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Re: [PATCH v2] Simplify handling of setup_git_directory_gently() failure cases.




Hi Peff,

On Tue, Dec 18, 2018 at 9:54 AM Jeff King <peff@xxxxxxxx> wrote:
>
> On Sat, Dec 15, 2018 at 05:05:08PM -0800, Erin Dahlgren wrote:
>
> > setup_git_directory_gently() expects two types of failures to
> > discover a git directory (e.g. .git/):
> > [...]
>
> When I read your subject line, I'll admit to having a funny feeling in
> the pit of my stomach. This setup code has historically been full of
> subtle corner cases, and I expected a very tricky review.
>
> However, your explanation was so well-written that it was a pleasure to
> read. :)

Thanks :)

>
> > Before this change are two misleading additional behaviors:
> >
> >   - GIT_DIR_HIT_CEILING: setup_nongit() changes to the cwd for no
> >       apparent reason. We never had the chance to change directories
> >       up to this point so chdir(current cwd) is pointless.
>
> That makes sense. I think this is a holdover from when we used to walk
> backwards via chdir(), prior to ce9b8aab5d (setup_git_directory_1():
> avoid changing global state, 2017-03-13). Back then, we needed to
> restore the state at this point, but now we don't.
>
> >   - GIT_DIR_HIT_MOUNT_POINT: strbuf_release() frees the buffer
> >       of a static struct strbuf (cwd). This is unnecessary because the
> >       struct is static so its buffer is always reachable. This is also
> >       misleading because nowhere else in the function is this buffer
> >       released.
>
> Makes sense.
>
> I do have one question, though:
>
> >       case GIT_DIR_HIT_CEILING:
> > -             prefix = setup_nongit(cwd.buf, nongit_ok);
> > -             break;
> > +             if (!nongit_ok)
> > +                     die(_("not a git repository (or any of the parent directories): %s"),
> >a +                                    DEFAULT_GIT_DIR_ENVIRONMENT);
> > +             *nongit_ok = 1;
> > +             strbuf_release(&dir);
> > +             return NULL;
>
> This case used to break out of the switch, and now returns early.
>
> So we do not execute the later code which clears GIT_PREFIX_ENVIRONMENT,
> and zeroes the fields in startup_info. Those probably don't matter in
> most cases, but I wonder if there are some obscure ones where it might.
>
> Might it make sense to make GIT_DIR_HIT_MOUNT_POINT more like
> GIT_DIR_HIT_CEILING currently is, rather than the other way around?
> I.e., something like:
>
>   case GIT_DIR_HIT_CEILING:
>         if (!nongit_ok)
>                 die(...);
>         *nongit_ok = 1;
>         prefix = NULL;
>         break;
>   case GIT_DIR_HIT_MOUNT_POINT:
>         if (!nongit_ok)
>                 die(...);
>         *nongit_ok = 1;
>         prefix = NULL;
>         break;
>
> ?

After some digging, there seems to be a documented guarantee that
"`GIT_PREFIX` is set as returned by running 'git rev-parse
--show-prefix'". See Documentation/config/alias.txt. If the
environment variable GIT_PREFIX is already set to something and we
don't clear it when prefix is NULL, then the two can be out of sync.
So that's a problem with my patch and the current handling of
GIT_DIR_HIT_MOUNT_POINT. I'm not sure how important of a guarantee it
is, but we should respect what's documented.

Side note: One of the secondary goals of my patch was to make it
really obvious that neither the GIT_DIR_HIT_CEILING nor the
GIT_DIR_HIT_MOUNT_POINT case can get us into the block protected by
(startup_info->have_repository || getenv(GIT_DIR_ENVIRONMENT)). With
your suggestion (and it's a fair one) I don't think that's feasible
without more significant refactoring. Should I just settle with a
comment that explains this?

Thanks,
Erin

>
> -Peff