#!/bin/bash
# Copyright 2021 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# Fixes a Copybara push that failed to create a merge commit, using the
# COPYBARA_TAG label to add a second parent to the HEAD commit. This should be
# run when Copybara exports a 'main -> google' commit, but fails to create a
# merge commit. The failure to create such a commit means that the
# COPYBARA_INTEGRATE_REVIEW tag is left in the commit message. It should only
# be run on the google branch. After running this script, you can verify the git
# log looks as expected, using something like:
#
# git log --left-right --graph --oneline --boundary google...main
#
# and then force push over the google branch. Force pushing is destructive. If
# you're uncertain, ask!

set -e

COPYBARA_TAG="COPYBARA_INTEGRATE_REVIEW"
UPSTREAM_REMOTE="${UPSTREAM_REMOTE:-upstream}"

if [[ -n "$(git status --porcelain)" ]]; then
  echo -e "\n\nWorking directory not clean. Aborting"
  git status
  exit 1
fi

# Technically this works anywhere, but our only current use case is to run it on
# the google branch and this is a weird and destructive change, so just be
# really picky about it.
CURRENT_BRANCH="$(git branch --show-current)"
if [[ "${CURRENT_BRANCH?}" != "google" ]]; then
  echo -e "\n\nCurrent branch ${CURRENT_BRANCH?} is not 'google'. Aborting"
  exit 1
fi

if [[ -n "$(git rev-list --merges HEAD^..HEAD)" ]]; then
  echo -e "\n\nHEAD commit is already a merge commit. Aborting"
  git log -n 1 HEAD
  exit 1
fi

# Get the commit message of the HEAD commit
MESSAGE="$(git log --format=%B -n 1 HEAD)"

# Extract the commit to merge from using the Copybara tag.
MERGE_FROM="$(echo "${MESSAGE?}" | awk -v pat="${COPYBARA_TAG?}" '$0~pat{print $NF}')"

if [[ -z "${MERGE_FROM?}" ]]; then
  echo -e "\n\nHEAD commit is not tagged with ${COPYBARA_TAG?}. Aborting"
  git log -n 1 HEAD
  exit 1
fi

echo "To revert the changes made by this script, run:"
echo "git reset --hard $(git rev-parse HEAD)"

# And create a new message with the tag removed
NEW_MESSAGE="$(echo "${MESSAGE?}" | sed "/${COPYBARA_TAG?}/d")"

# Make sure we actually have the commit we need to merge from.
echo -e "\n\nFetching ${MERGE_FROM?}"
git fetch "${UPSTREAM_REMOTE?}" "${MERGE_FROM?}"

echo -e "\n\nMerging from ${MERGE_FROM?}:"
git log -n 1 "${MERGE_FROM?}"

if ! git merge-base --is-ancestor "${MERGE_FROM?}" main; then
  echo -e "\n\nCommit to merge from is not on main branch. Aborting"
  exit 1
fi

# Add a tag to the commit to merge from so it is highlighted in the git log. If
# someone knows how to just highlight an individual commit with git log, that
# would be preferable.
git tag "merge-from-${MERGE_FROM?}" "${MERGE_FROM?}"

echo -e "\n\nCurrent git graph:"
git log --left-right --graph --oneline --boundary "HEAD...main"

# Create a new commit object `git commit-tree` based on the tree of the current
# HEAD commit with the parent of the HEAD commit as first parent and the commit
# to merge from as the second. Use the new message as the commit message. Reset
# the current branch to this commit.
# See https://stackoverflow.com/q/48560351
git reset --soft "$(git commit-tree -m "${NEW_MESSAGE?}" -p HEAD^ -p ${MERGE_FROM?} HEAD^{tree})"

echo -e "\n\nCreated fake merge. New git graph:"
git log --left-right --graph --oneline --boundary "HEAD...main"

# Delete the tag we created
git tag -d "merge-from-${MERGE_FROM?}"