|hg rename|d files no longer |hg merge|ing [(k)eep or (d)elete?]

[Matt Mackall]

On Tue, Aug 07, 2007 at 03:08:58AM -0700, Edward Lee wrote:
> On 8/7/07, Edward Lee <edilee at mozilla.com> wrote:
> > Are there any known issues with rename
> Good news and bad news. Seems like it's an issue of |hg merge| and not
> |hg rename|. By renaming the files back to original, commit, rename to
> the new name, commit, mercurial is able to merge the files correctly
> when pulling changes. However, this problem of mercurial forgetting
> about the rename can occur again fairly easily.
> 
> > some reason after merging [6] some changes from (1) into (2), pulling
> > new changes results in "[(k)eep or (d)elete?]"
> > [6] http://hg.mozilla.org/mozilla-central/?rev/d7e93861f3f3
> The interesting thing here is that it turns out the merge just
> happened to not touch any of the renamed files. So any merges that
> don't involve the renamed files will have mercurial forgetting about
> the rename. Similarly, even if some of the renamed files are touched,
> the remaining renamed files that are untouched are also forgotten in
> the merge.

Hmmm, you've found a case I hadn't considered.

Your changeset graph looks like:

 0-2-4
  \ \ \
   1-3-5

where the rename happens in 1, and the change to the rename happens in
4. But Mercurial currently only looks back as far as the least common
ancestor when looking for renames, so when merging 3 and 4, it only
looks back to 2. But the rename in question happened in 1, so it's not
visible at 2. Clearly it should search for renames all the way back to
0 in this case.

But if we look at another graph:

 0-1-2
    \ \
     3-4

..it's clear that there's no point in looking back any further than 1,
as any change in one is visible to both branches that get merged at 4.

And if we take our first graph and add an edge:

 0-2-x-4
  \ \ / \
   1-3-y-5       (extra changesets added to make merges meaningful)

..then we can stop our search when merging y and 4 at 3, because the
rename was already visible to both sides.

So we have to include all changesets that aren't common to both sides
in our rename search. Fun, a new graph algorithm.

-- 
Mathematics is the supreme nostalgia of our time.