service worker: Optimize response header creation.

content/browser/service_worker/service_worker_url_request_job.cc

 // Copyright 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 "content/browser/service_worker/service_worker_url_request_job.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <map>
@@ skipping 661 matching lines @@
   response_is_in_cache_storage_ = response.is_in_cache_storage;
   response_cache_storage_cache_name_ = response.cache_storage_cache_name;
 }
 
 void ServiceWorkerURLRequestJob::CreateResponseHeader(
     int status_code,
     const std::string& status_text,
     const ServiceWorkerHeaderMap& headers) {
   // TODO(kinuko): If the response has an identifier to on-disk cache entry,
   // pull response header from the disk.
-  std::string status_line(
-      base::StringPrintf("HTTP/1.1 %d %s", status_code, status_text.c_str()));
-  status_line.push_back('\0');
-  http_response_headers_ = new net::HttpResponseHeaders(status_line);
-  for (ServiceWorkerHeaderMap::const_iterator it = headers.begin();
-       it != headers.end();
-       ++it) {
-    std::string header;
-    header.reserve(it->first.size() + 2 + it->second.size());
-    header.append(it->first);
-    header.append(": ");
-    header.append(it->second);
-    http_response_headers_->AddHeader(header);
+
+  // Build a string instead of using HttpResponseHeaders::AddHeader on
+  // each header, since AddHeader has O(n^2) performance.
+  std::string buf(base::StringPrintf("HTTP/1.1 %d %s\r\n", status_code,
+                                     status_text.c_str()));
+  for (const auto& item : headers) {
+    buf.append(item.first);
+    buf.append(": ");
+    buf.append(item.second);
+    buf.append("\r\n");
   }
+  buf.append("\r\n");
+  http_response_headers_ = new net::HttpResponseHeaders(
+      net::HttpUtil::AssembleRawHeaders(buf.c_str(), buf.size()));
 }
 
 void ServiceWorkerURLRequestJob::CommitResponseHeader() {
   if (!http_response_info_)
     http_response_info_.reset(new net::HttpResponseInfo());
   http_response_info_->headers.swap(http_response_headers_);
   http_response_info_->vary_data = net::HttpVaryData();
   http_response_info_->metadata =
       blob_reader_ ? blob_reader_->response_metadata() : nullptr;
   NotifyHeadersComplete();
@@ skipping 201 matching lines @@
   DCHECK(!reported_navigation_preload_metrics_);
   reported_navigation_preload_metrics_ = true;
 
   ServiceWorkerMetrics::RecordNavigationPreloadResponse(
       worker_ready_time_ - worker_start_time_,
       navigation_preload_response_time_ - worker_start_time_,
       initial_worker_status_, worker_start_situation_);
 }
 
 }  // namespace content