Re: [PATCH v2] clone: Don't segfault on -b specifing a non-commit

From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: Davide Berardi <davide.berardi6@unibo.it>,
	"git\@vger.kernel.org" <git@vger.kernel.org>
Subject: Re: [PATCH v2] clone: Don't segfault on -b specifing a non-commit
Date: Wed, 06 Nov 2019 10:36:18 +0900	[thread overview]
Message-ID: <xmqqbltpyeel.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <20191105043749.GA27601@sigill.intra.peff.net> (Jeff King's message of "Mon, 4 Nov 2019 23:37:49 -0500")

Jeff King <peff@peff.net> writes:

> I like the logic flow in this function, which is IMHO clearer than the
> existing code. But the "err" thing puzzled me for a moment. I think you
> are trying to tell the difference between the case that both "our" and
> "remote" are NULL, and the case that we saw a non-commit. In either case
> we return NULL, but only one is an error.

Yup.

> But:
>
>   - I don't think that logic needs to extend down into
>     lookup_commit_reference_gently(); a NULL return from it would always
>     be an error, wouldn't it?

Yes.

>   - we could probably simplify this by just inlining it into
>     update_head(). Something like:
>
>       if (our)
>               tip = &our->old_oid;
>       else if (remote)
>               tip = &remote->old_oid;
>
>       if (!tip) {
> 	      /*
> 	       * We have no local branch requested with "-b", and the
> 	       * remote HEAD is unborn. There's nothing to update HEAD
> 	       * to, but this state is not an error.
> 	       */
>               return 0;
>       }

I somehow had an impression that Davide was protecting against the
case where tip->old_oid is null_oid (cloning from an empty repo?);
NULL return from lookup_commit_reference_gently(null_oid) would not
deserve a warning from this codepath, and should work just like the
way it has worked before these changes.

>       tip_commit = lookup_commit_reference_gently(...);
>       if (!tip_commit) {
>               /*
>> -	fetch_if_missing = 1;
>> -	err = checkout(submodule_progress);
>> +	if (!err) {
>> +		fetch_if_missing = 1;
>> +		err = checkout(submodule_progress);
>> +	}
>
> This part makes sense. We might want an explanatory comment along the
> lines of:
>
>   /*
>    * Only try to checkout if we successfully updated HEAD; otherwise
>    * HEAD isn't pointing to the thing the user wanted.
>    /
>    if (!err) {
>        ...

Yup.

>> diff --git a/t/t5609-clone-branch.sh b/t/t5609-clone-branch.sh
>
> I think it would be the case that the remote HEAD is pointing to a
> non-commit (since that's a corrupt repository, it might make sense
> create a separate sub-repository).

All good comments. 

Thanks