Re: [PATCH v5 2/6] refs/files-backend: stop setting errno from lock_ref_oid_basic

[Han-Wen Nienhuys <hanwen@google.com>]

On Sun, Jul 11, 2021 at 1:48 PM Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
>
>
> On Wed, Jul 07 2021, Han-Wen Nienhuys via GitGitGadget wrote:
>
> >  /*
> >   * Locks a ref returning the lock on success and NULL on failure.
> > - * On failure errno is set to something meaningful.
> >   */
> >  static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
> >                                          const char *refname,
> > @@ -922,7 +921,6 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
> >  {
> >       struct strbuf ref_file = STRBUF_INIT;
> >       struct ref_lock *lock;
> > -     int last_errno = 0;
> >       int mustexist = (old_oid && !is_null_oid(old_oid));
> >       int resolve_flags = RESOLVE_REF_NO_RECURSE;
> >       int resolved;
> > @@ -949,7 +947,6 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
> >                * to remain.
> >                */
> >               if (remove_empty_directories(&ref_file)) {
> > -                     last_errno = errno;
> >                       if (!refs_verify_refname_available(
> >                                           &refs->base,
> >                                           refname, extras, skip, err))
> > @@ -962,10 +959,13 @@ static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
> >                                                    &lock->old_oid, type);
> >       }
> >       if (!resolved) {
> > -             last_errno = errno;
> > +             int last_errno = errno;
> >               if (last_errno != ENOTDIR ||
> > -                 !refs_verify_refname_available(&refs->base, refname,
> > -                                                extras, skip, err))
> > +                 /* in case of D/F conflict, try to generate a better error
> > +                  * message. If that fails, fall back to strerror(ENOTDIR).
> > +                  */
> > +                 !refs_verify_refname_available(&refs->base, refname, extras,
> > +                                                skip, err))
> >                       strbuf_addf(err, "unable to resolve reference '%s': %s",
> >                                   refname, strerror(last_errno));
>
> I don't think it's some dealbreaker and we can move on, but just FWIW I
> think what I mentioned ending in your
> https://lore.kernel.org/git/CAFQ2z_NpyJQLuM70MhJ8K1h2v3QXFuAZRjN=SvSsjnukNRJ8pw@mail.gmail.com/
> is still outstanding.
>
> I.e. you added the comment, which is just says what the error emitting
> looks like, that's all well & good.
>
> But what I was pointing out that it didn't make sense to do any
> "last_errno" here at all anymore. You pointed to 5b2d8d6f218
> (lock_ref_sha1_basic(): improve diagnostics for ref D/F conflicts,
> 2015-05-11), we started setting "last_errno" there, but that was *not*
> to avoid clobbering between the !resolved and the
> strbuf_add(strerror(last_errno)) here, but rather to carry the
> "last_errno" forward to the end of this lock_ref_oid_basic(), because
> other things (after this hunk) might reset/clear errno.

I disagree. In your suggested change

> +       if (!resolved &&
> +           (errno != ENOTDIR ||
> +            /* in case of D/F conflict, try to generate a better error
> +             * message. If that fails, fall back to strerror(ENOTDIR).
> +             */
> +            !refs_verify_refname_available(&refs->base, refname, extras,
> +                                           skip, err))) {
> +               strbuf_addf(err, "unable to resolve reference '%s': %s",
> +                           refname, strerror(errno));
>                 goto error_return;

the refs_verify_refname_available() call happens only if
errno==ENOTDIR. The call might clobber the ENOTDIR errno and then
fail. Then we'd be printing the last errno that
refs_verify_refname_available() saw, which may be different from
ENOTDIR.

Other than that, for clarity's sake, it's always better to avoid the
use of global errno if we can.

--
Han-Wen Nienhuys - Google Munich
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