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[Monotone-devel] Re: cherrypicking in monotone (was: interesting thread.

From: William Uther
Subject: [Monotone-devel] Re: cherrypicking in monotone (was: interesting thread...)
Date: Mon, 11 Dec 2006 12:02:20 +1100

Nathaniel Smith wrote:

In case people are interested, there's a nice thread going on over on
the VCS-neutral revctrl@ mailing list about cherrypicking in
monotone-style DAG-of-snapshot systems, DaggyFixes, and more generally
the connection between darcs-style and monotone-style merging:

I was hoping to test my ideas and try to implement cherry-picking in monotone sometime soon. But I got distracted and wrote up http:// instead.

Cherry-picking for monotone:

Assume we're cherry-picking the changes in B-C and merging them into revision D.

a) Find the ancestor you'd use for three way merge, call it A.
b) Pluck B-C and make a new branch with this change from A. Call the new revision C'. If there are any conflicts then abort and tell the user what conflicted - the conflicting revision is a dependency on what they're trying to pull across and should probably be pulled across too.
c) either:
    i) Modify C so it has C' as a parent, but otherwise no change.
ii) Merge C' and C to make C''. C'' should be identical to C. Auto-resolve all conflicts in favour of C (there shouldn't be any, but there may be if you didn't abort in b). Any descendants of C should then be merged with C'' - this should always be a clean merge. I am wary of doing this (merging descendants with C'') automatically though.
d) Merge C' with D and allow the user to resolve conflicts.

The end result is something that looks like wiki/DaggyFixes . But done post-hoc. And it records the history correctly so that you shouldn't get future conflicts (I think).

Now for the monotone questions... It seems you're using *-merge for merging (in When it gets a conflict on file contents then fall back to resolve_merge_conflicts() (in That first tries to do three-way-merging (in, and if that fails calls out to a lua hook for the user to fix things.

a) I haven't traced much of monotone's three-way-merge yet. I'm guessing noone has thought about this, but does anyone know how monotone's merge behaves relative to property TP2 of http:// ? (I was going to have a think about it, but I thought I'd ask first :)

b) As far as implementing cherry-picking, it seems that the least common ancestor for merge is found about line 80 of Is that the right place to be looking?


Will         :-}

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