ZMQ_DISH socket type does not support ZMQ_FD getsockopt

[staticfloat]

Issue description

In trying to use the ZMQ_DISH draft socket type, I am unable to use getsockopt with ZMQ_FD to get the underlying file descriptor for usage with an external event system. This appears to be the case because _thread_safe is set to true in the parent socket_base_t constructor here. Setting _thread_safe implies a denied ZMQ_FD request, as seen here.

I do not understand the internals of ZMQ well enough to understand why thread safety denies ZMQ_FD, but my guess would be because _thread_safe originally meant ZMQ_PAIR, and running over the inproc transport would naturally not have an OS-level file descriptor. From what I can tell of the ZMQ_DISH type however, it appears a file descriptor could be expected.

Without the ability to integrate these sockets into an external event loop, efforts to port these draft socket types over to language bindings such as ZMQ.jl will be hampered, as we use the ability to watch file descriptors as a central part of our concurrency model.

Environment

• libzmq version: 4.3.2
• OS: Linux x86_64

[stale]

This issue has been automatically marked as stale because it has not had activity for 365 days. It will be closed if no further activity occurs within 56 days. Thank you for your contributions.

[staticfloat]

Please do not mark this as stale, stalebot! :)