Re: [PATCH 0/4] btrfs-progs: Compiling warning fixes for devel branch

[Omar Sandoval <osandov@osandov.com>]

On Mon, Nov 18, 2019 at 11:47:41PM +0000, Qu WenRuo wrote:
> 
> 
> On 2019/11/19 上午2:27, Omar Sandoval wrote:
> > On Mon, Nov 18, 2019 at 02:30:48PM +0800, Qu Wenruo wrote:
> >> We have several compiling errors, in devel branch.
> >> One looks like a false alert from compiler, the first patch will
> >> workaround it.
> >>
> >> 3 warning from libbtrfsutils are due to python3.8 changes.
> >> Handle it properly by using designated initialization, which also saves
> >> us quite some lines.
> >>
> >> Qu Wenruo (4):
> >>   btrfs-progs: check/lowmem: Fix a false alert on uninitialized value
> >>   btrfs-progs: libbtrfsutil: Convert to designated initialization for
> >>     BtrfsUtilError_type
> >>   btrfs-progs: libbtrfsutil: Convert to designated initialization for
> >>     QgroupInherit_type
> >>   btrfs-progs: libbtrfsutil: Convert to designated initialization for
> >>     SubvolumeIterator_type
> >>
> >>  check/mode-common.c             |  2 +-
> >>  libbtrfsutil/python/error.c     | 49 ++++++++-------------------------
> >>  libbtrfsutil/python/qgroup.c    | 43 ++++++-----------------------
> >>  libbtrfsutil/python/subvolume.c | 44 ++++++-----------------------
> >>  4 files changed, 30 insertions(+), 108 deletions(-)
> > 
> > Thanks for fixing the libbtrfsutil parts. For some reason, the
> > convention for Python C extensions is to not use designated
> > initializers, but after this breakage it's definitely safer to use them.
> > 
> > I guess Dave already merged these, but FWIW,
> > 
> > Reviewed-by: Omar Sandoval <osandov@fb.com>
> > 
> A small question inspired by this bug, but not specific to btrfs.
> 
> Does this mean each big python upgrade makes all c-binding binary
> incompatible? Or this is just a hiccup for this python3.8 release?
> 
> If it's the former case, that doesn't look correct to me...

In general, each major Python version may change the C extension ABI,
which is why the Python version is encoded in the file name:

/usr/lib/python3.8/site-packages/btrfsutil.cpython-38-x86_64-linux-gnu.so

Usually they maintain source-level compatibility, but it appears that
CPython itself uses 0 for its unused initializers rather than NULL, so
they probably just missed this.