Signals and Eventloop going wrong

This is a small repository to reproduce this error:

RuntimeError: cannot release un-acquired lock

It is based on an actual bug I saw in production systems, it took some time to debug, and the explanation is quite interesting. You can read more in the article here.

Getting started

requirements

You will need to have python and poetry installed on your system.

installing the dependencies

To install the python dependencies run

$ poetry install

run the examples

To execute the example run

$ poetry run gevent_example.py

Eventually you will see one of the following exceptions on the console. The first one will stop the program, and happen eventually, within seconds, but it can take up to a minute.

Traceback (most recent call last):
  File "gevent_example.py", line 41, in <module>
    thread.join()
  File "src/gevent/greenlet.py", line 831, in gevent._gevent_cgreenlet.Greenlet.join
  File "src/gevent/greenlet.py", line 857, in gevent._gevent_cgreenlet.Greenlet.join
  File "src/gevent/greenlet.py", line 846, in gevent._gevent_cgreenlet.Greenlet.join
  File "src/gevent/_greenlet_primitives.py", line 61, in gevent._gevent_c_greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "src/gevent/_greenlet_primitives.py", line 61, in gevent._gevent_c_greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "src/gevent/_greenlet_primitives.py", line 65, in gevent._gevent_c_greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "src/gevent/_gevent_c_greenlet_primitives.pxd", line 35, in gevent._gevent_c_greenlet_primitives._greenlet_switch
gevent.exceptions.LoopExit: This operation would block forever
	Hub: <Hub '' at 0x7f4ffacb6ac0 epoll default pending=0 ref=0 fileno=3 thread_ident=0x7f4ffffda180>
	Handles:
[]

The other two are more rare, you might see them every 10 runs, althouh I haven't collected enough samples to have high confidency in this frequency. They also don't halt the program.

Traceback (most recent call last):
  File "src/gevent/greenlet.py", line 906, in gevent._gevent_cgreenlet.Greenlet.run
  File "gevent_example2.py", line 21, in continuous_print
    _LOCK.release()
  File "/home/lucas/.pyenv/versions/3.8.5/lib/python3.8/threading.py", line 179, in release
    raise RuntimeError("cannot release un-acquired lock")
RuntimeError: cannot release un-acquired lock
2021-09-13T19:31:17Z <Greenlet at 0x7fac5ae05040: continuous_print(2)> failed with RuntimeError

Traceback (most recent call last):
  File "src/gevent/greenlet.py", line 906, in gevent._gevent_cgreenlet.Greenlet.run
  File "gevent_example.py", line 20, in continuous_counting
    _LOCK.release()
  File "/home/lucas/.pyenv/versions/3.8.5/lib/python3.8/threading.py", line 183, in release
    self._block.release()
  File "src/gevent/_semaphore.py", line 496, in gevent._gevent_c_semaphore.BoundedSemaphore.release
  File "src/gevent/_semaphore.py", line 502, in gevent._gevent_c_semaphore.BoundedSemaphore.release
RuntimeError: Semaphore released too many times
2021-09-13T20:13:40Z <Greenlet at 0x7f2a76d7ebf0: continuous_counting(0)> failed with RuntimeError