Support coverage plugins with sys.monitoring

[scoder]

Is your feature request related to a problem? Please describe.

Running coverage.py on Cython code with sys.monitoring in Py3.13 disables the Cython coverage plugin:
https://github.com/nedbat/coveragepy/blob/7f336228d78d8338239d4da4d4de3ade0abd0901/coverage/control.py#L565-L568

Describe the solution you'd like
Plugins should work the same with the old tracing and new monitoring infrastructure.

Additional context
I'm currently working on getting Cython to use the monitoring C-API, but found that coverage.py doesn't support both together as it stands.

[scoder]

Ping @nedbat. Any hint on what's missing to make this work?

[nedbat]

Hi, sorry to let this sit. What plugin behavior in particular are you looking for? The key idea of sys.monitoring that makes it fast is to disable an event once it's fired, but some plugin methods would need the events to be re-enabled. This is extra choreography that I haven't really thought through yet, and I hope it doesn't completely nullify the speed benefits.

[scoder]

Our coverage plugin is here:
https://github.com/cython/cython/blob/master/Cython/Coverage.py

We don't need access to events, we just do file lookups and line matching.

[oscarbenjamin]

With Python 3.14 this now just crashes:

$ python --version
Python 3.14.0rc2
$ coverage --version
Coverage.py, version 7.10.6 with C extension
Full documentation is at https://coverage.readthedocs.io/en/7.10.6
$ cython --version
Cython version 3.1.3
$ PYTHONPATH=src coverage run --rcfile=.coveragerc.setuptools -m flint.test
~/python-flint/.venv_314/lib/python3.14/site-packages/coverage/control.py:619: CoverageWarning: Plugin file tracers (Cython.Coverage.Plugin) aren't supported with SysMonitor
  self._warn(
Traceback (most recent call last):
  File "/stuff/current/active/python-flint/.venv_314/bin/coverage", line 10, in <module>
    sys.exit(main())
             ~~~~^^
  File "/stuff/current/active/python-flint/.venv_314/lib/python3.14/site-packages/coverage/cmdline.py", line 1135, in main
    status = CoverageScript().command_line(argv)
  File "/stuff/current/active/python-flint/.venv_314/lib/python3.14/site-packages/coverage/cmdline.py", line 834, in command_line
    return self.do_run(options, args)
           ~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/stuff/current/active/python-flint/.venv_314/lib/python3.14/site-packages/coverage/cmdline.py", line 1023, in do_run
    runner.run()
    ~~~~~~~~~~^^
  File "/stuff/current/active/python-flint/.venv_314/lib/python3.14/site-packages/coverage/execfile.py", line 174, in run
    self._prepare2()
    ~~~~~~~~~~~~~~^^
  File "/stuff/current/active/python-flint/.venv_314/lib/python3.14/site-packages/coverage/execfile.py", line 139, in _prepare2
    pathname, self.package, self.spec = find_module(self.modulename)
                                        ~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/stuff/current/active/python-flint/.venv_314/lib/python3.14/site-packages/coverage/execfile.py", line 49, in find_module
    spec = importlib.util.find_spec(modulename)
  File "<frozen importlib.util>", line 90, in find_spec
  File "/stuff/current/active/python-flint/src/flint/__init__.py", line 1, in <module>
    from .pyflint import ctx
  File "src/flint/pyflint.pyx", line 1, in init flint.pyflint
    """
  File "src/flint/flint_base/flint_base.pyx", line 1, in init flint.flint_base.flint_base
    from flint.flintlib.types.flint cimport (
  File "src/flint/flint_base/flint_context.pyx", line 203, in init flint.flint_base.flint_context
    cdef FlintContext thectx = FlintContext()
  File "src/flint/flint_base/flint_context.pyx", line 12, in flint.flint_base.flint_context.FlintContext.__init__
    def __init__(self):
  File "/stuff/current/active/python-flint/.venv_314/lib/python3.14/site-packages/coverage/sysmon.py", line 364, in sysmon_py_start
    sys_monitoring.set_local_events(self.myid, code, local_events)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
SystemError: cannot instrument shim code object '__init__'

Is that a different issue?