block push of trivial merge to git server

Question

A while back I asked our developers to use rebase instead of merge before pushing. Eliminating trivial merges makes for a much easier to follow commit graph (ie: gitk, git log).

Sometimes folks still accidentally do trivial merges, then push. Does anyone have handy or have tips for writing a server-side hook that blocks trivial merges?

By "trivial merge", I mean a merge without conflicts. Here's an example, and here's a better explanation of a trivial merge in git.

Update Wed Nov 10 01:26:41 UTC 2010: great comments, all! Thank you.

• Consider the following: all I'm really asking folks to do is this:
  • if git pull --ff-only fails, do git pull --rebase instead of git pull
• git.git only has one or two committers, right? In theory, it should be easy to follow the commit graph, but it looks pretty messy to me.

Update Thu Nov 11 23:49:35 UTC 2010:

• here's a gist with some in-progress work
• another idea is to prevent trivial merges on the client end, with a "git push" wrapper

Update Wed Dec 15 18:34:52 UTC 2010:

• adymitruk is close! Just one case is still unresolved: non-trivial merges must still work.
• A rather complete test suite is available, check it out.
• I asked for help on a (the?) git mailing list.

Answer 1

I came across this piece of code, while trying to find a solution. It doesn't do exactly what you want, but it should be ez to add extra branch names on the if statement.

Works for me, so far. it forces pull --rebase for the same branch and lets regular merges with other branches go through.

All credits go to the original author.

#!/bin/bash
#
# This git update hook will refuse unnecessary merge commits caused by pulling
# from being pushed to a shared repository. These commits make following the
# history of a project difficult and are always avoidable with rebasing.
#
# by Scott Kyle (appden)
# modified by Olivier Refalo (orefalo)

refname="$1"
oldrev="$2"
newrev="$3"

# if this is not run as a hook, you may have messed up
if [ -z "$GIT_DIR" -o -z "$refname" -o -z "$oldrev" -o -z "$newrev" ]; then
    echo "Usage: GIT_DIR=<path> $0 <ref> <oldrev> <newrev>" >&2
    exit 1
fi

# if the new revision is all 0's then it's a commit to delete a ref
zero="0000000000000000000000000000000000000000"
# also check if the new revision is not a commit or is not a fast forward
# detect branch deletion, branch creation... and more
if [ "${refname#refs/heads/}" = "master" ] || [ "$newrev" = "$zero" ] || [ "$oldrev" = "$zero" ] || [ $(git cat-file -t $newrev) != "co
mmit" ] || [ $(git merge-base $oldrev $newrev) != "$oldrev" ]; then
    exit 0
fi

# loop through merges in the push only following first parents
for merge in $(git rev-list --first-parent --merges $oldrev..$newrev --); do
    # lazily create the revision list of this branch prior to the push
    [ -z "$revlist" ] && revlist=$(git rev-list $oldrev)
    # check if the second parent of the merge is already in this branch
    if grep -q $(git rev-parse $merge^2) <<< "$revlist"; then
        cat >&2 <<-EOF
            *** PUSH REJECTED ***
            *** TRIVIAL merge detected on local branch ${refname#refs/heads/}
            *** To fix: git rebase origin/${refname#refs/heads/}
            ***
            *** Next time use: git pull --rebase
            ***
            *** Permanent fix: git config [--global] branch.autosetuprebase always
            *** Then for existing branches: git config branch.<name>.rebase true
        EOF
        exit 1
    fi
done

echo -Info- Clean history successfully preserved!
exit 0

Answer 2

This update hook will check if you are pushing to specific branches (it allows trivial merges in wip, topic and other branches).

This does not bother with the rest of the parents on octopus merges as it only references the 2nd parent in each merge commit being pushed. Please feel free to update the script.

UPDATE: Reserved branches are required to exist on the remote.

#!/bin/bash
refname="$1"
oldrev="$2"
newrev="$3"
branches="refs/heads/hotfixes refs/heads/dev refs/heads/qa refs/heads/master"
cont="no"
for branch in $branches ; do
  if [[ $refname == $branch ]] ; then
    cont="yes"
  fi
done
if [[ $cont == "no" ]] ; then
  exit 0
fi
echo "inspecting branch $refname for trivial merges" >&2
hashes="$(git log --format=%H --merges $oldrev..$newrev)"
for hash in $hashes ; do
  echo "checking merge commit $hash" >&2
  cont="no"
  for branch in $branches ; do
    if [[ $refname == $branch ]] ; then
      continue
    fi
    # if [[ "$(git log --format=%H $hash^2 ^$branch | wc -l)" == "0" ]] ; then
    if [[ "$(git log --format=%H $hash^2 ^$branch | wc -l)" == "    0" ]] ; then
      cont="yes"
    fi
  done
  if [[ $cont == "no" ]] ; then
    echo "No trivial merges allowed. Please rebase and push again." >&2
    exit 1
  fi
done
exit 0