Provide mutable members of UrlRequestContext via pure-virtual interface

chrome/browser/net/chrome_http_user_agent_settings.cc

+// Copyright (c) 2012 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/net/chrome_http_user_agent_settings.h"
+
+#include "chrome/browser/prefs/pref_service.h"
+#include "chrome/common/pref_names.h"
+#include "content/public/browser/browser_thread.h"
+#include "content/public/common/content_client.h"
+#include "net/http/http_util.h"
+
+ChromeHttpUserAgentSettings::ChromeHttpUserAgentSettings(PrefService* prefs) {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+  pref_accept_language_.Init(prefs::kAcceptLanguages, prefs, NULL);
+  pref_accept_charset_.Init(prefs::kDefaultCharset, prefs, NULL);
+  last_pref_accept_language_ = *pref_accept_language_;
+  last_http_accept_language_ =
+      net::HttpUtil::GenerateAcceptLanguageHeader(last_pref_accept_language_);
+  last_pref_accept_charset_ = *pref_accept_charset_;
+  last_http_accept_charset_ =
+      net::HttpUtil::GenerateAcceptCharsetHeader(last_pref_accept_charset_);
+  pref_accept_language_.MoveToThread(content::BrowserThread::IO);
+  pref_accept_charset_.MoveToThread(content::BrowserThread::IO);
+}
+
+ChromeHttpUserAgentSettings::~ChromeHttpUserAgentSettings() {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::IO));
+}
+
+void ChromeHttpUserAgentSettings::CleanupOnUIThread() {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));

[erikwright (departed), 2012/10/26 21:10:46]

Is there any kind of guarantee that GetAcceptLanguage or GetAcceptCharset won't
be called anymore?

Technically PrefMember says you shouldn't "use" it after calling Destroy.

[pauljensen, 2012/10/26 22:04:14]

No.
True.  Given the current implementation it will work fine after calling
Destroy() (given that we do unconditionally call operator*() prior to this) but
we're going against the comment so there's no guarantee this will continue to
work.  Sadly I see no test for PrefMember::Destroy() at all :(

I wonder if this is a problem for the other PrefMembers in ProfileIOData.
Possible resolutions:
1. Add some sort of guarantee we don't read the PrefMembers after Destroy()ing
them.  This would be easy and cheap if we had the ThreadSafeBool class I've
wanted.
2. Change PrefMember::Destroy() comments and add a unit test for this behavior. 
This is a little icky because I think it only continues to work if you've called
GetValue() prior.

+  pref_accept_language_.Destroy();
+  pref_accept_charset_.Destroy();
+}
+
+std::string ChromeHttpUserAgentSettings::GetAcceptLanguage() const {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::IO));
+  std::string new_pref_accept_language = *pref_accept_language_;
+  if (new_pref_accept_language != last_pref_accept_language_) {
+    last_http_accept_language_ =
+        net::HttpUtil::GenerateAcceptLanguageHeader(new_pref_accept_language);
+    last_pref_accept_language_ = new_pref_accept_language;
+  }
+  return last_http_accept_language_;
+}
+
+std::string ChromeHttpUserAgentSettings::GetAcceptCharset() const {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::IO));
+  std::string new_pref_accept_charset = *pref_accept_charset_;
+  if (new_pref_accept_charset != last_pref_accept_charset_) {
+    last_http_accept_charset_ =
+        net::HttpUtil::GenerateAcceptCharsetHeader(new_pref_accept_charset);
+    last_pref_accept_charset_ = new_pref_accept_charset;
+  }
+  return last_http_accept_charset_;
+}
+
+std::string ChromeHttpUserAgentSettings::GetUserAgent(const GURL& url) const {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::IO));
+  return content::GetUserAgent(url);
+}
+