[net] Add AsyncDNS.TTL histogram.

net/base/host_resolver_impl.cc

Index: net/base/host_resolver_impl.cc
diff --git a/net/base/host_resolver_impl.cc b/net/base/host_resolver_impl.cc
index d9e1aa2b7e79fa2e2eafe49c6e54f2ec36fee623..27bc0e5e3a1aa3cc76a3f19bfdd5f45729fdf834 100644
--- a/net/base/host_resolver_impl.cc
+++ b/net/base/host_resolver_impl.cc
@@ -205,6 +205,10 @@ void RecordTotalTime(bool had_dns_config,
   }
 }
 
+void RecordTTL(base::TimeDelta ttl) {
+  UMA_HISTOGRAM_CUSTOM_COUNTS("AsyncDNS.TTL", ttl.InSeconds(), 1, 86400, 100);
+}
+
 //-----------------------------------------------------------------------------
 
 // Wraps call to SystemHostResolverProc as an instance of HostResolverProc.
@@ -1464,12 +1468,7 @@ class HostResolverImpl::Job : public PrioritizedDispatcher::Job {
       }
     }
 
-    base::TimeDelta ttl = base::TimeDelta::FromSeconds(
-        kNegativeCacheEntryTTLSeconds);
-    if (net_error == OK)
-      ttl = base::TimeDelta::FromSeconds(kCacheEntryTTLSeconds);
-
-    CompleteRequests(net_error, addr_list, ttl);
+    CompleteRequests(net_error, addr_list, base::TimeDelta::FromSeconds(-1));

[cbentzel, 2012/10/11 18:27:30]

At the point that the kNegativeCacheEntry/kCacheEntry replacement is done in
CompleteRequests, you lose a lot of context for why this is done.

It might be better to add an original_ttl or "real_ttl" arguments to
CompleteRequests to make it clearer.

   }
 
   void StartDnsTask() {
@@ -1958,6 +1957,8 @@ bool HostResolverImpl::ServeFromCache(const Key& key,
 
   *net_error = cache_entry->error;
   if (*net_error == OK) {
+    if (cache_entry->original_ttl >= base::TimeDelta())
+      RecordTTL(cache_entry->original_ttl);
     *addresses = cache_entry->addrlist;
     EnsurePortOnAddressList(info.port(), addresses);
   }
@@ -2001,8 +2002,25 @@ void HostResolverImpl::CacheResult(const Key& key,
                                    int net_error,
                                    const AddressList& addr_list,
                                    base::TimeDelta ttl) {
-  if (cache_.get())
-    cache_->Set(key, net_error, addr_list, base::TimeTicks::Now(), ttl);
+  bool have_ttl = (ttl >= base::TimeDelta());
+  if (!have_ttl) {
+    if (net_error == OK) {
+      ttl = base::TimeDelta::FromSeconds(kCacheEntryTTLSeconds);
+    } else {
+      ttl = base::TimeDelta::FromSeconds(kNegativeCacheEntryTTLSeconds);
+    }
+  }
+
+  if (cache_.get()) {
+    HostCache::Entry* entry = cache_->Set(
+        key, net_error, addr_list, base::TimeTicks::Now(), ttl);
+    if (have_ttl) {
+      RecordTTL(ttl);
+      entry->original_ttl = ttl;
+    } else {
+      entry->original_ttl = base::TimeDelta::FromSeconds(-1);
+    }
+  }
 }
 
 void HostResolverImpl::RemoveJob(Job* job) {