Reduce the false positive rate for SafeBrowsing gethash requests

chrome/browser/safe_browsing/protocol_manager.cc

 // Copyright (c) 2006-2008 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 "chrome/browser/safe_browsing/protocol_manager.h"
 
 #include "base/file_version_info.h"
 #include "base/histogram.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
@@ skipping 178 matching lines @@
   }
 
   HashRequests::iterator it = hash_requests_.find(source);
   if (it != hash_requests_.end()) {
     // GetHash response.
     fetcher.reset(it->first);
     SafeBrowsingService::SafeBrowsingCheck* check = it->second;
     std::vector<SBFullHashResult> full_hashes;
     bool can_cache = false;
     if (response_code == 200 || response_code == 204) {
+      // For tracking our GetHash false positive (204) rate, compared to real
+      // (200) responses.
+      if (response_code == 200)
+        UMA_HISTOGRAM_COUNTS("SB2.GetHash200", 1);
+      else
+        UMA_HISTOGRAM_COUNTS("SB2.GetHash204", 1);
       can_cache = true;
       gethash_error_count_ = 0;
       gethash_back_off_mult_ = 1;
       bool re_key = false;
       SafeBrowsingProtocolParser parser;
       parsed_ok = parser.ParseGetHash(data.data(),
                                       static_cast<int>(data.length()),
                                       client_key_,
                                       &re_key,
                                       &full_hashes);
@@ skipping 414 matching lines @@
 void SafeBrowsingProtocolManager::HandleGetHashError() {
   int next = GetNextBackOffTime(&gethash_error_count_, &gethash_back_off_mult_);
   next_gethash_time_ = Time::Now() + TimeDelta::FromSeconds(next);
 }
 
 void SafeBrowsingProtocolManager::UpdateFinished(bool success) {
   UMA_HISTOGRAM_COUNTS("SB2.UpdateSize", update_size_);
   update_size_ = 0;
   sb_service_->UpdateFinished(success);
 }