bpo-34170: Rework _PyCoreConfig_Read() to avoid side effect

[vstinner]

Rework _PyCoreConfig_Read() function which reads core configuration
to not modify the path configuration.

A new _PyCoreConfig_SetPathConfig() function now recreates the path
configuration from the core configuration. This function is now
called very late in _Py_InitializeCore(), just before calling
initimport().

Changes:

• Add _PyCoreConfig.dll_path
• Rename _PyPathConfig_Calculate() to _PyPathConfig_Calculate_impl()
• Replace _PyPathConfig_Init() with _PyPathConfig_Calculate(): the
  function now requires a _PyPathConfig
• Add _PyPathConfig_SetGlobal() to set the _Py_path_config global
  variable.
• Add _PyCoreConfig_InitPathConfig(): compute the path configuration
• Add _PyCoreConfig_SetPathConfig(): set path configuration from core
  configuration
• Rename wstrlist_append() to _Py_wstrlist_append()

https://bugs.python.org/issue34170

[vstinner]

I don't think that "Add _PyCoreConfig.dll_path" needs a public changelog entry, since the structure is still private.

[ncoghlan]

+1 for avoiding side effects when reading the configuration, and the general structure here looks good to me (I didn't look closely at the internals of the relocated functions, though)

[vstinner]

Hum, maybe copy_wstr() should be used. I'm not sure if these fields can be NULL or not.

[ncoghlan]

While I can't think of a way this could cause an externally visible change in behaviour, if you wanted to backport this to Python 3.7 to keep the code structure consistent, then I think it would need a public change log entry so it appeared in the 3.7.1 notes.

If it remains 3.8 only though, then yeah, I agree it would be covered by the eventual change of making the new initialisation API public.

[vstinner]

I see this change as an enhancement rather than a bugfix. The change is large, I would be risky to backport it. I don't want to backport this change, I wrote it for the master branch only. It's part of a larger effort to fix Py_Main() called after Py_Initialize(): apply the new Py_Main() configuration (not sure if this PR really makes things easier, but I wanted to do this change anyway :-D).

[ZackerySpytz]

This commit causes a warning with Clang 5.0.0 (as seen in the TravisCI build):

In file included from Parser/pgenmain.c:20:
./Include/internal/pystate.h:55:3: warning: redefinition of typedef '_PyPathConfig' is a C11 feature [-Wtypedef-redefinition]
} _PyPathConfig;
  ^
./Include/pylifecycle.h:123:30: note: previous definition is here
typedef struct _PyPathConfig _PyPathConfig;
                             ^
1 warning generated.

[vstinner]

This commit causes a warning with Clang 5.0.0 (as seen in the TravisCI build):

It should have been fixed now ;-)

[fsrrt]

https://github.com/toy/rbenv-update-rubies.wiki.git