Add a script to fix Copybara failed merges (#4718)
This should be usable by build cops as-is, but the next step would be
integrating it to run on commits to the google branch to automatically
fix them up.
diff --git a/scripts/git/fix_copybara_merge.sh b/scripts/git/fix_copybara_merge.sh
new file mode 100755
index 0000000..21e4fad
--- /dev/null
+++ b/scripts/git/fix_copybara_merge.sh
@@ -0,0 +1,104 @@
+#!/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?}"
