[zeromq-dev] Propose removal of ZMQ_IDENTITY_FD socket option from libzmq 4.1rc and trunk

[Thomas Rodgers]

I would like to propose removing this option before it becomes part of an
officially released API, but I would like to reassurance that this is an
appropriate course of action before doing so.

My reasoning for removing it is as follows -

* It is the only option to zmq_getsockopt() that treats option_value as a
value/result parameter, all others treat option_value as strictly a result
parameter.

* A brief survey of the Posix getsockopt() API shows a similar lack of
using option_value as a value/result parameter.

* The original implementation requires the caller to ensure that the
returned buffer is sufficient to hold an fd_t, but fd_t is not part of
ZeroMQ's public API. It is conditionally defined by platform and on Windows
has two potential definitions.

* I'm not comfortable with this blind faith assignment through a pointer,
so I submitted a length check change. Unfortunately this introduces a whole
new class of potential usage problems for this option.

* I don't know what the intended use case for the option is, but I infer
from the test_id2fd test case, that is to build a map of router identity to
fd, perhaps to pass fd to getpeername(2) and build a map of identity to
peername. If this is indeed the case, this use case is already handled by
calling zmq_msg_get(&part, ZMQ_SRCFD), on the message part containing the
identity frame.

What am I missing here?
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