#!/bin/sh

#
# This enforces 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--otherwise this changes
# would not have appeared in QA.
#

# Command line
refname="$1"
oldrev="$2"
newrev="$3"

if expr "$oldrev" : '0*$' >/dev/null ; then
	exit 0
fi

if [ "$refname" != "refs/heads/stable" ] ; then
	exit 0
fi

# 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
	echo "----------------------------------------------------"
	echo
	echo "Moving stable must entail a single commit"
	echo
	echo "----------------------------------------------------"
	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
	echo "----------------------------------------------------"
	echo
	echo "Moving stable must entail a merge commit"
	echo
	echo "----------------------------------------------------"
	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
	echo "----------------------------------------------------"
	echo
	echo "Moving stable must have the previous stable as the first parent"
	echo
	echo "----------------------------------------------------"
	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
	echo "----------------------------------------------------"
	echo
	echo "Moving stable must not result in any changes"
	echo
	echo "----------------------------------------------------"
	exit 1
fi