Adding cache hit/miss histograms to DiskBasedCertCache.

net/http/disk_based_cert_cache.cc

Index: net/http/disk_based_cert_cache.cc
diff --git a/net/http/disk_based_cert_cache.cc b/net/http/disk_based_cert_cache.cc
index 606407999e65eb9339f67504aca2f1fb924573bf..a04bb9b0ff9331d1c025c11ac1dd9f4737b042b3 100644
--- a/net/http/disk_based_cert_cache.cc
+++ b/net/http/disk_based_cert_cache.cc
@@ -9,6 +9,7 @@
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/memory/ref_counted.h"
+#include "base/metrics/histogram.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "net/base/io_buffer.h"
@@ -32,6 +33,14 @@ std::string GetCacheKeyToCert(const X509Certificate::OSCertHandle cert_handle) {
          base::HexEncode(fingerprint.data, arraysize(fingerprint.data));
 }
 
+void RecordMemCacheHits(bool hit) {
+  UMA_HISTOGRAM_BOOLEAN("DiskBasedCertCache.MemCacheHits", hit);
+}
+
+void RecordDiskCacheHits(bool hit) {
+  UMA_HISTOGRAM_BOOLEAN("DiskBasedCertCache.DiskCacheHits", hit);
+}
+
 }  // namespace
 
 // WriteWorkers represent pending Set jobs in the DiskBasedCertCache. Each
@@ -428,8 +437,13 @@ int DiskBasedCertCache::ReadWorker::DoOpen() {
 }
 
 int DiskBasedCertCache::ReadWorker::DoOpenComplete(int rv) {
+  // Errors other than cache miss are not recorded as either a hit
+  // or a miss.
+  if (rv == ERR_CACHE_MISS)
+    RecordDiskCacheHits(false);
   if (rv < 0)
     return rv;

[wtc, 2014/07/09 14:55:36]

Nit: I would structure this as follows:

  if (rv < 0) {
    // Errors other than cache miss are not recorded as either a hit
    // or a miss.
    if (rv == ERR_CACHE_MISS)
      RecordDiskCacheHits(false);
    return rv;
  }

so that we don't do the rv == ERR_CACHE_MISS test when rv is OK. This is a very
minor issue, so please pick the form that you think is more readable.

+  RecordDiskCacheHits(true);

[wtc, 2014/07/09 14:55:36]

This means a corrupted disk cache entry is still reported as a cache hit. I
think this is reasonable.

 
   state_ = STATE_READ;
   return OK;
@@ -506,10 +520,12 @@ void DiskBasedCertCache::Get(const std::string& key, const GetCallback& cb) {
   // list in the MRU cache.
   MRUCertCache::iterator mru_it = mru_cert_cache_.Get(key);
   if (mru_it != mru_cert_cache_.end()) {
+    RecordMemCacheHits(true);
     ++mem_cache_hits_;
     cb.Run(mru_it->second);
     return;
   }
+  RecordMemCacheHits(false);
   ++mem_cache_misses_;
 
   ReadWorkerMap::iterator it = read_worker_map_.find(key);