How to automatically fix foxtrot-merges in my the 'master' branch in git?

Question

The problem of foxtrot merges explained in details in this question. In a nutshell: When working on the same branch (remote and local), the simple git merge command hides the remote merged commits under 2nd parent, and replaces the first-parents history of the remote (in the regular case - "origin"). And we consider it to be bad.

The linked-above question asks for a way to detect this situation, and reject the push. My question is how to automatically fix it.

I don't really care about the neat-tidiness of the full tree history, but I want that the first-parents tree will be perfect. And it is important to not disturb the other users, so the problem should solve itself automatically.

The goal: When the git origin hook detects foxtrot commit push, it will add another merge-commit, above the old-HEAD, and the new-HEAD, putting the old-HEAD as first-parent.

What is the safest way to achieve it?

Answer 1

In the inspiration of the answers to the mentioned above question and to this question (merges in bare repo), I came out with the solution below.

This script should be put in the post-receive hook file. As requested, the script creates new-merge-commit that doesn't hide the old-HEAD.

The script also:

1. Collects all the commit-msgs of the first-parent line of the new-HEAD, and sets it as the new-merge-commit msg.
2. Fakes the author and committer of the new-merge-commit, to be as the author of the new-HEAD (while taking care cases of " or ' in the name).

Commands explanation:

1. The git read-tree command stages the exact content of the new-HEAD commit. The new-merge-commit should be with the same exact content.
2. The git log command collects the needed commit messages.
3. The git ... commit-tree command creates the new-merge-commit with the author name/email, the staged data, the 2 parents, and the combined commit msg.
4. The git update-ref command updates the relevant ref (in this case - master) to point to the new-merge-commit.

If there are problems with this script, it would be very appreciated if you'll spot them. :)

The script:

#!/bin/bash

while read oldrev newrev refname
do
if [ "$refname" = "refs/heads/master" ]; then
    echo "Fix Foxtrot-Merges Hook..."
    MATCH=`git log --first-parent --pretty='%H %P' $oldrev..$newrev | grep $oldrev | awk '{ print \$2 }'`
    if [ "$oldrev" = "$MATCH" ]; then
        echo "...All is OK"
    else
        echo "...Fixing"
        
        authorName=`git log --first-parent --pretty='%an' $newrev^..$newrev`
        authorEmail=`git log --first-parent --pretty='%ae' $newrev^..$newrev`
        
        authorName=${authorName//"/\\"}
        authorEmail=${authorEmail//"/\\"}
        
        git read-tree -i --reset $newrev
        
        git log --first-parent --pretty='%B' $oldrev..$newrev > COMMIT_EDITMSG
        
        newCommitHash=$(git -c user.name="$authorName" -c user.email="$authorEmail" commit-tree $(git write-tree) -p $oldrev -p $newrev < COMMIT_EDITMSG)
        
        git update-ref $refname $newCommitHash
    fi
fi
done