Synchronous requests that get a 304 clobber the cached response body.

third_party/WebKit/Source/core/fetch/ResourceLoader.cpp

 /*
  * Copyright (C) 2006, 2007, 2010, 2011 Apple Inc. All rights reserved.
  *           (C) 2007 Graham Dennis (graham.dennis@gmail.com)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 498 matching lines @@
         }
         didFail(0, errorOut);
         return;
     }
     didReceiveResponse(0, responseOut);
     if (m_state == Terminated)
         return;
     RefPtr<ResourceLoadInfo> resourceLoadInfo = responseOut.toResourceResponse().resourceLoadInfo();
     int64_t encodedDataLength = resourceLoadInfo ? resourceLoadInfo->encodedDataLength : WebURLLoaderClient::kUnknownEncodedDataLength;
     m_fetcher->didReceiveData(m_resource, dataOut.data(), dataOut.size(), encodedDataLength);
-    m_resource->setResourceBuffer(dataOut);
+    if (dataOut.size())

[Yoav Weiss, 2015/12/31 06:29:17]

Isn't there a scenario where the response *has* changed to be a zero sized
response and should be clobbered? Wouldn't it be better the special case 304s?

[Nate Chapin, 2016/01/04 22:15:54]

In all non-revalidation cases, Resource::m_data will be empty at this point. If
a revalidation attempt returns something other than a 304,
Resource::revalidationFailed() will clear the buffer automatically, so calling
setResourceBuffer() again with an empty buffer will be a noop.

However, I just noticed that I accidnetally left the m_fetcher->didReceiveData()
call outside of the if(), which seems wrong for the 304 case. Updated that.

[Yoav Weiss, 2016/01/05 14:13:03]

What's the relationship between Resource::m_data and dataOut?
so `dataOut.size()` is non-zero in all cases other than 304?
Could you add a comment making that explicit?

+        m_resource->setResourceBuffer(dataOut);
     didFinishLoading(0, monotonicallyIncreasingTime(), encodedDataLength);
 }
 
 ResourceRequest& ResourceLoader::applyOptions(ResourceRequest& request) const
 {
     request.setAllowStoredCredentials(m_options.allowCredentials == AllowStoredCredentials);
     return request;
 }
 
 }