Adding cache hit/miss histograms to DiskBasedCertCache.

net/http/disk_based_cert_cache.cc

 // Copyright (c) 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/http/disk_based_cert_cache.h"
 
 #include <vector>
 
 #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"
 #include "net/base/net_errors.h"
 #include "net/disk_cache/disk_cache.h"
 
 namespace net {
 
 namespace {
 
 // TODO(brandonsalmon): change this number to improve performance.
 const size_t kMemoryCacheMaxSize = 30;
 
 // Used to obtain a unique cache key for a certificate in the form of
 // "cert:<hash>".
 std::string GetCacheKeyToCert(const X509Certificate::OSCertHandle cert_handle) {
   SHA1HashValue fingerprint =
       X509Certificate::CalculateFingerprint(cert_handle);
 
   return "cert:" +
          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
 // certificate requested to be cached is assigned a Writeworker on a one-to-one
 // basis. The same certificate should not have multiple WriteWorkers at the same
 // time; instead, add a user callback to the existing WriteWorker.
 class DiskBasedCertCache::WriteWorker {
  public:
   // |backend| is the backend to store |certificate| in, using
   // |key| as the key for the disk_cache::Entry.
@@ skipping 376 matching lines @@
 
   return rv;
 }
 
 int DiskBasedCertCache::ReadWorker::DoOpen() {
   state_ = STATE_OPEN_COMPLETE;
   return backend_->OpenEntry(key_, &entry_, io_callback_);
 }
 
 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;
 }
 
 int DiskBasedCertCache::ReadWorker::DoRead() {
   state_ = STATE_READ_COMPLETE;
   io_buf_len_ = entry_->GetDataSize(0 /* index */);
   buffer_ = new IOBuffer(io_buf_len_);
   return entry_->ReadData(
@@ skipping 56 matching lines @@
 }
 
 void DiskBasedCertCache::Get(const std::string& key, const GetCallback& cb) {
   DCHECK(!key.empty());
 
   // If the handle is already in the MRU cache, just return that (via callback).
   // Note, this will also bring the cert_handle to the front of the recency
   // 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);
 
   if (it == read_worker_map_.end()) {
     ReadWorker* worker =
         new ReadWorker(backend_,
                        key,
                        base::Bind(&DiskBasedCertCache::FinishedReadOperation,
                                   weak_factory_.GetWeakPtr(),
@@ skipping 41 matching lines @@
 
 void DiskBasedCertCache::FinishedWriteOperation(
     const std::string& key,
     X509Certificate::OSCertHandle cert_handle) {
   write_worker_map_.erase(key);
   if (!key.empty())
     mru_cert_cache_.Put(key, X509Certificate::DupOSCertHandle(cert_handle));
 }
 
 }  // namespace net