How do I rework a git merge commit?

Question

I would like to amend a merge commit which failed a unit test on our continuous integration server, but I don't want to leave a commit which fails tests in the history.

Since this commit was only used by the CI server (and would not have been pulled down by anyone else) I would like to fix up the merge and then --force push it to replace the existing failed merge commit.

My problem is, I can't work out how to easily get back to the state immediately before the original commit was made, so that I can fix the failing test, recommit the merge and then force push that change up.

I don't really want to have to redo the whole merge, since there are quite a few files with conflicts which had to be resolved and all but one was resolved successfully.

What I have tried so far

Attempt 1: git reset

My first attempt was to do a mixed reset back to the commit before the merge commit, fix the bug introduced by the merge and then recommit.

git reset HEAD^
# Fix the failing test
git commit

Unfortunately this results in a non-merge commit which silently incorporates all of the branch changes - not really what I want. *8')

Attempt 2: reset, stash, merge & apply

I then attempted to use stash to save the changes I made during my first attempt to merge:

git reset HEAD^
git add .               # Since there were untracked files
git stash
git merge branch
git stash apply         # Fails
git add .
git stash apply         # Merged but with conflicts

Unfortunately when git stash apply attempts to merge my stashed changes with the existing conflicted merge files it fails with the error

Cannot apply to a dirty working tree, please stage your changes

If I dutifully make my working directory clean by using git add . then the git stash apply runs but now not only do I have conflicts, I have conflicts with conflicts in them and any files which should have been removed have been added back in.

Attempt 3: amend and/or rebase.

I tried the nice simple suggestion by qqx to just fix the problem and git commit --amend the original commit, but without actually going back to the mid-merge state you lose access to the merge tools.

Once I've committed, I lose the ability to run git mergetool {file} as it just returns with the error:

{file}: file does not need merging

When trying to fix up a merge conflict, I really like being able to see my base, left, right and current revisions all together using tools like kdiff3.

using rebase suffers the same issue.

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Any hints on a workflow which could allow me to achieve my desired result would be appreciated.

Answer 1

git branch old_merge
git checkout -b new_merge HEAD~
git merge branch

Would get you back into pending merge state, where you can use the normal merge tools. Since the old merge is saved as the old_merge branch you can also use git checkout old_merge {file} to use the choices that you made for a particular file on your previous attempt. Once you're done you can commit that as normal, then checkout the branch that you were previously working on and use git reset --hard new_merge to switch to using the new merge commit.