zmq_proxy and optional parameter

[albestro]

I'm not an expert both in Julia and zeromq and I was playing with this wrapper. My example uses the zmq_proxy, which is missing in the wrapper currently, and I managed to make it work.

I would have open a PR with a minimal proposal, but I faced an issue that I'd like to know from you Julia and ZMQ.jl experts about how to address it.

The wrapper function would be something similar to

function proxy(frontend::Socket, backend::Socket, context::Socket)
  ccall((:zmq_proxy, libzmq),
    Cint, (Ptr{Cvoid}, Ptr{Cvoid}, Ptr{Cvoid}),
    frontend, backend, context)
end

I have a few concerns about this function per se, but the main problem keeping me away from opening a PR is the fact that context may be NULL or a Socket, and I was not able to figure out a way in the ZMQ.jl design to create a "null" Socket (e.g. I may evaluate to add an additional Socket constructor that sets data = C_NULL, but then the problem is the Socket.pollfd, which in turn I don't know anything about).

[stevengj]

If you want to be able to pass C_NULL for context, you could just declare a separate two-argument method:

function proxy(frontend::Socket, backend::Socket)
  ccall((:zmq_proxy, libzmq),
    Cint, (Ptr{Cvoid}, Ptr{Cvoid}, Ptr{Cvoid}),
    frontend, backend, C_NULL)
end

[albestro]

If you want to be able to pass C_NULL for context, you could just declare a separate two-argument method:

function proxy(frontend::Socket, backend::Socket)
  ccall((:zmq_proxy, libzmq),
    Cint, (Ptr{Cvoid}, Ptr{Cvoid}, Ptr{Cvoid}),
    frontend, backend, C_NULL)
end

Aha! Thanks for the reply! I discarded that option because it does not follow DRY principle, but maybe it is the right way in Julia for a simple thing like this.

I will eventually open a PR with this simple proposal.
Thanks again!

[JamesWrigley]

So we now have an automatically-wrapped lib.zmq_proxy:

ZMQ.jl/src/bindings.jl

Lines 358 to 360 in e66c836

	function zmq_proxy(frontend_, backend_, capture_)
	ccall((:zmq_proxy, libzmq), Cint, (Ptr{Cvoid}, Ptr{Cvoid}, Ptr{Cvoid}), frontend_, backend_, capture_)
	end

Which you should be able to call like e.g. lib.zmq_proxy(s1, s2, s3) or lib.zmq_proxy(s1, s2, C_NULL). Do you still need a high-level wrapper? If so I'd probably PR something like:

function proxy(frontend::Socket, backend::Socket, capture=Union{Nothing, Socket})
    capture_arg = isnothing(capture) ? C_NULL : capture
    lib.zmq_proxy(frontend, backend, capture_arg)
end

[stevengj]

Because zmq_proxy never returns, it's a bit tricky to call in a useful way. See the hoops that IJulia jumps through to call it in a separate thread: https://github.com/JuliaLang/IJulia.jl/blob/60568fabbe8abd54ae26b2acf2c426c830ed801b/src/heartbeat.jl#L13-L32

Might be nice to have a wrapper that helps with some of this in ZMQ.jl.

[JamesWrigley]

Hum, interesting. What do you think about a pure Julia implementation of proxy()? It's a little unfortunate to duplicate the functionality but it might be a bit safer.