#!/bin/bash
MIN_AGE_SECONDS=60
CONFIG_DIR=/etc/btrfs/remotes

die()
{
        printf 'Error: %s\n' "$*" >&2
        exit 1
}

read_config()
{
        eval config="($(jq -r '. | to_entries | .[] | "[\"" + .key + "\"]=" + (.value | @sh)'))"
}

check_dependencies()
{
        for c in flock jq btrfs pv realpath stat egrep date mv ln rm
        do
                command -v "$c" >/dev/null || die "missing dependency: $c"
        done
}

check_user_is_root()
{
        [ "$UID" = 0 ]
}

is_subvolume()
{
        btrfs subvolume show -- "$1" >/dev/null 2>&1
}

is_readonly_subvolume()
{
        btrfs subvolume show -- "$1" | egrep -q '^	Flags:.*\breadonly\b'
}

get_age()
{
        now=$(date +%s)
        then=$(date +%s -d "${1##*.snapshot~}")
        echo $((now - then))
}

btrfs_receive()
{
    ssh_cmdline=(ssh -- "${1%%:*}")
    "${ssh_cmdline[@]}" bash -c $(printf %q 'set -x; ([ -d "$0" ] || mkdir -p "$0") && btrfs receive -- "$0"') $(printf %q "${1#*:}")
}

set -e
check_dependencies
check_user_is_root

if [ $# = 1 ]
then
        case "$1" in
                ''     ) die 'config file name is blank' ;;
                */*    ) die 'config file name must not contain "/"' ;;
                .*     ) die 'config file name must not begin "."' ;;
                *.json ) ;;
                *      ) die 'config file name must end ".json"' ;;
        esac
        config_file=$CONFIG_DIR/$1
        config_file_temp=$CONFIG_DIR/.$1~tmp
else
        echo "Usage: $0 <config.json>" >&2
        exit 1
fi

[ -r "$config_file" ] || die "config file does not exist: $config_file"
exec 3<>"$config_file"
flock -n 3
declare -A config
read_config <&3

src=${config[source]}
dst=${config[destination]}
remote_head=${config[head]}

case "$dst" in
        *:/*) ;;
        /*) dst=localhost:$dst ;;
        *) die "Invalid destination: $dst" ;;
esac

is_subvolume "$src"

if [ "$remote_head" ]
then
        is_readonly_subvolume "$remote_head"

        AGE=$(get_age "$remote_head")
        if [ "$AGE" -le "$MIN_AGE_SECONDS" ]
        then
                echo "Up-to-date." >&2
                exit
        fi
fi

new_snapshot=${src%/}/.snapshot~$(date -Ins)
btrfs subvolume snapshot -r -- "$src" "$new_snapshot"
if ! btrfs send ${remote_head:+ -p "$remote_head"} -- "$new_snapshot" | btrfs_receive "$dst"
then
        btrfs subvolume delete "$new_snapshot"
        exit 1
fi
jq --arg h "$new_snapshot" '. | .head=$h' <"$config_file" >"$config_file_temp"
mv -T -- "$config_file_temp" "$config_file"

if [ "$remote_head" ]
then
        btrfs subvolume delete "$remote_head"
fi

### OK, basic idea:
#
## the system has a list of source subvolumes
## each source subvolume has a snapshot period
## each source subvolume has a list of subscribers
## each source subvolume has an ordered list of its snapshots
## each subscriber has a last-received snapshot (if any)

# when a subvolume's last snapshot is older than the configured period, a new snapshot is created
# when a subvolume has subscribers whose last snapshot is out-of-date, a snapshot is pushed (using the common ancestor)
# the subscriber's last snapshot is then updated
# if there are no subscriber's holding open a snapshot which is not the latest snapshot, then it is deleted