`remote_exec` hangs forever when remotely using `gevent` model only at python3

The problem only occurs at python3.

Environment:

• OS: Linux faa410b1f09b 4.19.121-linuxkit #1 SMP Thu Jan 21 15:36:34 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
• Python version: Python 3.7.5

Dependencies:

• gevent version: 21.1.2
• execnet version: 1.8.0

Code:

import execnet
# hangs forever when remote is gevent
execnet.set_execmodel('thread', 'gevent')
gw = execnet.makegateway()
print(gw)
print(gw.remote_status())

print('channel.send')
print (gw.remote_exec("channel.send(333)").receive())
print('everything done')

Problem: Above code hangs forever.

Stdout:

<Gateway id='gw0' receive-live, thread model, 0 active channels>
<RInfo 'execmodel=gevent, numchannels=0, numexecuting=0'>
channel.send

Debug log:

[89653] gw0 sent <Message STATUS channel=1 lendata=0>
[89653] gw0 [receiver-thread] RECEIVERTHREAD: starting to run
[89655] creating workergateway on <execnet.gateway_base.Popen2IO object at 0x1106a5820>
[89655] gw0-worker [serve] spawning receiver thread
[89655] gw0-worker [serve] joining receiver thread
[89655] gw0-worker waiting for receiver thread to finish
[89655] gw0-worker [receiver-thread] RECEIVERTHREAD: starting to run
[89655] gw0-worker [receiver-thread] received <Message STATUS channel=1 lendata=0>
[89655] gw0-worker sent <Message CHANNEL_DATA channel=1 lendata=73>
[89653] gw0 [receiver-thread] received <Message CHANNEL_DATA channel=1 lendata=73>
[89653] gw0 [receiver-thread] received <Message CHANNEL_CLOSE channel=1 lendata=0>
[89655] gw0-worker sent <Message CHANNEL_CLOSE channel=1 lendata=0>
[89653] gw0 1 channel.__del__
[89653] gw0 sent <Message CHANNEL_EXEC channel=3 lendata=31>
[89655] gw0-worker [receiver-thread] received <Message CHANNEL_EXEC channel=3 lendata=31>

The code hangs while executing remote_exec. The problem only occurs when remote thread model is gevent.

I guess the problem occurred because while gevent is rapidly syncing with python3, execnet was not maintained :C.

Maybe related to function mapping when making ExecModel:
https://github.com/pytest-dev/execnet/blob/894533a515baf526c35ea0fe525b4866c6824ab8/execnet/gateway_base.py#L112-L125

I know this repo is unmaintained. Hoping for receiving someone's help!

Reproduce the hang with the Python 3 example using execnet's thread-to-gevent configuration and the versions listed in the issue. Start by inspecting the ExecModel mapping in execnet/gateway_base.py around lines 112-125, then trace remote_exec through the receiver and worker threads. Done means the remote call returns 333 and the program reaches “everything done” without hanging.