#!/bin/bash

# Copyright (c) 2016 G. Sylvie Davies. http://bit-booster.com/
# Copyright (c) 2016 torek. http://stackoverflow.com/users/1256452/torek
# Copyright (c) 2021 Alfred Melch. https://melch.pro
# License: MIT license. https://opensource.org/licenses/MIT

# From: https://blog.developer.atlassian.com/stop-foxtrots-now/

NULL_REV="0000000000000000000000000000000000000000"

# refname (e.g. refs/head/master) will be updated from oldrev (commit hash) to newrev (commit hash)
while read oldrev newrev refname
do
  if [ $oldrev = $NULL_REV ]; then
    # pushing a new branch
    exit 0
  fi
  if [ $newrev = $NULL_REV ]; then
    # deleting a branch 
    exit 0
  fi
  if [ "$refname" = "refs/heads/master" ] || [ "$refname" = "refs/heads/main" ]; then
    # Find the first parent of the commit where oldrev is a parent
    # git log: list commits from newrev to oldrev
    #   %H: commit hash
    #   %P: parent hashes (list of parents)
    # grep: filter for oldrev
    #   Should appear once as a parent
    # awk: print the second word 
    #   The second word is the first parent. Third word would be second parent, ...
    MATCH=`git log --first-parent --pretty='%H %P' $oldrev..$newrev |
      grep $oldrev |
      awk '{ print $2 }'`
          # First parent should be the oldrev
          if [ "$oldrev" = "$MATCH" ]; then
            exit 0
          else
            echo >&2 "*** Push rejected! Foxtrot merge blocked! ***"
            echo "See: https://blog.developer.atlassian.com/stop-foxtrots-now/"
            exit 1
          fi
  fi
done