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

[Thomas Huth <thuth@redhat.com>]

On 10/02/2023 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 "${PYTHON-python3}" python python3.{11..6}
>   do
>       if has "$binary"
>       then
>           python=$(command -v "$binary")
> -        break
> +        if test -z "$first_python"; then
> +           first_python=$python
> +        fi
> +        if check_py_version "$python"; then
> +            # This one is good.
> +            first_python=
> +            break
> +        fi
>       fi
>   done
>   
> +# If first_python is set, we didn't find a suitable binary.
> +# Use this one for possible future error messages.
> +if test -n "$first_python"; then
> +    python="$first_python"
> +fi

>   # Check for ancillary tools used in testing
>   genisoimage=
> @@ -1037,9 +1056,7 @@ then
>       error_exit "GNU make ($make) not found"
>   fi
>   
> -# Note that if the Python conditional here evaluates True we will exit
> -# with status 1 which is a shell 'false' value.
> -if ! $python -c 'import sys; sys.exit(sys.version_info < (3,6))'; then
> +if ! check_py_version "$python"; then
>     error_exit "Cannot use '$python', Python >= 3.6 is required." \
>         "Use --python=/path/to/python to specify a supported Python."
>   fi

I like the idea!

Reviewed-by: Thomas Huth <thuth@redhat.com>