Using mercurial to merge a cvs branch

Question

Currently my company is using cvs for version control. We have an old branch of code which has been used specifically for one client (don't ask) that we'd like to merge to the head.

Due to the delta between this branch and the head I think the merging capabilities of mercurial should make my job a bit easier. My line of reasoning is:

1. Create mercurial repositories of the branch and the current head.
2. Do a merge of the branch repo to the trunk repo.

At this stage I'm expecting mercurial to provide better merge support than cvs.

Then I would commit my changes to the trunk repository back into cvs.

Is this approach sound? Will this strategy result in a less painful merge as I think it will, or is there something I'm missing?

Answer 1

The reason Mercurial merges better than CVS or Subversion is because it tracks the most recent common ancestor of the two heads/branches, so you'll to make sure you're providing that information accurately or you'll probably end up with a worse merge.

If you do something like this:

hg init newrepo
cd newrepo
cvs checkout POINT_OF_DIVERGENCE
hg commit --addremove -m 'commiting point of divergence'
cvs checkout BRANCH
hg addremove --similarity 90 # let Mercurial discover any renames
hg commit -m 'committing branch head'
hg update -r 0               # jump back to point of divergence
cvs checkout HEAD
hg addremove --similarity 90 # let Mercurial discover any renames
hg commit -m 'commiting HEAD head'
hg merge

then the common ancestor for the two heads will be the point of divergence (sorry if the CVS commands are wrong, it's been a mercifully long time).

The trouble is knowing where the POINT_OF_DIVERGENCE is in the CVS repo -- CVS doesn't keep track of that at all, which is why its own merge doesn't use it.

CVS best practices always suggest that whenever you branch you create a tag that marks the point of divergence, but if you didn't do that then there's some tricky hunting ahead of you.

Without a correct most recent common ancestor the Mercurial merge won't be any better than CVS's.

Answer 2

Yes, that should work but there might be tiny little problems along the way (like how to get the source from CVS into Mercurial: Using hg convert or doing a manual import of a certain revision, etc).

Please create a new question if you run into a problem.

Answer 3

The built-in convert extension won't do well with CVS except for very simple histories. Use the fork of cvs2svn called cvs2hg

http://hg.gerg.ca/cvs2svn