Re: Segfault: git show-branch --reflog refs/pullreqs/1

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 22, 2024, 16:32 UTC
Message-ID: <xmqq34tkiho9.fsf@gitster.g>
In-Reply-To: <ZdcNtxw04MtybTWZ@tanuki>

Patrick Steinhardt <ps@pks.im> writes:

Show 11 quoted lines

>> Even though it may feel wrong to successfully resolve foo@{0} when
>> reflog for foo does not exist at the mechanical level (read: the
>> implementors of reflog mechanism may find the usability hack a bad
>> idea), I suspect at the end-user level it may be closer to what
>> people expect out of foo@{0} (i.e. "give me the latest").
>
> Hum, I dunno. I don't really understand what the benefit of this
> fallback is. If a user wants to know the latest object ID of the ref
> they shouldn't ask for `foo@{0}`, they should ask for `foo`. On the
> other hand, if I want to know "What is the latest entry in the ref's
> log", I want to ask for `foo@{0}`.

The usability hack helps small things like "List up to 4 most recent states from a branch", e.g.

    for nth in $(seq 0 3)
    do
	git rev-parse --quiet --verify @$nth || break
	git show -s --format="@$nth %h %s" @$nth
    done

vs

    for rev in HEAD @{1} @{2} @{3}
    do
	git rev-parse --quiet --verify "$rev" || break
	git show -s --format="$rev %h %s" "$rev"
    done

by not forcing you to special case the "current".

Ideally, "foo@{0}" should have meant "the state immediately before the current state of foo" so that "foo" is the unambiguous and only way to refer to "the current state of foo", but that was not how we implemented the reflog, allowing a subtle repository corruption where the latest state of a branch according to the reflog and the current commit pointed by the branch can diverge. But that wasn't what we did, and instead both "foo@{0}" and "foo" mean to refer to "the latest state of foo". We can take advantage of that misdesign and allow "foo@{0}" to refer to the same commit as "foo", at least at the get_oid_basic() level, whether a reflog actually exists or not, and that would make the whole thing more consistent.

In any case, I do not know how this "usability" actually helps in the field, and I wouldn't personally shed tears if it gets removed. The above is just an explanation why it exists.

Thanks.