current 
performance, where I didn't persist any info, and only pruned (actually
discarded) all learned info at shutdown.

r=mbelshe
Review URL: http://codereview.chromium.org/21374

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@9912 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 29 insertions, 3 deletions

diff --git a/chrome/browser/net/dns_global.cc b/chrome/browser/net/dns_global.cc
index b34de8a..67e4265 100644
--- a/chrome/browser/net/dns_global.cc
+++ b/chrome/browser/net/dns_global.cc
@@ -60,6 +60,7 @@ void OnTheRecord(bool enable) {
 
 void RegisterPrefs(PrefService* local_state) {
   local_state->RegisterListPref(prefs::kDnsStartupPrefetchList);
+  local_state->RegisterListPref(prefs::kDnsHostReferralList);
 }
 
 void RegisterUserPrefs(PrefService* user_prefs) {
@@ -73,8 +74,7 @@ static DnsMaster* dns_master;
 // This API is only used in the browser process.
 // It is called from an IPC message originating in the renderer.  It currently
 // includes both Page-Scan, and Link-Hover prefetching.
-// TODO(jar): Separate out link-hover prefetching, and histogram results
-// separately.
+// TODO(jar): Separate out link-hover prefetching, and page-scan results.
 void DnsPrefetchList(const NameList& hostnames) {
   DnsPrefetchMotivatedList(hostnames, DnsHostInfo::PAGE_SCAN_MOTIVATED);
 }
@@ -221,7 +221,7 @@ void PrefetchObserver::OnFinishResolutionWithStatus(bool was_resolved,
     return;
   // TODO(jar): Don't add host to our list if it is a non-linked lookup, and
   // instead rely on Referrers to pull this in automatically with the enclosing
-  // page load.
+  // page load (once we start to persist elements of our referrer tree).
   StartupListAppend(navigation_info);
 }
 
@@ -466,6 +466,32 @@ void DnsPrefetchHostNamesAtStartup(PrefService* user_prefs,
                          DnsHostInfo::STARTUP_LIST_MOTIVATED);
 }
 
+//------------------------------------------------------------------------------
+// Functions to persist and restore host references, that are used to direct DNS
+// prefetch of names (probably) used in subresources when the major resource is
+// navigated towards.
+
+void SaveSubresourceReferrers(PrefService* local_state) {
+  if (NULL == dns_master)
+    return;
+  ListValue* referral_list =
+      local_state->GetMutableList(prefs::kDnsHostReferralList);
+  dns_master->SerializeReferrers(referral_list);
+}
+
+void RestoreSubresourceReferrers(PrefService* local_state) {
+  if (NULL == dns_master)
+    return;
+  ListValue* referral_list =
+      local_state->GetMutableList(prefs::kDnsHostReferralList);
+  dns_master->DeserializeReferrers(*referral_list);
+}
+
+void TrimSubresourceReferrers() {
+  if (NULL == dns_master)
+    return;
+  dns_master->TrimReferrers();
+}
 
 }  // namespace chrome_browser_net