Do not sniff mime type for fetch() requests

content/browser/loader/resource_dispatcher_host_impl.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.
 
 // See http://dev.chromium.org/developers/design-documents/multi-process-resource-loading
 
 #include "content/browser/loader/resource_dispatcher_host_impl.h"
 
 #include <stddef.h>
 
@@ skipping 1697 matching lines @@
 
   // Add the post mime sniffing throttles.
   handler.reset(new ThrottlingResourceHandler(
       std::move(handler), request, std::move(post_mime_sniffing_throttles)));
 
   PluginService* plugin_service = nullptr;
 #if defined(ENABLE_PLUGINS)
   plugin_service = PluginService::GetInstance();
 #endif
 
-  // Insert a buffered event handler to sniff the mime type.
-  // Note: all ResourceHandler following the MimeSniffingResourceHandler should
-  // expect OnWillRead to be called *before* OnResponseStarted as part of the
-  // mime sniffing process.
-  handler.reset(new MimeSniffingResourceHandler(
-      std::move(handler), this, plugin_service, intercepting_handler, request));
+  // fetch() ignores mime sniffing.
+  if (fetch_request_context_type != REQUEST_CONTEXT_TYPE_FETCH) {

[mmenke, 2016/09/08 14:19:18]

Can we put this logic in MimeTypeHandler?  It already has some no-sniff paths,
and I'd rather not mix mime type magic here and there.

Also, what's the difference between "fetch_request_context_type ==
REQUEST_CONTEXT_TYPE_FETCH" and "resource_type == RESOURCE_TYPE_XHR"?

Doesn't look like there's any documentation for RequestContextType.

[yhirano, 2016/09/14 13:22:18]

Done.

RESOURCE_TYPE_XHR contains REQUEST_CONTENT_TYPE_FETCH. See
https://cs.chromium.org/chromium/src/content/child/web_url_request_util.cc?q=...
( WebURLRequest::RequestContext corresponds to ResourceContextType).

[mmenke, 2016/09/15 15:08:39]

Can we disable it for XHRs, too?  (There's actually an old filed bug about
that).  No idea about EventSources.

[yhirano, 2016/09/20 12:12:31]

I hope we can, but I expect that disabling the feature for fetch API is much
less controversial than disabling it for XHR because fetch is relatively new.
Separating changes would be good in case we need to re-enable it for XHR, I
think.

+    // Insert a buffered event handler to sniff the mime type.
+    // Note: all ResourceHandler following the MimeSniffingResourceHandler
+    // should expect OnWillRead to be called *before* OnResponseStarted as
+    // part of the mime sniffing process.
+    handler.reset(new MimeSniffingResourceHandler(
+        std::move(handler), this, plugin_service, intercepting_handler,
+        request));
+  }
 
   // Add the pre mime sniffing throttles.
   handler.reset(new ThrottlingResourceHandler(
       std::move(handler), request, std::move(pre_mime_sniffing_throttles)));
 
   return handler;
 }
 
 void ResourceDispatcherHostImpl::OnReleaseDownloadedFile(int request_id) {
   UnregisterDownloadedTempFile(filter_->child_id(), request_id);
@@ skipping 996 matching lines @@
         request_info->GetRouteID(), is_content_initiated, true, &throttles);
     if (!throttles.empty()) {
       handler.reset(new ThrottlingResourceHandler(std::move(handler), request,
                                                   std::move(throttles)));
     }
   }
   return handler;
 }
 
 }  // namespace content