Measure DNS resolution waste from not always performing AF_UNSPEC lookups.

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 9ec0a379e197f59bebc3e506545fc55a973e4f6c..01d49fca75cd7e644607c23418bdc9603787458a 100644
--- a/net/base/host_resolver_impl.cc
+++ b/net/base/host_resolver_impl.cc
@@ -1822,8 +1822,43 @@ bool HostResolverImpl::ServeFromCache(const Key& key,
   if (!info.allow_cached_response() || !cache_.get())
     return false;
 
-  const HostCache::Entry* cache_entry = cache_->Lookup(
-      key, base::TimeTicks::Now());
+  base::TimeTicks current_time = base::TimeTicks::Now();
+  const HostCache::Entry* cache_entry = cache_->Lookup(key, current_time);
+
+  bool found = cache_entry != NULL;

[szym, 2012/08/07 23:17:32]

I suggest adding braces around this whole block so that it's apparent that all
of this serves just one histogram, and is easier to remove in the future.

+  bool ipv4 = key.address_family == ADDRESS_FAMILY_IPV4;
+  // If we couldn't find the desired address family, check to see if the other
+  // family is in the cache, which indicates waste, and we should fix
+  // crbug.com/139811.
+  bool found_other_family = false;
+  if (!found && default_address_family_ == ADDRESS_FAMILY_UNSPECIFIED) {
+    Key other_family_key = key;
+    other_family_key.address_family = ipv4 ?
+        ADDRESS_FAMILY_UNSPECIFIED : ADDRESS_FAMILY_IPV4;
+    found_other_family =
+        cache_->Lookup(other_family_key, current_time) != NULL;
+  }
+  enum {  // Used in HISTOGRAM_ENUMERATION.
+    CACHE_IPV4_ONLY_FOUND,
+    CACHE_IPV4_ONLY_MISS,
+    CACHE_FOUND_IPV4,
+    CACHE_FOUND_UNSPEC,
+    CACHE_WASTE_IPV4,
+    CACHE_WASTE_UNSPEC,
+    CACHE_MISS_IPV4,
+    CACHE_MISS_UNSPEC,
+    CACHE_MAX,  // Bounding value.
+  } category = CACHE_MAX;
+  if (default_address_family_ != ADDRESS_FAMILY_UNSPECIFIED)
+    category = found ? CACHE_IPV4_ONLY_FOUND : CACHE_IPV4_ONLY_MISS;
+  else if (found)

[szym, 2012/08/07 23:17:32]

nit: Use braces whenever if has an else clause.

+    category = ipv4 ? CACHE_FOUND_IPV4 : CACHE_FOUND_UNSPEC;
+  else if (found_other_family)
+    category = ipv4 ? CACHE_WASTE_IPV4 : CACHE_WASTE_UNSPEC;
+  else
+    category = ipv4 ? CACHE_MISS_IPV4 : CACHE_MISS_UNSPEC;
+  UMA_HISTOGRAM_ENUMERATION("DNS.ResolveCacheCategory", category, CACHE_MAX);
+
   if (!cache_entry)
     return false;