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List: git
Subject: Re: [PATCH] Documentation/git-filter-branch.txt: Fix description of --commit-filter
From: Kevin Ballard <kevin () sb ! org>
Date: 2008-05-31 23:50:00
Message-ID: CEA5A26A-9109-4D22-9D3F-8FFF8305DBEE () sb ! org
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On May 31, 2008, at 3:33 PM, Junio C Hamano wrote:
> Junio C Hamano <gitster@pobox.com> writes:
>
>> Kevin Ballard <kevin@sb.org> writes:
>>
>>> You're still talking about the parent-filter here. I think you're
>>> quite confused.
>>
>> Blush. I should go to bed.
>
> Now after following the codepath, your original
>
> diff --git a/Documentation/git-filter-branch.txt b/Documentation/
> git-filter-branch.txt
> index 506c37a..541bf23 100644
> --- a/Documentation/git-filter-branch.txt
> +++ b/Documentation/git-filter-branch.txt
> @@ -113,8 +113,8 @@ OPTIONS
> stdin. The commit id is expected on stdout.
> +
> As a special extension, the commit filter may emit multiple
> -commit ids; in that case, ancestors of the original commit will
> -have all of them as parents.
> +commit ids; in that case, the rewritten children of the original
> commit will
> +have all of them as parents. You probably don't want to do this.
> +
> You can use the 'map' convenience function in this filter, and
> other
> convenience functions, too. For example, calling 'skip_commit
> "$@"'
>
> does make sense to me. Except for "You probably don't want to do
> this."
> part. It is just "the utility of this feature is unknown to us" ;-)
>
> I dug the code with "git blame" and the basic logic has been the same
> since its introduction to git with 6f6826c (Add git-filter-branch,
> 2007-06-03). The commit-filter itself appeared first in Cogito as
> d690516
> (cg-admin-rewritehist --commit-filter for omitting commits,
> 2006-03-26),
> and the commit log message claims that it was primarily meant to
> _omit_
> unwanted commits from the history, but at the same time it
> advertises the
> multiple commits case as a "feature" without telling why somebody
> wants to
> do so.
>
> Except for this gem, which may have been lost in our copy:
>
> # ... Note that this handles merges properly! In case Darl
> # committed a merge between P1 and P2, it will be propagated
> properly
> # and all children of the merge will become merge commits with
> P1,P2
> # as their parents instead of the merge commit.
>
> IOW, to rewrite this history:
>
> ---A---C---D---E
> /
> B
>
> to pretend C never happened, you would give A' and B' back when you
> rewrite C, to end up with this history:
>
> ---A'--D'--E'
> /
> B'
>
> I'd agree with "You probably don't want to do this", but perhaps it
> needs
> a bit of clarification as to _why_ you would not:
>
> - If the history is being rewritten for the whole tree, this will
> make D' an evil merge that contains difference between C to D.
>
> - If the filtering of the history is done to ignore parts of the tree
> that is touched between C and D (iow, history simplification would
> leave trees C and D the same), you would want to simplify away D'
> not
> C'. IOW, you would want the resulting history to look like:
>
> ---A'--C'--E'
> /
> B'
>
> and for that you do not need to use this "feature".
Yeah, this utility of omitting commits occurred to me last night after
I went to bed. It does seem pretty limited in use, but I guess someone
might want to do it. For example, if C resolved merge conflicts
incorrectly and D fixed it, and then later somebody said "why do I
have two commits when I should just have one?" and wanted to omit C
and leave D behind as the merge.
I'll submit a new patch later that has better wording and perhaps a
diagram or two.
-Kevin Ballard
--
Kevin Ballard
http://kevin.sb.org
kevin@sb.org
http://www.tildesoft.com
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