Downloading a big file from QNetworkDiskCache can be very slow and CPU-intensive

When developing for a QtWebKit2 -based browser I noticed that a file download can take 100% CPU for many seconds. This happens when the file is downloaded from QNetworkDiskCache and isn't fully read in a directly connected slot to QNetworkReply::readyRead() signal. I attached a test app demonstrating this code path (that is hit in QtWebKit2) as "slowcachedownload.tar.gz".

The problem comes down to this: the file is read into a QByteArray which is appended into QNetworkReplyHttpImplPrivate::downloadMultiBuffer. From there it is read via QIODevice::readAll() 16kB at a time, and the remaining data is copied to the start of the array each time.

I made a fix that eliminates unnecessary copying by adding a read position variable to QByteDataBuffer. A fix and a unit test that includes benchmarks is attached as "qbytedatabuffer.tar.gz". The fix adds very little overhead to the best case scenario and improves the worst case scenario I encountered with a ~30MB file from 10s user time to practically 0s.