Certificate Transparency: Collect metrics on age of SCT vs STH

components/certificate_transparency/single_tree_tracker.cc

Index: components/certificate_transparency/single_tree_tracker.cc
diff --git a/components/certificate_transparency/single_tree_tracker.cc b/components/certificate_transparency/single_tree_tracker.cc
index 7946208ca753303a7589260e7a60fd34a58bb5e0..dcd28a52335dfdb0ad4d5bdf0da687a96fd765ec 100644
--- a/components/certificate_transparency/single_tree_tracker.cc
+++ b/components/certificate_transparency/single_tree_tracker.cc
@@ -6,6 +6,7 @@
 
 #include <utility>
 
+#include "base/metrics/histogram_macros.h"
 #include "net/cert/ct_log_verifier.h"
 #include "net/cert/signed_certificate_timestamp.h"
 #include "net/cert/x509_certificate.h"
@@ -14,6 +15,22 @@ using net::ct::SignedTreeHead;
 
 namespace certificate_transparency {
 
+namespace internal {
+
+const char kCanCheckForInclusionHistogramName[] =
+    "Net.CertificateTransparency.CanInclusionCheckSCT";
+
+}  // namespace internal
+
+namespace {
+
+void LogCanBeCheckedForInclusionToUMA(bool can_be_checked) {
+  UMA_HISTOGRAM_BOOLEAN(internal::kCanCheckForInclusionHistogramName,
+                        can_be_checked);
+}
+
+}  // namespace
+
 SingleTreeTracker::SingleTreeTracker(
     scoped_refptr<const net::CTLogVerifier> ct_log)
     : ct_log_(std::move(ct_log)) {}
@@ -34,15 +51,18 @@ void SingleTreeTracker::OnSCTVerified(
   if (verified_sth_.timestamp.is_null() ||
       (verified_sth_.timestamp <
        (sct->timestamp + base::TimeDelta::FromHours(24)))) {
-    // TODO(eranm): UMA - how often SCTs have to wait for a newer STH for
-    // inclusion check.
     entries_status_.insert(
         std::make_pair(sct->timestamp, SCT_PENDING_NEWER_STH));
+
+    // Do not log histogram if there's no STH for this log yet, as it does
+    // not provide any meaningful data on how fresh SCTs usually are.
+    if (!verified_sth_.timestamp.is_null())
+      LogCanBeCheckedForInclusionToUMA(false);
     return;
   }
 
+  LogCanBeCheckedForInclusionToUMA(true);
   // TODO(eranm): Check inclusion here.
-  // TODO(eranm): UMA - how often inclusion can be checked immediately.
   entries_status_.insert(
       std::make_pair(sct->timestamp, SCT_PENDING_INCLUSION_CHECK));
 }