support gevent-using apps when workers>2 after #2183

[markjm]

Hi - thank you so much for this projects, its been great for me to work with!

Below is a reported issue and suggested fix for compatibility with the (not-so-new anymore) multiprocessing work from #2183 and applications which use gevent.

Issue

I have this gist which demonstrated the issue https://gist.github.com/markjm/7d63260afa0012532151c935ca3b1640

Notice when running uv run demo.py --patch-parent (to simulate a project where gevent monkeypatches are in use), we see an error like:

Exception in thread Thread-1 (always_pong):
Traceback (most recent call last):
  File "/home/mmolinaro/.local/share/uv/python/cpython-3.12-linux-x86_64-gnu/lib/python3.12/threading.py", line 1075, in _bootstrap_inner
    self.run()
  File "/home/mmolinaro/.local/share/uv/python/cpython-3.12-linux-x86_64-gnu/lib/python3.12/threading.py", line 1012, in run
    self._target(*self._args, **self._kwargs)
  File "/home/mmolinaro/code/uvicorn/uvicorn/supervisors/multiprocess.py", line 51, in always_pong
    self.pong()
  File "/home/mmolinaro/code/uvicorn/uvicorn/supervisors/multiprocess.py", line 46, in pong
    self.child_conn.recv()
  File "/home/mmolinaro/.local/share/uv/python/cpython-3.12-linux-x86_64-gnu/lib/python3.12/multiprocessing/connection.py", line 250, in recv
    buf = self._recv_bytes()
          ^^^^^^^^^^^^^^^^^^
  File "/home/mmolinaro/.local/share/uv/python/cpython-3.12-linux-x86_64-gnu/lib/python3.12/multiprocessing/connection.py", line 430, in _recv_bytes
    buf = self._recv(4)
          ^^^^^^^^^^^^^
  File "/home/mmolinaro/.local/share/uv/python/cpython-3.12-linux-x86_64-gnu/lib/python3.12/multiprocessing/connection.py", line 395, in _recv
    chunk = read(handle, remaining)
            ^^^^^^^^^^^^^^^^^^^^^^^
BlockingIOError: [Errno 11] Resource temporarily unavailable

I believe this is due to gevent patching (which makes most multiprocessing/threading primitives cooperative) interacting with multiprocess.Pipe which (according to this comment) can not be patched.

Note also that we can’t delay monkeypatching into the worker thread because the spawned process inherits a copy of the parent’s sys.modules + modules uvicorn imports before getting to user code. These already-imported modules need to be imported after patching, but that is not possible. This can be simulated via uv run demo.py --patch-worker

Suggested solution

Instead of ping-ponging with a multiprocess.Pipe, use a socket.socketpair instead. Sockets are gevent-patch-supported, so this change in management communication should be enough to ensure any projects like mine to work again after the multiprocessing changes.

I have a draft of something which I think has potential viable here main...markjm:uvicorn:markjm/multi-processing

If im in the right track, change seems not-too intrusive to me!

Alternative solutions

TBH I am no expert in this area, so also open to other ideas, suggestions, or you can just tell me this is outside the realm of something youre interested in fixing