bpo-1635741: port pyexpat to multi-phase init (PEP 489)

[koubaa]

https://bugs.python.org/issue1635741

[koubaa]

@vstinner @corona10 @shihai1991 please review.

There's PyCapsule_New usage but the capsule methods did not depend on any globals so it isn't a concern in this case.

[shihai1991]

compile warning in here:

##[warning]/home/runner/work/cpython/cpython/Modules/pyexpat.c:1185:1: warning: ‘xmlparse_dealloc’ defined but not used [-Wunused-function]
 xmlparse_dealloc(xmlparseobject *self)
 ^~~~~~~~~~~~~~~~

[koubaa]

compile warning in here:

##[warning]/home/runner/work/cpython/cpython/Modules/pyexpat.c:1185:1: warning: ‘xmlparse_dealloc’ defined but not used [-Wunused-function]
 xmlparse_dealloc(xmlparseobject *self)
 ^~~~~~~~~~~~~~~~

Oops, I forgot to add this to the type slots. Should be fixed now

[vstinner]

DECREF is needed on error, no? Same comment for the following PyModule_AddObject calls.

[vstinner]

I would prefer to "goto error" if PyModule_New() fails.

[vstinner]

Maybe it would be simpler to extract changes which add error handling to the exec function, and write a second PR to convert the module to multi-phase init. Since the existing code basically has no code to handle errors :-(

You can rename pyexpat_exec() to PyInit_pyexpat() and just add Py_DECREF(m) in "goto error".

[vstinner]

I would prefer to get PR #22489 merged before this one, to ease review ;-)

[koubaa]

@vstinner rebased and I think ready to go. Please review

[erlend-aasland]

Shouldn't handler_info also be a part of the module state?

[vstinner]

@shihai1991 made expat_CAPI per module instance in commit 7c83eaa.

I merged the PR, thanks @koubaa!

Shouldn't handler_info also be a part of the module state?

Hum. It doesn't contain anything dynamically allocated. It's a static array of static data. I don't think that we have to make it per instance.