Ruby 3.4: `zmq_proxy` thread never terminates, process hangs

[kojix2]

Hello Chuck Remes (@chuckremes),

Thank you very much for maintaining ffi-rzmq.
Since Ruby 3.4, I have observed unexpected behavior with ffi-rzmq.

When running IRuby’s test suite under Ruby 3.4, the process hangs after all tests pass and never terminates.
Dumping thread backtraces shows that the zmq_proxy call remains blocked and does not return.

Below is a minimal reproduction without using IRuby.

Steps to Reproduce

1. Install Ruby:

   rbenv install 3.3.8
   rbenv install 3.4.5

2. Save the following code as hang.rb:

   require "ffi-rzmq"
   
   ctx    = ZMQ::Context.new
   front  = ctx.socket(ZMQ::REP)
   back   = ctx.socket(ZMQ::REQ)
   front.bind("inproc://front")
   back.bind("inproc://back")
   
   Thread.new do
     ZMQ::Device.new(front, back)
   end
   
   sleep 1
   puts "Exiting main thread"

3. Run:

   ruby hang.rb

4. Compare the results:

   • Ruby 3.3.8: Process exits normally after printing Exiting main thread.
   • Ruby 3.4.5: Process does not exit and remains blocked in the zmq_proxy call.

This issue does not occur on Ruby 3.3.x.
It may be related to thread or shutdown handling changes in Ruby 3.4.
Any guidance or insights on this behavior would be greatly appreciated.

Thanks in advance for looking into this!

[chuckremes]

This is tricky. It's been at least a decade since I had to debug a problem like that. My recollection from when earlier versions of the gem hung on MRI it was usually related to the finalizer. See here:

https://github.com/chuckremes/ffi-rzmq/blob/master/lib/ffi-rzmq/context.rb#L123

If you want to work on this, I recommend commenting out the context finalizer code and running the test to see if it hangs.

Similarly, comment out the finalizer code on the socket and see if that helps.

https://github.com/chuckremes/ffi-rzmq/blob/master/lib/ffi-rzmq/socket.rb#L598

If the hang goes away, then the finalizer behavior has changed or closure behavior has changed.