Seek to beginning of IOBuffer before returning it

[ikirill]

It’s impossible to read from an IOBuffer that was just written to: its
position is set to its end and reading from it results in EOFError.

[ViralBShah]

@ikirill Do we need to merge this?

[vtjnash]

An IOStream and IOBuffer are incompatible types

[ViralBShah]

So close this?

[vtjnash]

Suggested change

	function Base.convert(::Type{IOStream}, zmsg::Message)
	s = IOBuffer()
	write(s, zmsg)
	seek(s, 0)
	function Base.convert(::Union{Type{IO},Type{IOBuffer}}, zmsg::Message)
	s = IOBuffer()
	write(s, zmsg)
	seekstart(s)

[vtjnash]

I am not sure, but convert seems like an odd predicate here. IOBuffer usually takes things like Message as an argument to the constructor to make an "IO-like" view of something

[JamesWrigley]

I'm with @vtjnash here, it seems like a bug (or at least a footgun) that convert(IOStream, msg) returns an IOBuffer. I would suggest that we:

1. Deprecate the Base.convert(::IOStream, ::Message) method entirely since it's just not doing the right thing.
2. Document that people can call IOBuffer(msg) to create a zero-copy IOBuffer of a Message. This works out of the box because Message <: AbstractArray.

@ikirill, are you able to finish the PR? I can if not.

[ViralBShah]

@ikirill doesn't seem to be too active in the last year.

[ikirill]

Hi!

I don't remember anything about this PR at this point, and I gave up on trying to use ZeroMQ a very long time ago.