exewrapper: find the proper python3X.dll in the registry

Authored by mharbison72.


exewrapper: find the proper python3X.dll in the registry

Previously, we relied on the default library lookup[1], which for us is
essentially to look on PATH. That has issues- the Python installations are
not necessarily on PATH, so I started copying the DLLs locally in 2960b7fac966
and ed286d150aa8 during the build to work around that. However, it's been
discovered that causes python3.dll and python3X.dll to get slipped into the
wheel that gets distributed on PyPI. Additionally, Mercurial would fail to run
in a venv if the Python environment that created it isn't on PATH, because
venv creation doesn't copy the DLLs locally.

The logic here is inspired by the py.exe launcher[2], though this is simpler
because we don't care about the architecture- if this is a 32 bit process
running on Win64, the registry reflection will redirect to where the 32 bit
Python process wrote its keys. A nice unintended side effect is to also make
venvs that don't have their root Python on PATH work without all of the code
required to read pyvenv.cfg[3]. I don't see any reasonable way to create a
venv without Python being installed (other than maybe building Python from
source?), so punt on trying to read that file for now and save a bunch of string
manipulation code.

I somehow managed to corrupt my Windows user profile, and that makes the
Microsoft Store python not run (even loading the DLL gives an access error), so
I'm giving priority to both global and user specific python.org installations.

Loading python3.dll is new, but when I went down the rabbit hole of implementing
pyvenv.cfg support, I saw a comment[4] that led me to think we could have
trouble if we don't. The comment in ed286d150aa8 confirms this, so we should
probably bail out completely if Python3 can't be loaded from the registry,
rather than getting something random on PATH. But I'll leave that for the
default branch.

[1] https://docs.microsoft.com/en-us/windows/win32/Dlls/dynamic-link-library-search-order#standard-search-order-for-desktop-applications
[2] https://github.com/python/cpython/blob/adcd2205565f91c6719f4141ab4e1da6d7086126/PC/launcher.c#L249
[3] https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/PC/getpathp.c#L707
[4] https://github.com/python/cpython/blob/bb3e0c240bc60fe08d332ff5955d54197f79751c/PC/getpathp.c#L1098

Differential Revision: https://phab.mercurial-scm.org/D11454