Clarify that const is being cast away

[tacaswell]

Closes #301

I should preface this with a disclaimer that my c++ is very rusty and this is the first time I have looked at this code base. I expect this is not the correct fix, but it compiles! I get one failure locally (see below).

python/cpython#91959 switch to using "new-style" c++ casts which are (apparently) stricter about casts.

One set of changes is to explicitly strip const when passing into the C API (to increase / decrease reference counts). I think this used to work because const-ness was being ignored before. Although this was fixed upstream to work, I think it is better to still have the explicit casting.

The other set of changes is to borrow the PyObject pointer from the greenlet wrapper classes. I think this used to work because the pointer was the first element in the wrapped objected so it is safe to directly cast it, however the new-style casting balks. (dropped this commit)

[tacaswell]

I get one test failure locally:

===================================================================================================== FAILURES =====================================================================================================
___________________________________________________________________________ TestGreenlet.test_dealloc_catches_GreenletExit_throws_other ____________________________________________________________________________

self = <greenlet.tests.test_greenlet.TestGreenlet testMethod=test_dealloc_catches_GreenletExit_throws_other>

    def test_dealloc_catches_GreenletExit_throws_other(self):
        def run():
            try:
                greenlet.getcurrent().parent.switch()
            except greenlet.GreenletExit:
                raise SomeError
    
        g = greenlet(run)
        g.switch()
        # Destroying the only reference to the greenlet causes it
        # to get GreenletExit; when it in turn raises, even though we're the parent
        # we don't get the exception, it just gets printed.
        # When we run on 3.8 only, we can use sys.unraisablehook
        oldstderr = sys.stderr
        try:
            from cStringIO import StringIO
        except ImportError:
            from io import StringIO
        stderr = sys.stderr = StringIO()
        try:
            del g
        finally:
            sys.stderr = oldstderr
    
        v = stderr.getvalue()
>       self.assertIn("Exception", v)
E       AssertionError: 'Exception' not found in ''

src/greenlet/tests/test_greenlet.py:214: AssertionError
================================================================================================= warnings summary =================================================================================================
src/greenlet/tests/test_greenlet.py::TestGreenlet::test_dealloc_catches_GreenletExit_throws_other
  /home/tcaswell/.virtualenvs/bleeding/lib/python3.11/site-packages/_pytest/unraisableexception.py:78: PytestUnraisableExceptionWarning: Exception ignored in: <greenlet.greenlet object at 0x7f6e668eddc0 (otid=0x7f6e668589f0) dead>
  
  Traceback (most recent call last):
    File "/home/tcaswell/source/p/python-greenlet/greenlet/src/greenlet/tests/test_greenlet.py", line 192, in run
      greenlet.getcurrent().parent.switch()
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  greenlet.GreenletExit: Killing the greenlet because all references have vanished.
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/home/tcaswell/source/p/python-greenlet/greenlet/src/greenlet/tests/test_greenlet.py", line 194, in run
      raise SomeError
      ^^^^^^^^^^^^^^^
  greenlet.tests.test_greenlet.SomeError
  
    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
============================================================================================= short test summary info ==============================================================================================
FAILED src/greenlet/tests/test_greenlet.py::TestGreenlet::test_dealloc_catches_GreenletExit_throws_other - AssertionError: 'Exception' not found in ''

[tacaswell]

9fe8746 is no longer needed due to CPython considering this a regression and fixing in python/cpython#92138

[tacaswell]

This looks like it going to be fully fixed upstream, but I think you should keep the first two commits (to make in clear in the code that const is being discarded even though CPython will do the discarding for you).

[jamadden]

Alright, thank you! I should actually have some time to look closely at this "soon"; I'm in the process of getting the time allotted by $WORK.

[tacaswell]

python/cpython#92951 is the upstream PR to watch.

[vstinner]

python/cpython#92951 is the upstream PR to watch.

It got merged, so I suggest closing this PR which will no longer be needed with the future Python 3.11 beta2.

[tacaswell]

I dropped the commit that did the borrrowing, but I still think that it would be good to the explicit const casting in greenlets for clarity (rather than compatibility).

Leaving this to the greenlet developers to take or leave at their discretion.

[vstinner]

I suggest you dropping the usage of const PyObject* in greenlet. See python/cpython#91768