Re: [PATCH] sha1_name: don't resolve refs when core.warnambiguousrefs is false

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 7, 2014, 20:31 UTC
Message-ID: <xmqqd2k3cz42.fsf@gitster.dls.corp.google.com>
In-Reply-To: <20140107195844.GA21812@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

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> On Tue, Jan 07, 2014 at 11:38:15AM -0800, Junio C Hamano wrote:
>
>> >> > Alternatively, I guess "cat-file
>> >> > --batch" could just turn off warn_ambiguous_refs itself.
>> >> 
>> >> Sounds like a sensible way to go, perhaps on top of this change?
>> >
>> > The downside is that we would not warn about ambiguous refs anymore,
>> > even if the user was expecting it to. I don't know if that matters much.
>> 
>> That is true already with or without Brodie's change, isn't it?
>> With warn_on_object_refname_ambiguity, "cat-file --batch" makes us
>> ignore core.warnambigousrefs setting.  If we redo 25fba78d
>> (cat-file: disable object/refname ambiguity check for batch mode,
>> 2013-07-12) to unconditionally disable warn_ambiguous_refs in
>> "cat-file --batch" and get rid of warn_on_object_refname_ambiguity,
>> the end result would be the same, no?
>
> No, I don't think the end effect is the same (or maybe we are not
> talking about the same thing. :) ).
>
> There are two ambiguity situations:
>
>   1. Ambiguous non-fully-qualified refs (e.g., same tag and head name).
>
>   2. 40-hex sha1 object names which might also be unqualified ref names.
>
> Prior to 25ffba78d, cat-file checked both (like all the rest of git).
> But checking (2) is very expensive,...

Ahh, of course.  Sorry for forgetting about 1.

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> The two options I was musing over earlier today were (all on top of
> Brodie's patch):
>
>   a. Revert 25ffba78d. With Brodie's patch, core.warnAmbiguousRefs
>      disables _both_ warnings. So we default to safe-but-slow, but
>      people who care about performance can turn off ambiguity warnings.
>      The downside is that you have to know to turn it off manually (and
>      it's a global config flag, so you end up turning it off
>      _everywhere_, not just in big queries where it matters).

Or "git -c core.warnambiguousrefs=false cat-file --batch", but I think a more important point is that it is no longer automatic for known-to-be-heavy operations, and I agree with you that it is a downside.

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>   b. Revert 25ffba78d, but then on top of it just turn off
>      warn_ambiguous_refs unconditionally in "cat-file --batch-check".
>      The downside is that we drop the safety from (1). The upside is
>      that the code is a little simpler, as we drop the extra flag.
>
> And obviously:
>
>   c. Just leave it at Brodie's patch with nothing else on top.
>
> My thinking in favor of (b) was basically "does anybody actually care
> about ambiguous refs in this situation anyway?". If they do, then I
> think (c) is my preferred choice.

OK. I agree with that line of thinking. Let's take it one step at a time, i.e. do c. and also use warn_on_object_refname_ambiguity in "rev-list --stdin" first and leave the simplification (i.e. b.) for later.

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>> > I kind of feel in the --batch situation that it is somewhat useless (I
>> > wonder if "rev-list --stdin" should turn it off, too).
>> 
>> I think doing the same as "cat-file --batch" in "rev-list --stdin"
>> makes sense.  Both interfaces are designed to grok extended SHA-1s,
>> and full 40-hex object names could be ambiguous and we are missing
>> the warning for them.
>
> I'm not sure I understand what you are saying. We _do_ have the warning
> for "rev-list --stdin" currently. We do _not_ have the warning for
> "cat-file --batch", since my 25ffba78d.

What I wanted to say was that we would be discarding the safety for "rev-list --stdin" with the same argument as we did for "cat-file --batch". If the argument for the earlier "cat-file --batch" were "this interface only takes raw 40-hex object names", then the situation would have been different, but that is not the case.

> I was wondering if rev-list should go the same way as 25ffba78d,
> for efficiency reasons (e.g., think piping to "rev-list --no-walk
> --stdin").

Yes, and I was trying to agree with that, but apparently I failed ;-)