Apple

I have a Mac OS X application running on 10.11 that sometimes hangs while performing a network upload. The upload is performed over HTTPs using NSURLConnection that has a bodyStream associated with it. The body stream is one end of a CFStream created with CFStreamCreateBoundPair. The other end of the stream is written to periodically as data becomes available. When I get an NSStreamHasSpaceAvailable delegate message, I append the data I have available in my stream.

This all works well 90% of the time, but around 1 in 20 uploads will deadlock with the backtrace below. It looks like CFStream is calling my delegate recursively—frames 10 and 4 are both in my delegate callback, for the same stream, with the same event code (hasBytesAvailable). This in itself is weird, but it causes a deadlock because boundPairWrite must be implemented with a mutex that is taken recursively.

In frame 10, I write all the data I have at the moment—152 bytes. This isn’t large, and doesn’t fill up the transfer size associated with the bound stream (which is 4K at the moment). Best I can tell, CFStream decides that since I didn’t consume all the space, it can send another hasBytesAvailable delegate message.