mkvenv: add console script entry point generation

When creating a virtual environment that inherits system packages,
script entry points (like "meson", "sphinx-build", etc) are not
re-generated with

mkvenv: add console script entry point generation

When creating a virtual environment that inherits system packages,
script entry points (like "meson", "sphinx-build", etc) are not
re-generated with the correct shebang. When you are *inside* of the
venv, this is not a problem, but if you are *outside* of it, you will
not have a script that engages the virtual environment appropriately.

Add a mechanism that generates new entry points for pre-existing
packages so that we can use these scripts to run "meson",
"sphinx-build", "pip", unambiguously inside the venv.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-9-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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mkvenv: add --diagnose option to explain "ensure" failures

This is a routine that is designed to print some usable info for human
beings back out to the terminal if/when "mkvenv ensure" fails to loc

mkvenv: add --diagnose option to explain "ensure" failures

This is a routine that is designed to print some usable info for human
beings back out to the terminal if/when "mkvenv ensure" fails to locate
or install a package during configure time, such as meson or sphinx.

Since we are requiring that "meson" and "sphinx" are installed to the
same Python environment as QEMU is configured to build with, this can
produce some surprising failures when things are mismatched. This method
is here to try and ease that sting by offering some actionable
diagnosis.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-8-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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mkvenv: add ensure subcommand

This command is to be used to add various packages (or ensure they're
already present) into the configure-provided venv in a modular fashion.

Examples:

mkvenv ensure

mkvenv: add ensure subcommand

This command is to be used to add various packages (or ensure they're
already present) into the configure-provided venv in a modular fashion.

Examples:

mkvenv ensure --online --dir "${source_dir}/python/wheels/" "meson>=0.61.5"
mkvenv ensure --online "sphinx>=1.6.0"
mkvenv ensure "qemu.qmp==0.0.2"

It's designed to look for packages in three places, in order:

(1) In system packages, if the version installed is already good
enough. This way your distribution-provided meson, sphinx, etc are
always used as first preference.

(2) In a vendored packages directory. Here I am suggesting
qemu.git/python/wheels/ as that directory. This is intended to serve as
a replacement for vendoring the meson source for QEMU tarballs. It is
also highly likely to be extremely useful for packaging the "qemu.qmp"
package in source distributions for platforms that do not yet package
qemu.qmp separately.

(3) Online, via PyPI, ***only when "--online" is passed***. This is only
ever used as a fallback if the first two sources do not have an
appropriate package that meets the requirement. The ability to build
QEMU and run tests *completely offline* is not impinged.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-7-jsnow@redhat.com>
[Use distlib to lookup distributions. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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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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