[URLSession]Fix a crash in _HTTPBodyDataSource.getNextChunk() #854
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I came across this bug while investigating another URLSession failure. URLSession uses the CURLOPT_READFUNCTION callback to write data to a socket during uploads. This writing happens in chunks of 16K. The
getNextChunkis used to split the body which is wrapped in DispatchData into a writable chunk (typically 16K with libcurl) and the remaining data. When the remaining data is lesser than 16K, we do not split the data and send it as is, making sure that the next call togetNextChunkwill indicate completion (return length = 0). However, because we set data tonil, the last call togetNextChunkin a large transfer is bound to crash here.Instead of setting data to
nilwe can set it toDispatchData.empty. It looks like this was the intention in the original PR but we seemed to have introduced this bug while updating PR 299 for the new Dispatch API in Swift 3.