Only use "sysmon" core when available (Python 3.12+)

[hugovk]

If COVERAGE_CORE=sysmon, but we don't have PEP 669's sys.monitoring available (i.e. Python 3.11 or older), then don't use sysmon, but use the other defaults as if it hadn't been set.

This allows easily setting a global environment variable on a CI to benefit from the performance improvement where available, and 3.11 and older won't error like this:

Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pytest/__main__.py", line 5, in <module>
    raise SystemExit(pytest.console_main())
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 197, in console_main
    code = main()
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 155, in main
    config = _prepareconfig(args, plugins)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 337, in _prepareconfig
    config = pluginmanager.hook.pytest_cmdline_parse(
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_hooks.py", line 501, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_manager.py", line 119, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 138, in _multicall
    raise exception.with_traceback(exception.__traceback__)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 121, in _multicall
    teardown.throw(exception)  # type: ignore[union-attr]
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/helpconfig.py", line 104, in pytest_cmdline_parse
    config = yield
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 102, in _multicall
    res = hook_impl.function(*args)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1094, in pytest_cmdline_parse
    self.parse(args)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1446, in parse
    self._preparse(args, addopts=addopts)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/config/__init__.py", line 1346, in _preparse
    self.hook.pytest_load_initial_conftests(
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_hooks.py", line 501, in __call__
    return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_manager.py", line 119, in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 138, in _multicall
    raise exception.with_traceback(exception.__traceback__)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 121, in _multicall
    teardown.throw(exception)  # type: ignore[union-attr]
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/warnings.py", line 148, in pytest_load_initial_conftests
    return (yield)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 121, in _multicall
    teardown.throw(exception)  # type: ignore[union-attr]
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/_pytest/capture.py", line 151, in pytest_load_initial_conftests
    yield
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pluggy/_callers.py", line 102, in _multicall
    res = hook_impl.function(*args)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pytest_cov/plugin.py", line 152, in pytest_load_initial_conftests
    plugin = CovPlugin(options, early_config.pluginmanager)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pytest_cov/plugin.py", line 203, in __init__
    self.start(engine.Central)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pytest_cov/plugin.py", line 225, in start
    self.cov_controller.start()
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pytest_cov/engine.py", line 44, in ensure_topdir_wrapper
    return meth(self, *args, **kwargs)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pytest_cov/engine.py", line 241, in start
    self.cov.start()
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/coverage/control.py", line 652, in start
    self._collector.start()
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/coverage/collector.py", line 373, in start
    self._start_tracer()
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/coverage/collector.py", line [32](https://github.com/hugovk/Pillow/actions/runs/7854326439/job/21434925668#step:10:33)7, in _start_tracer
    fn = tracer.start()
  File "/opt/hostedtoolcache/Python/3.10.13/x[64](https://github.com/hugovk/Pillow/actions/runs/7854326439/job/21434925668#step:10:65)/lib/python3.10/site-packages/coverage/sysmon.py", line 225, in start
    sys_monitoring.use_tool_id(self.myid, "coverage.py")
AttributeError: 'NoneType' object has no attribute 'use_tool_id'

[asmeurer]

Without this, you need to use something like this (in the jobs section) to enable this on GitHub Actions

      # Enable experimental faster sys.monitoring coverage for Python 3.12
      - name: Set COVERAGE_CORE=sysmon
        run: echo "COVERAGE_CORE=sysmon" >> $GITHUB_ENV
        if: matrix.python-version == 3.12

[nedbat]

It feels a bit odd to me to silently ignore the setting when it can't be used. Maybe we could issue a warning?

[asmeurer]

A warning seems like the best solution to me. That would alert people who might not realize that the option does nothing in Python 3.11, but still let you set the flag on CI without using complex workarounds.

[nedbat]

Thanks! I'll add the warning in the next commits.

[nedbat]

This is now released as part of coverage 7.4.2.

[hugovk]

Thanks! We get between 9% and 27% speedup for Pillow!

python-pillow/Pillow#7820