Re: [PATCHv2 6/7] builtin/describe.c: describe a blob

From: Jacob Keller <jacob.keller@gmail.com>
To: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Stefan Beller <sbeller@google.com>,
	Junio C Hamano <gitster@pobox.com>, git <git@vger.kernel.org>,
	Kevin Daudt <me@ikke.info>
Subject: Re: [PATCHv2 6/7] builtin/describe.c: describe a blob
Date: Wed, 1 Nov 2017 23:05:00 -0700	[thread overview]
Message-ID: <CA+P7+xqw9zKNUkn9P-qA57ADSB5G_7Sd0JCc6SaK6bBf9-Jhtg@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1.1711012340290.6482@virtualbox>

On Wed, Nov 1, 2017 at 3:41 PM, Johannes Schindelin
<Johannes.Schindelin@gmx.de> wrote:
> Hi Stefan,
>
> On Wed, 1 Nov 2017, Stefan Beller wrote:
>
>> >> I know id prefer the first commit that introduced the blob. That's
>> >> what describing a commit does, it finds the oldest tag prior to the
>> >> commit, while --contains finds the first tag that contains the commit
>> >> as an ancestor.
>> >
>> > It is very easy to wish for "the oldest commit introducing a blob". But
>> > since we're in a DAG, this question is not necessarily easy to answer:
>> >
>> > - A - B - C
>> >     \   /
>> >       D
>> >
>> > Let's assume that all commits except A have the blob, and that B and D
>> > (for some reason) have the same author/committer dates.
>> >
>> > Do you identify B or D as the commit introducing the blob?
>>
>> The current implementation gives C, though.
>> (assuming C is HEAD, and A is ancient)
>>
>> With the --reverse flag one of B or D is given (not sure which,
>> depends on the exact order).
>
> Sure, but it is still a tricky thing. Imagine
>
> - A1 - B1 - A2 - B2 - B3
>
> where all the B* commits have the blob. Do you really want to report B1
> rather than B2 as the commit introducing the blob? (I would prefer B2...)
>
> Ciao,
> Dscho

Yes, but I'd rather we find either B1 or B2, rather than B3, or in the
first case, I'd rather we *at least* provide B or D, rather than C.

Or:

A - B1 - C -E - ... - Z
  \ B2 /

In this example, if everything except a had the blob, we'd find z,
which I think isn't as useful as finding one of B or D.

Sure we can't find a perfect answer, but I'd rather find a commit
which introduced as blob as one that adds the blob, rather than one
that happens to point to it because it has history which includes it.

I think both questions are valuable, the first is "which commit last
had this blob", the second  is "which commit first introduced this
blob", neither of which can always give a definitive answer. It really
depends on what question you're asking up front.

Thanks,
Jake