#!/usr/bin/env zsh

# Bring the default branch up to date in a fork-model repo, and keep my fork's
# copy of it aligned: fast-forward it from whichever remote owns it, then push
# it to origin. Named git-sync and installed on the $PATH, so git finds it as
# `git sync`.

set -e

git rev-parse --git-dir > /dev/null

# The branch belongs to upstream in a fork, and to origin when there is no
# fork in play.
if git remote get-url upstream > /dev/null 2>&1; then
    source_remote=upstream
else
    source_remote=origin
fi

if ! git remote get-url origin > /dev/null 2>&1; then
    echo "git sync: no 'origin' remote to sync" >&2
    exit 1
fi

# Ask the server which branch is the default rather than reading
# refs/remotes/<remote>/HEAD. That ref is a cache written at clone time, and
# git does not refresh it when the default branch changes upstream - which is
# the whole reason this script exists.
branch=$(git ls-remote --symref $source_remote HEAD |
    awk '/^ref:/ { sub("refs/heads/", "", $2); print $2; exit }')

if [[ -z $branch ]]; then
    echo "git sync: could not work out the default branch of $source_remote" >&2
    exit 1
fi

echo "Syncing $branch from $source_remote"

git fetch --prune $source_remote

# Re-point the cached HEADs at what the server just told us, so `git
# default-branch` and nvim's diff-against-branch prompt agree with this script.
git remote set-head origin --auto > /dev/null
if [[ $source_remote != origin ]]; then
    git remote set-head $source_remote --auto > /dev/null
fi

# Be explicit about what the local branch tracks. A bare `git switch` has to
# guess, and refuses to when several remotes carry the same branch name - which
# they do as soon as a colleague's fork is added as a remote.
if git show-ref --verify --quiet refs/heads/$branch; then
    git switch $branch
elif git show-ref --verify --quiet refs/remotes/origin/$branch; then
    # Track origin, not upstream: this script keeps the fork's copy current, and
    # it means a stray `git push` here goes somewhere I can actually write to.
    git switch --create $branch --track origin/$branch
else
    git switch --create $branch --track $source_remote/$branch
fi

# Never a merge commit: if the default branch has diverged locally, that is
# something to look at, not something to paper over.
git merge --ff-only $source_remote/$branch

if [[ $source_remote != origin ]]; then
    git push origin $branch
fi