Re: [Qemu-devel] [PATCH 1/4] configure: keep track of Python version

[Peter Maydell <peter.maydell@linaro.org>]

On Fri, 9 Nov 2018 at 15:09, Cleber Rosa <crosa@redhat.com> wrote:
>
> Some functionality is dependent on the Python version
> detected/configured on configure.  While it's possible to run the
> Python version later and check for the version, doing it once is
> preferable.  Also, it's a relevant information to keep in build logs,
> as the overall behavior of the build can be affected by it.
>
> Signed-off-by: Cleber Rosa <crosa@redhat.com>
> ---
>  configure | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/configure b/configure
> index 74e313a810..67fff0290d 100755
> --- a/configure
> +++ b/configure
> @@ -1740,6 +1740,9 @@ if ! $python -c 'import sys; sys.exit(sys.version_info < (2,7))'; then
>        "Use --python=/path/to/python to specify a supported Python."
>  fi
>
> +# Preserve python version since some functionality is dependent on it
> +python_version=$($python -V 2>&1 | sed -e 's/Python\ //')
> +

Hi. Somebody on IRC has just fallen over a problem where
their python's "-V" output prints multiple lines, which
means that "$python_version" here is multiple lines, which
means that the eventual config-host.mak has invalid syntax
because we assume here:

> @@ -6823,6 +6826,7 @@ echo "INSTALL_DATA=$install -c -m 0644" >> $config_host_mak
>  echo "INSTALL_PROG=$install -c -m 0755" >> $config_host_mak
>  echo "INSTALL_LIB=$install -c -m 0644" >> $config_host_mak
>  echo "PYTHON=$python" >> $config_host_mak
> +echo "PYTHON_VERSION=$python_version" >> $config_host_mak
>  echo "CC=$cc" >> $config_host_mak
>  if $iasl -h > /dev/null 2>&1; then
>    echo "IASL=$iasl" >> $config_host_mak

that it's only one line, and will generate bogus makefile
syntax if it's got an embedded newline. (Problem system
seems to be Fedora 29.)

I've reread this thread, where there seems to have been
some discussion about just running Python itself to
get the sys.version value (which is how we check for
"is this python too old" earlier in the configure script).
But I'm not really clear why trying to parse -V output is better:
it's definitely less reliable, as demonstrated by this bug.

Given that the only thing as far as I can tell that we
do with PYTHON_VERSION is use it in tests/Makefile.inc
to suppress a bit of test functionality if we don't have
Python 3, could we stop trying to parse -V output and run
python to print sys.version_info instead, and/or just
have the makefile variable track "is this python 2",
since that's what we really care about and would mean we
don't have to then search the string for "v2"  ?

thanks
-- PMM