Intermittent checksum failures

[arnsholt]

When running the kernel, every so often my frontend craps out complaining about checksum failures. They come in two flavours: either the frontend expects the same checksum as the kernel claims to have transmitted (that is, the frontend gets the same checksum as was sent by the Perl code, but for some reason what it gets off the wire is different), but at other times the frontend gets the same checksum off the wire as the kernel sent, but computes a different checksum.

I'm not entirely sure what the cause is, but I suspect it's a NativeCall problem. The problem is basically that ZMQ doesn't copy anything in a call to zmq_msg_init_data. Instead it assumes that the caller will pass a callback which is invoked to free the data when ZMQ is done with it. The way Net::ZMQ does it currently, if the data can get clobbered if a GC gets triggered between the sending function returning and ZMQ completing the send process.