Re: git rebase --preserve-merges seems to not work

On 2010.09.08 23:17:39 -0600, Joshua Jensen wrote:

# Commit 'd' is something I would pull --rebase from the server
before I push.
$ git lg --all
*   472fd93 - (HEAD, bc-merge) Merge commit 'c' into HEAD (3 seconds ago)
|\
| * d132c87 - (c) c (2 minutes ago)
| * 4a88fd1 - b (2 minutes ago)
|/
| * 6abf527 - (d) d (2 minutes ago)
|/
* b576660 - (a, master) a (3 minutes ago)


# Here is the rebase with the --preserve-merges option.
$ git rebase --preserve-merges d
Rebasing (n/3)
Successfully rebased and updated detached HEAD.

# Of course, bc-merge is an orphaned branch.  Its changes got
replayed on top of 'd'.

Yup, and the "problem" is, that both sides of the merge got replayed.

# Note the merge commit with the description "Merge commit 'c' into
HEAD" is missing.

Because the merge could be resolved as a fast-forward, as it merges the
replayed commits b' and c', not the originals.

$ git lg --all
* 2c93a8a - (HEAD) c (11 seconds ago)
* 2b98bc6 - b (11 seconds ago)
* 6abf527 - (d) d (3 minutes ago)
| *   472fd93 - (bc-merge) Merge commit 'c' into HEAD (74 seconds ago)
| |\
|/ /
| * d132c87 - (c) c (3 minutes ago)
| * 4a88fd1 - b (3 minutes ago)
|/
* b576660 - (a, master) a (4 minutes ago)

What you actually wanted to replay is just the first-parent history,
redoing the merges with their respective original second parent.
Jonathan (Cc'ed) had the same problem about a month ago.

The "first parent" thing isn't that well defined, criss-cross merges
combined with some unfortunate fast-forwards will easily make the
first-parent history become the one that you didn't mean (ask any
git-svn users that dared to use "git merge" without fully understanding
how git-svn uses the history ;-)), but it works in this special case.
And with that special case in mind (and admittedly probably not thinking
much further) I came up with this patch a few years ago:

http://marc.info/?l=git&m=119379735525213&w=2

It was not my itch to scratch though, so I never got around to get it
included, and of course it doesn't apply cleanly anymore. I tried to
give some hints to Jonathan ("hachi") on #git on how to update the patch
to make it apply again:

http://colabti.de/irclogger/irclogger_log/git?date=2010-08-20#l1241

Feel free to pick up from there...

Björn