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Re: Fixing merge - Subtree, Cyclic, and Tree Change cases

From: Folker Schamel <schamel23_at_spinor.com>
Date: Mon, 18 Jul 2011 22:37:11 +0200

Hi Andy,

two thoughts about cyclic merges:

1. Merging should not skip cyclic merges (like this old
svnmerge tool), but must subtract (reverse-merge) the original
change first, and then add (merge) the cyclic merge, in order
to not loose adaptions of changes.

For example, suppose you have two branches A and B.
c100 is a change in A.
c101 is a change in B.
B merges c1 into B (maybe with or without conflict),
but has to adapt this change to get it compatible with c101,
resulting into c102.
Now, A merges all changes from B to A.
Then just merging c101 would loose the adaptions made in c102.
So the correct behavior is to subtract c100 and then add c101 and c102.
Note that if the changesets are not overlapping, the order
of the reverse-merges and merges does not matter.
But if the changesets are overlapping, then the correct of
reverse-merges and merges can matter.

2. Supporting cyclic merges correctly requires that merge-info
only records the direct merge info without carrying over
existing merge info.

See for example


> To start the discussion, I will refer to this blog article by Mark Phippard:
> http://blogs.collab.net/subversion/2008/07/subversion-merg/
> I found the article to be a good overview of the issues.I think that we
> need help from Mark.On the other hand, I have seen that Mark sometimes
> makes discouraging comments. My work is apparently “hand wavey” and
> “proprietary”.I’m used to this treatment because I have 25 developers
> who work for me who often think that I am full of crap.However, it might
> have a discouraging effect on other contributors.For example, you can
> see in this great ticket thread -
> http://subversion.tigris.org/issues/show_bug.cgi?id=2897 - he states "I
> do not think it is possible in this design....I think we need to accept
> the limitations of the current design and work towards doing the best we
> can within that design” Apparently that was enough to kill progress.I
> think we should keep a more open mind going forward.
> I’m going to make some claims that some problems have “straightforward”
> solutions.That doesn’t mean they are simple solutions.Handling all of
> the merge cases is going to be hard.However, they are straightforward in
> the sense that we can discuss the strategy at the high level used in
> Mark’s article.
> Let’s consider three issues:Subtree merginfo, cyclic merge, and tree
> change operations
> Mark notes that reintegrate does not work if you have subtree merginfo.
> The subtrees potentially make the top-level mergeinfo inaccurate.So,
> basically everyone that has looked at merge problems in the past four
> years, including Mark, has tried to get rid of subtree merginfo.It’s
> amazing that Subversion still tries to support this feature.It can’t be
> supported in NewMerge.
> In the following sections, we will also see that the merginfo data is
> too sparse, and we need to replace it with something bigger and more
> extensible.
> The case where we merge back and forth between a development or
> deployment branch, and trunk, is the base case for merge.It should be
> supported.Subversion only supports it with special instructions.This is
> the “cyclic merge” problem.
> It seems that we have two basic ways to do a merge.We can grab all of
> the changes that we are trying to merge in one big diff between the
> branch we are merging from and the branch we are merging into - the
> reintegrate merge as described in Mark’s article.Or, we can sequentially
> apply or “replay” each of the changes that we want to merge into our
> working copy - the “recursive” strategy that is the default for git.
> It seems to me that the “one big diff” and the replay strategy are
> closely related.When you are replaying, you grab all of the changes in
> any sequence of revisions that doesn’t include a merge as one big
> diff.So, the current “one big diff” strategy is a special case of the
> replay strategy that applies when there are no intermediate merges from
> other branches or cherrypicks.
> But wait!According to this article, we can’t use the replay strategy
> because we are missing part of the replay.We lose information that was
> used to resolve a merge when composing merge commits.If we had that
> information, we could replay individual merges, and handle a higher
> percentage of the cyclic merge cases.
> This problem seems to have a straightforward solution.When we commit the
> merge, we can stuff the changeset that represents the difference between
> the merge, and the commit, into the merge_history.We just need an
> extensible merge_history format to hold it.
> It’s totally not clear to me why you need to say “reintegrate” when you
> merge to trunk, and why you need to update the branch after you do a
> reintegrate merge from it.The computer should be able to remember the
> history of merges and it should be obvious which things have been merged
> and which revisions have been committed on both branches.The only reason
> that I can think if is that that the mergeinfo is so sparse that the
> computer doesn’t remember enough about the merge history.Would a bigger
> and more extensible data format give us a straightforward way to solve
> that problem?
> We can identify tree changes by pattern matching.This is the same tactic
> that git uses, without any other tree change tracking.We can identify
> when this match is successful because the match is applied, examined by
> the merger, and then the merge is committed.In this case we could write
> thetree map into the merge_history so thatwe can map changes
> bi-directionally during future merges without guessing again.This is
> another case of saving information that we need to replay a merge.
> I think we could get a similar effect by generating a move operation
> (normal copy & delete form) as part of the merge.I think that this
> mapping would need to be done by updates as well as by explicit merges.
> Who on this list knows enough about the core algorithm used in merge to
> critique these suggestions and point to places in the code or documentation?
Received on 2011-07-18 22:37:48 CEST

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