#!/bin/sh

#
# This enforces stable moving in approved ways.
#
# Specifically:
#
# * stable must move by only 1 commit-per-push
# * the stable commit must have 2 and only 2 parents
#   * The first parent must be the previous stable commit
#   * The second parent is the tip of the candidate branch being released
# * the stable commit must have the same contents as the candidate tip
#   * Any merge conflicts should have been resolved in the candidate tip
#     by pulling stable into the candidate and having qa/tests done--pulling
#     candidate into stable should then apply cleanly
#
# For DAG aesthetics, we prefer stable only moving in the approved way,
# which is via empty (no change) merge commits. The rationale is that
# in the DAG we want a simple, one-commit move from each release to the
# next.
#
# We started out with:
#
# * -- A                stable
#  \    \
#   \    * -- * -- B    topic1
#    \           /
#     * -- * -- *       topic2
#
# And then publishing stable was a matter of fast-forwarding
# from A to B.
#
# In a complicated (non-rebased) DAG, this becomes hard to follow,
# so want we want instead is:
#
# * -- A ----------- C  stable
#  \    \           /
#   \    * -- * -- B    topic1
#    \           /
#     * -- * -- *       topic2
#
# Where commit C lists as it's first parent the prior stable
# commit and as it's second parent the release candidate. No
# other parents are allowed (e.g. no octopus merges here, which
# would insinuate qa didn't happen on the merged result).
#
# Also, we want to enforce that C does not actually introduce
# any diffs to the files between B and C--as they would be changes
# that QA does not see.
#

. $(dirname $0)/functions

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

case "$refname" in
	refs/heads/*)
		short_refname=${refname##refs/heads/}
		;;
	*)
		exit 0
		;;
esac

set_change_type
if [ "$change_type" == "delete" ] ; then
	exit 0
fi

# create/delete is okay
if [ "$change_type" != "update" ] ; then
	exit 0
fi

if [ "$short_refname" == "stable" ] ; then
	# Stable enforcement

	# read backwards:
	# - all commits from old..new
	# - unless they were already pointed to by a branch
	# = all new commits on stable
	count=$(git rev-parse --not --branches | git rev-list --stdin $oldrev..$newrev | wc -l)
	if [ "$count" -ne "1" ] ; then
		display_error_message "Moving stable must entail a single commit"
		exit 1
	fi

	number_of_parents=$(git rev-list --no-walk --parents $newrev | sed 's/ /\n/g' | grep -v $newrev | wc -l)
	if [ "$number_of_parents" -ne "2" ] ; then
		display_error_message "Moving stable must entail a merge commit"
		exit 1
	fi

	first_parent=$(git rev-list --no-walk --parents $newrev | sed 's/ /\n/g' | grep -v $newrev | head --lines=1)
	if [ "$first_parent" != "$oldrev" ] ; then
		display_error_message "Moving stable must have the previous stable as the first parent"
		exit 1
	fi

	second_parent=$(git rev-list --no-walk --parents $newrev | sed 's/ /\n/g' | grep -v $newrev | tail --lines=1)
	changed_lines=$(git diff $second_parent..$newrev | wc -l)
	if [ "$changed_lines" -ne "0" ] ; then
		display_error_message "Moving stable must not result in any changes from $second_parent"
		exit 1
	fi
fi