Re: [PATCHv5 7/7] builtin/describe.c: describe a blob

From: Junio C Hamano <gitster@pobox.com>
Date: Nov 16, 2017, 03:24 UTC
Message-ID: <xmqqwp2qx5w6.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20171116020039.17810-8-sbeller@google.com>

Stefan Beller <sbeller@google.com> writes:

Show 6 quoted lines

> When describing commits, we try to anchor them to tags or refs, as these
> are conceptually on a higher level than the commit. And if there is no ref
> or tag that matches exactly, we're out of luck.  So we employ a heuristic
> to make up a name for the commit. These names are ambiguous, there might
> be different tags or refs to anchor to, and there might be different
> path in the DAG to travel to arrive at the commit precisely.

I am not sure if "And if there is ..." is adding much value here (I do not think it is even technically correct for that matter). If there are more than one tag that point at the commit the user is interested in, we use one of the tags, as tags conceptually sit at a higher level. And we use a heuristic to use one or the other tag to make up a name for the commit, so the same commit can have two valid names. ---So what? Neither of these two valid names is "ambigous"; the commit object the user wanted to name _is_ correctly identified (I would assume that we are not discussing a hash collision).

Lucikly, if we remove "And if...precisely", the logic still flows nicely, if not more, to the next paragraph.

Show 7 quoted lines

> When describing a blob, we want to describe the blob from a higher layer
> as well, which is a tuple of (commit, deep/path) as the tree objects
> involved are rather uninteresting.  The same blob can be referenced by
> multiple commits, so how we decide which commit to use?  This patch
> implements a rather naive approach on this: As there are no back pointers
> from blobs to commits in which the blob occurs, we'll start walking from
> any tips available, listing the blobs in-order of the commit and once we

Is "any tips" still the case? I was wondering why you start your traversal at HEAD and nothing else in this iteration. There seems to be no mention of this design decision in the documentation and no justification in the log.

Show 10 quoted lines

> found the blob, we'll take the first commit that listed the blob. For
> example
>
>   git describe --tags v0.99:Makefile
>   conversion-901-g7672db20c2:Makefile
>
> tells us the Makefile as it was in v0.99 was introduced in commit 7672db20.
>
> The walking is performed in reverse order to show the introduction of a
> blob rather than its last occurrence.

The reversing may improve the chance of an older commit to be chosen rather than the newer one, but it does not even guarantee to show the "introduction".

What this guarantees is that a long history will be traversed fully before we start considering which commit has the blob of interest, I am afraid. Is this a sensible trade-off?

> +	argv_array_pushl(&args, "internal: The first arg is not parsed",
> +		"--objects", "--in-commit-order", "--reverse", "HEAD",
> +		NULL);