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

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

Index: third_party/WebKit/Source/core/fetch/ResourceLoader.cpp
diff --git a/third_party/WebKit/Source/core/fetch/ResourceLoader.cpp b/third_party/WebKit/Source/core/fetch/ResourceLoader.cpp
index 0c6b3611c2b427cdc9156678b8d948ea885aeda8..a3ae8717540a5edba593bc480e1399bdefff9679 100644
--- a/third_party/WebKit/Source/core/fetch/ResourceLoader.cpp
+++ b/third_party/WebKit/Source/core/fetch/ResourceLoader.cpp
@@ -515,8 +515,15 @@ void ResourceLoader::requestSynchronously()
         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);
+
+    // Follow the async case convention of not calling didReceiveData or
+    // appending data to m_resource if the response body is empty. Copying the
+    // empty buffer is a noop in most cases, but is destructive in the case of
+    // a 304, where it will overwrite the cached data we should be reusing.
+    if (dataOut.size()) {
+        m_fetcher->didReceiveData(m_resource, dataOut.data(), dataOut.size(), encodedDataLength);
+        m_resource->setResourceBuffer(dataOut);
+    }
     didFinishLoading(0, monotonicallyIncreasingTime(), encodedDataLength);
 }