[WIP] bpo-35081: Include/internal/ becomes Include/pycore/

[vstinner]

• Rename Include/internal/ directory to Include/pycore/
• Rename Include/pycore/xxx.h to Include/pycore/pycore_xxx.h
• Include/internal/mem.h renamed to Include/pycore/pycore_objimpl.h
• "xxx.h" now includes "pycore/pycode_xxx.h" if Py_BUILD_CORE is
  defined: it is no longer needed to include directly "pycore_xxx.h".
  For example, include "pycore/pycore_objimpl.h" from "objimpl.h".
• objimpl.h now includes pycore_objimpl.h just after defining
  PyGC_Head type. Replace Remove _PyGC_generation0: replaced by
  _PyRuntime.gc.generation0.

https://bugs.python.org/issue35081

[vstinner]

I'm not sure if header files in Include/pycore/ must require or not Py_BUILD_CORE to be defined.

[vstinner]

I added [WIP] to my PR title and the DO-NOT-MERGE label because I wrote this PR to "show" my proposed change, but I'm not sure about the overall directory hierarchy. I would prefer to wait the approval of a few core devs before fixing/reworking this PR :-)

[ncoghlan]

I like the direction you're taking this, and I like the idea of having an "#ifndef PY_BUILD_CORE -> compile error" stanza in each of the pycore internal headers - that way the compiler will help us out in keeping private APIs private, rather than relying on good intentions and human code review.

[vstinner]

I like the direction you're taking this, and I like the idea of having an "#ifndef PY_BUILD_CORE -> compile error" stanza in each of the pycore internal headers - that way the compiler will help us out in keeping private APIs private, rather than relying on good intentions and human code review.

Ah thanks! I wasn't 100% sure about this change.

See discussion on python-dev. It was decided to not automatically include the internal API from the public API (Python.h), like:

#ifdef Py_BUILD_CORE
#  include "pycore/pycore_context.h"
#endif

Instead, we have to explicitly include the private header: #include "internal/pystate.h".

Since this PR implemented an approach which has been rejected, I close it.

I started to push smaller changes with the decided approach: see https://bugs.python.org/issue35081