Re: [PATCH v2 3/7] configure: Look for auxiliary Python installations

[Paolo Bonzini <pbonzini@redhat.com>]

On 2/10/23 01:31, 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.
> 
> We can be kind and instead make a cursory attempt to locate a suitable
> Python binary ourselves, looking for the remaining well-known binaries.
> 
> This configure loop will prefer, in order:
> 
> 1. Whatever is specified in $PYTHON
> 2. python3
> 3. python
> 4. python3.11 down through python3.6
> 
> Notes:
> 
> - Python virtual environment provides 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 | 33 +++++++++++++++++++++++++--------
>   1 file changed, 25 insertions(+), 8 deletions(-)
> 
> diff --git a/configure b/configure
> index 64960c6000f..ea8c973d13b 100755
> --- a/configure
> +++ b/configure
> @@ -592,20 +592,39 @@ 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
> +# A bare 'python' is traditionally python 2.x, but some distros
> +# have it as python 3.x, so check in both places.
> +for binary in "${PYTHON-python3}" python python3.{11..6}

This is not available in e.g. dash, so we need to use {11,10,9,8,7,6}.
Just a nit, I can fix it myself.

Paolo