Cronet: Use stale DNS cache entries experimentally.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 1fc712068a1667c17ac96b19d86f566c563fcfaa..5b2d2473e8f62a0272a70c69dd7997ce41726604 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9898,6 +9898,43 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="DNS.StaleHostResolver.NetworkEarly" units="ms">
+  <owner>juliatuttle@chromium.org</owner>
+  <summary>
+    When a DNS request made through StaleHostResolver returns, a stale result
+    was available, and the network responded before the stale delay, how much

[Mark P, 2016/08/08 19:16:21]

is it possible that the network responded at the same time?  In other words, is
there a hole into which some replies can fall that are neither the earlier nor
the later bucket?

[Julia Tuttle, 2016/08/09 12:50:32]

Yes; I've ensured that's included in NetworkEarly, and tweaked the text to note.

+    earlier it responded.
+  </summary>
+</histogram>
+
+<histogram name="DNS.StaleHostResolver.NetworkLate" units="ms">
+  <owner>juliatuttle@chromium.org</owner>
+  <summary>
+    When a DNS request made through StaleHostResolver returns, a stale result
+    was available, and the network responded later than the stale delay, how
+    much later it responded.
+  </summary>
+</histogram>
+
+<histogram name="DNS.StaleHostResolver.RequestOutcome"
+    enum="DNS.StaleHostResolverRequestOutcome">
+  <owner>juliatuttle@chromium.org</owner>
+  <summary>
+    When a DNS request made through StaleHostResolver returns or is canceled,
+    the outcome of the request.
+  </summary>
+</histogram>
+

[Mark P, 2016/08/08 19:16:21]

I don't see anything about errors here.  Is there basically no possibility of an
error in fetching a stale result?

[Julia Tuttle, 2016/08/09 12:50:32]

There's never the possibility of getting an error *fetching* it -- it's just
from an in-memory cache.

There *is* the possibility that the stale cached data *was* an error result
(although I think we store those with 0 TTL), or that the network result
eventually resulted in an error, but those aren't included in this histogram.

[Mark P, 2016/08/11 22:58:22]

Considering this, perhaps you want to add the word "successfully" to the above
histogram descriptions, e.g., "network successfully responded"?

[Julia Tuttle, 2016/08/16 13:53:38]

I have "both the stale and network results were successful" already. Is that
okay?

+<histogram name="DNS.StaleHostResolver.StaleAddressListDelta"
+    enum="DNS.AddressListDeltaType">
+  <owner>juliatuttle@chromium.org</owner>
+  <summary>
+    When a DNS request made through StaleHostResolver had a stale result and
+    both the stale and network results were successful, the difference between
+    the old and new address lists.
+  </summary>
+</histogram>
+
 <histogram name="DNS.TotalTime" units="ms">
   <owner>juliatuttle@chromium.org</owner>
   <summary>
@@ -71933,6 +71970,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="Updated stale entry"/>
 </enum>
 
+<enum name="DNS.StaleHostResolverRequestOutcome" type="int">
+  <int value="0"
+      label="Returned synchronously (cache, hosts, IP literal etc.)"/>
+  <int value="1" label="Returned network result; no stale result available."/>
+  <int value="2" label="Returned network result; stale result available."/>
+  <int value="3" label="Returned stale result; network was too slow."/>
+  <int value="4" label="Canceled; no stale result available."/>
+  <int value="5" label="Canceled; stale result available."/>

[Mark P, 2016/08/08 19:16:21]

I'm confused.  If the request was canceled, how would you know whether a stale
result was available, i.e., distinguish between 4 and 5?

[Julia Tuttle, 2016/08/09 12:50:32]

The stale results come from the cache, not from the network. I've clarified by
saying "stale cached result".

[Mark P, 2016/08/11 22:58:22]

Your clarified is nice for those histograms.  However, I was asking about the
DNS.StaleHostResolver.RequestOutcome histogram and this associated enum, neither
of which you revised/clarified.

[Julia Tuttle, 2016/08/16 13:53:38]

Oops! Clarified these in the same way (noted that "stale" was "cached").

+</enum>
+
 <enum name="DNSEmptyAddressListAndNoError" type="int">
   <int value="0" label="Error reported or Address List is not empty"/>
   <int value="1" label="Success reported but Address List is empty"/>