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Re: A modest proposal: No index or "log -g" in Subversion 1.5

From: Daniel Rall <dlr_at_collab.net>
Date: 2007-12-02 22:26:37 CET

On Sat, 01 Dec 2007, Blair Zajac wrote:

> Ben Collins-Sussman wrote:
> >On Nov 30, 2007 4:23 PM, Blair Zajac <blair@orcaware.com> wrote:
> >
> >>It just works. I mean, you still can get conflicts when you merge back
> >>and you
> >>resolve those just as you normally would.
> >>
> >>Thinking about it, I guess you might have to redo conflict resolution
> >>again when
> >>you merge changes back to trunk.
> >
> >Yeah, I wouldn't categorize that as 'just works'. Why should a user
> >have to resolve the same conflicts twice? That's an indication that
> >there are some changes *not* being tracked that should be.
 
What svnmerge.py does is calculate "merged", "blocked", "phantom", and
"reflected" revisions, and subtract those from the list of revisions requested
for merge (see the contrib/client-side/svnmerge/svnmerge.py:action_merge()
function). Issue #2897 is about proper handling of "reflected" revisions;
that is, revisions from the merge source which carry mergeinfo from the merge
target as their payload.

There are two potential flavors of reflected revisions:

a) "Merging revisions", which themselves also carry mergeinfo.
b) "Merged revisions", which carry no mergeinfo (a leaf node in a merge DAG).

In Subversion's core Merge Tracking, we've decided that (a) requires
additional parents in the DAG to properly represent, and currently punt by
repeating merges in this case for the sake of correctness. The plan has
been to address this post-1.5. (b) is what Kamesh has been working on, and
is something that needs to be addressed before releasing 1.5. Without (a),
we're still in *much* better shape than in pre-1.5 releases; you can avoid
specifying revision ranges when performing merges, and the only repeated
merges you hit are when performing cyclic merging (M -> F -> M).

svnmerge.py makes no distinction between these two types of reflected
revisions, instead filtering out both (a) and (b), which can miss merging
changes when pushing a branch which has been kept in sync with a mainline back
into that mainline.

> Normally, this hasn't been an issue in my work. In fact, I didn't even
> think this was much of an issue on any svnmerge.py discussion until this
> discussion came about, so the conflict resolution wasn't as painful as
> dealing with determining the revisions to merge back into the
> branch-source. This suggests that the conflict resolution hasn't been that
> painful for people.
...
 
Indeed, svnmerge.py wasn't designed to handle cyclic merging; it's
--bidirectional option was bolted on after its initial design to offer some
degree of support.

In my experience, svnmerge.py works best for keeping a feature branch in
sync with a mainline, and does a good job of this. However, its handling of
cyclic merging is not something that I'd like to emulate.

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Received on Sun Dec 2 21:25:12 2007

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