rust-cpython: implement Graph using C parents function
We introduce the `Index` struct that wraps the C index. It is
not intrinsically protected by the GIL (see the lengthy
discussion in its docstring). Improving on this seems
prematurate at this point.
A pointer to the parents function is stored on the parsers
C extension module as a capsule object.
This is the recommended way to export a C API for consumption
from other extensions.
See also: https://docs.python.org/2.7/c-api/capsule.html
In our case, we use it in cindex.rs, retrieving function
pointer from the capsule and storing it within the CIndex
struct, alongside with a pointer to the index. From there,
the implementation is very close to the one from hg-direct-ffi.
The naming convention for the capsule is inspired from the
one in datetime:
>>> import datetime
>>> datetime.datetime_CAPI
<capsule object "datetime.datetime_CAPI" at 0x7fb51201ecf0>
although in datetime's case, the capsule points to a struct holding
several type objects and methods.
Differential Revision: https://phab.mercurial-scm.org/D5438
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).