#!/bin/bash

# git-mirror: Mirror a git repo in leader-followers configuration
# Copyright © 2019 Midgard
#
# This program is free software: you can redistribute it and/or modify it under the terms of the
# GNU Affero General Public License as published by the Free Software Foundation, either version 3
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
# even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
# Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License along with this program.
# If not, see <http://www.gnu.org/licenses/>.

set -euo pipefail
IFS=$'\n'

if [[ $# -lt 3 ]]; then
	echo "Usage: $0 <workdir> <leader> <follower...>"
	exit 1
fi

cd "$1"

leader="$2"
shift 2
followers=( "$@" )

echo "Working directory: $PWD"
echo "Leader remote:     $leader"
echo "Follower remotes:  ${followers[*]}" | tr '\n' ' '
echo; echo

torify git fetch --prune --multiple "$leader"
git fetch --prune --multiple "${followers[@]}"
echo

echo "Leader remote information:"
git remote show -n "$leader"
echo
echo "Follower remotes information:"
git remote show -n "${followers[@]}"
echo

branches=( $(git show-ref | sed -n 's| refs/remotes/'"$leader"'/|\t|p') )
echo -e "${#branches[*]} branches at leader:"
echo "${branches[*]}"

IFS=$'\t'

errors=( )

error() {
	echo "XXX $1" >&2
	errors[${#errors[*]}]="$1"
}

ref_exists() {
	git show-ref --verify --quiet "$1"
}

is_ref_at_sha() {
	sha1="$(git show-ref --verify --hash "$1")"
	sha2="$2"
	[[ $sha1 == $sha2 ]]
}

for branchspec in "${branches[@]}"; do
	set $branchspec
	leader_sha="$1"
	branch="$2"

	for remote in "${followers[@]}"; do
		echo
		echo "Updating $remote/$branch"

		follower_ref="refs/remotes/$remote/$branch"

		if ! ref_exists "$follower_ref"; then
			echo "Branch doesn't exist yet at remote, creating"
			git reset --hard "$leader_sha" --
			git push "$remote" "+HEAD:$branch" || { error "$remote $branch: failed to push"; continue; }

		elif is_ref_at_sha "$follower_ref" "$leader_sha"; then
			echo "Branch up to date"

		else
			echo "Branch existed at remote, updating"
			git reset --hard "$follower_ref" --
			git merge --ff-only "$leader_sha" || { error "$remote $branch: FF not possible"; continue; }
			git push "$remote" "+HEAD:$branch" || { error "$remote $branch: failed to push"; continue; }
		fi

	done
done

echo
if [[ "${#errors[*]}" -gt 0 ]]; then
	echo "Errors:"
	echo "${errors[@]}"
else
	echo "No errors."
fi