Re: [PATCH v3 1/6] configure: Look for auxiliary Python installations

[Daniel P . Berrangé]

On Tue, Feb 21, 2023 at 12:37:43PM -0500, John Snow wrote:
> On Tue, Feb 21, 2023, 6:03 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
> > On 2/21/23 02:24, John Snow wrote:
> > > At the moment, we look for just "python3" and "python", which is good
> > > enough almost all of the time. But ... if you are on a platform that
> > > uses an older Python by default and only offers a newer Python as an
> > > option, you'll have to specify --python=/usr/bin/foo every time.
> > >
> > > As a courtesy, we can make a cursory attempt to locate a suitable Python
> > > binary ourselves, looking for the remaining well-known binaries. This
> > > also has the added benefit of making configure "just work" more often
> > > on various BSD distributions that do not have the concept of a
> > > "platform default python".
> > >
> > > This configure loop will prefer, in order:
> > >
> > > 1. Whatever is specified in $PYTHON
> > > 2. python3
> > > 3. python (Which is usually 2.x, but might be 3.x on some platforms.)
> > > 4. python3.11 down through python3.6
> > >
> > > Notes:
> > >
> > > - Python virtual environments provide binaries for "python3", "python",
> > >    and whichever version you used to create the venv,
> > >    e.g. "python3.8". If configure is invoked from inside of a venv, this
> > >    configure loop will not "break out" of that venv unless that venv is
> > >    created using an explicitly non-suitable version of Python that we
> > >    cannot use.
> > >
> > > - In the event that no suitable python is found, the first python found
> > >    is the version used to generate the human-readable error message.
> > >
> > > - The error message isn't printed right away to allow later
> > >    configuration code to pick up an explicitly configured python.
> > >
> > > Signed-off-by: John Snow <jsnow@redhat.com>
> > > ---
> > >   configure | 34 ++++++++++++++++++++++++++--------
> > >   1 file changed, 26 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/configure b/configure
> > > index cf6db3d5518..6abf5a72078 100755
> > > --- a/configure
> > > +++ b/configure
> > > @@ -592,20 +592,40 @@ esac
> > >
> > >   : ${make=${MAKE-make}}
> > >
> > > -# We prefer python 3.x. A bare 'python' is traditionally
> > > -# python 2.x, but some distros have it as python 3.x, so
> > > -# we check that too
> > > +
> > > +check_py_version() {
> > > +    # We require python >= 3.6.
> > > +    # NB: a True python conditional creates a non-zero return code
> > (Failure)
> > > +    "$1" -c 'import sys; sys.exit(sys.version_info < (3,6))'
> > > +}
> > > +
> > >   python=
> > > +first_python=
> > >   explicit_python=no
> > > -for binary in "${PYTHON-python3}" python
> > > +# Check for $PYTHON, python3, python, then explicitly-versioned
> > interpreters.
> > > +for binary in "${PYTHON-python3}" ${PYTHON:+python3} python \
> > > +                                  python3.11 python3.10 python3.9 \
> > > +                                  python3.8 python3.7 python3.6
> >
> > I think if PYTHON is set we shouldn't look at anything else.
> >
> > Paolo
> >
> 
> PYTHON is one we made up, right?

$PYTHON is explicitly set in all our dockerfiles. We should
ensure we honour it and not fallback to anything else when
it is set. ie it would be a user error to set it to point
to a python that is missing/broken, so the user should
expect an error, not fallback to another version.


With regards,
Daniel
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