Certificate Transparency: Collect metrics on age of SCT vs STH

components/certificate_transparency/single_tree_tracker.h

Index: components/certificate_transparency/single_tree_tracker.h
diff --git a/components/certificate_transparency/single_tree_tracker.h b/components/certificate_transparency/single_tree_tracker.h
index f7afe72c3b449ed80396f12f54744cce005f38cf..8bc380bdc8ce06b789bc5b77dd6ca04b93c90239 100644
--- a/components/certificate_transparency/single_tree_tracker.h
+++ b/components/certificate_transparency/single_tree_tracker.h
@@ -26,6 +26,10 @@ struct SignedCertificateTimestamp;
 
 namespace certificate_transparency {
 
+namespace internal {
+extern const char kCanCheckForInclusionHistogramName[];

[Ryan Sleevi, 2016/07/20 20:34:00]

This does feel unnecessary, and it doesn't sound like Alexei was requiring it,
so I'd like to push back on this a little bit.

While I agree it's coupling a unittest to an implementation detail, my main
concerns are:

Putting it into an internal namespace still preserves global symbol visibility.
That is, it doesn't allow the compiler to help protect you from any mistakes.
While a static class member (private, with the unit test friended) would be
better for this, in that it provides a better guarantee (through
compiler-enforce visibility), the fact remains that if the unittest looks at
this histogram, it still remains that the unittest is testing the implementation
details of the class.

As such, I don't think there's much of a line between embedding the histogram
name itself in the test, without exposing any symbol, as you're still testing
implementation details. Arguably, by duplicating the symbol name, you're making
it clearer the contract of the unit tests expects a particular histogram named a
particular way to be set.

That's my longer justification for why I would say to duplicate in the test.

At the risk of code golfing, which may be an anti-pattern, it does seem the
duplication is the dominant form.

[Eran Messeri, 2016/07/21 17:53:43]

Done, duplicated in the test.

+}
+
 // Tracks the state of an individual Certificate Transparency Log's Merkle Tree.
 // A CT Log constantly issues Signed Tree Heads, for which every older STH must
 // be incorporated into the current/newer STH. As new certificates are logged,