Do not sniff mime type for fetch() requests

content/browser/loader/resource_dispatcher_host_impl.cc

Index: content/browser/loader/resource_dispatcher_host_impl.cc
diff --git a/content/browser/loader/resource_dispatcher_host_impl.cc b/content/browser/loader/resource_dispatcher_host_impl.cc
index 21183c34ab6026001aeeccdfca30332555a1e1f1..34c2591daad77bd5c7126a9efe54c72981fcaca2 100644
--- a/content/browser/loader/resource_dispatcher_host_impl.cc
+++ b/content/browser/loader/resource_dispatcher_host_impl.cc
@@ -1715,12 +1715,16 @@ ResourceDispatcherHostImpl::AddStandardHandlers(
   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(