mkvenv: add nested venv workaround

Python virtual environments do not typically nest; they may inherit from
the top-level system packages or not at all.

For our purposes, it would be convenient to

mkvenv: add nested venv workaround

Python virtual environments do not typically nest; they may inherit from
the top-level system packages or not at all.

For our purposes, it would be convenient to emulate "nested" virtual
environments to allow callers of the configure script to install
specific versions of python utilities in order to test build system
features, utility version compatibility, etc.

While it is possible to install packages into the system environment
(say, by using the --user flag), it's nicer to install test packages
into a totally isolated environment instead.

As detailed in https://www.qemu.org/2023/03/24/python/, Emulate a nested
venv environment by using .pth files installed into the site-packages
folder that points to the parent environment when appropriate.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-6-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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mkvenv: add better error message for broken or missing ensurepip

Debian debundles ensurepip for python; NetBSD debundles pyexpat but
ensurepip needs pyexpat. Try our best to offer a helpful error me

mkvenv: add better error message for broken or missing ensurepip

Debian debundles ensurepip for python; NetBSD debundles pyexpat but
ensurepip needs pyexpat. Try our best to offer a helpful error message
instead of just failing catastrophically.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-5-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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python: add mkvenv.py

This script will be responsible for building a lightweight Python
virtual environment at configure time. It works with Python 3.6 or
newer.

It has been designed to:
- work *of

python: add mkvenv.py

This script will be responsible for building a lightweight Python
virtual environment at configure time. It works with Python 3.6 or
newer.

It has been designed to:
- work *offline*, no PyPI required.
- work *quickly*, The fast path is only ~65ms on my machine.
- work *robustly*, with multiple fallbacks to keep things working.
- work *cooperatively*, using system packages where possible.
(You can use your distro's meson, no problem.)

Due to its unique position in the build chain, it exists outside of the
installable python packages in-tree and *must* be runnable without any
third party dependencies.

Under normal circumstances, the only dependency required to execute this
script is Python 3.6+ itself. The script is *faster* by several seconds
when setuptools and pip are installed in the host environment, which is
probably the case for a typical multi-purpose developer workstation.

In the event that pip/setuptools are missing or not usable, additional
dependencies may be required on some distributions which remove certain
Python stdlib modules to package them separately:

- Debian may require python3-venv to provide "ensurepip"
- NetBSD may require py310-expat to provide "pyexpat" *
(* Or whichever version is current for NetBSD.)

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-4-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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