#!/bin/bash
# Copyright (C) 2016 Ian Kelling
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# usage: $0 SUBVOL_MOUNTPOINT...
#
# In git, this is not not executable because it's meant to be installed
# using ./install-my-scripts
#
# If latest subvols $@ are not mounted, exit 1, print message, and touch
# /nocow/btrfs-stale/$subvol
#
# Either SUBVOL_MOUNTPOINT is a snapshot of the latest, or
# the latest snapshot is snapshot of SUBVOL_MOUNTPOINT.

[[ $EUID == 0 ]] || exec sudo -E "$BASH_SOURCE" "$@"

set -eE -o pipefail
trap 'echo "$0:$LINENO:error: \"$BASH_COMMAND\" returned $?" >&2' ERR

shopt -s nullglob

if [[ ! $@ ]]; then
    echo "$0: error: expected mountpoint argument"
fi

stale-file() {
    stale_dir=/nocow/btrfs-stale
    stale_file=$stale_dir/$vol
    if $stale; then
        mkdir -p $stale_dir
        printf "%s\n" $freshest_snap > $stale_file
        ret=1
    else
        rm -f $stale_file
    fi

}

ret=0
for d; do
    vol=${d##*/}
    dev=$(sed -rn "s,^\s*([^#]\S*)\s+$d\s.*,\1,p" /etc/fstab|head -n1)
    subvol_dir=$(sed -rn "s,^\s*[^#]\S*\s+$d\s.*\bsubvol=([a-zA-A/]+).*,\1,p" /etc/fstab|head -n1)
    # note, we need $dev because $d might not be mounted, and we do this loop
    # because the device in fstab for the rootfs can be different.
    for devx in $(btrfs fi show $dev| sed -rn 's#.*path (/\S+)$#\1#p'); do
        root_dir=$(sed -rn "s,^\s*$devx\s+(\S+).*\bsubvolid=0\b.*,\1,p" /etc/fstab|head -n1)
        if [[ $root_dir ]]; then break; fi
    done
    svp=$root_dir/$subvol_dir # subvolume path

    snaps=($root_dir/btrbk/$subvol_dir.20*) # Assumes we are in the 21st century.
    if [[ ! $snaps ]]; then
        # no snapshots yet
        echo "$0: warning. no snapshots found. this is expected for a brand new volume"
        continue
    fi

    # get info on last received sub
    last_received_gen=0
    for f in ${snaps[@]}; do
        show="$(btrfs sub show $f)"
        if echo "$show" | grep -E "Received UUID:\s+[[:alnum:]]" &>/dev/null; then
            cgen=$(echo "$show" | sed -rn 's,^\s*Gen at creation:\s+([0-9]+).*,\1,p')
            if [[ $cgen -gt $last_received_gen ]]; then
                last_received_cgen=$cgen
                last_received=$f
            fi
        fi
    done

    # Get last_snap by date.
    # when a btrbk bugfix makes it into the distro,
    # we might replace this with btrbk list latest /mnt/root/$vol | ...
    last_snap=$(
        for s in ${snaps[@]}; do
            f=${s##*/}
            unix_time=$(date -d $(sed -r  's/(.{4})(..)(.{5})(..)(.*)/\1-\2-\3:\4:\5/' <<<${f#$vol.}) +%s)
            printf "%s %s\n" $unix_time $s
        done | sort -r | head -n 1 | awk '{print $2}'
             )
    if [[ ! $last_snap ]]; then
        # should not happen.
        echo "$0: error: could not find latest snapshot for $svp among ${snaps[*]}"
        ret=1
        continue
    fi

    if [[ ! -e $svp ]]; then
        echo "$0: warning: subvol does not exist: $svp"
        echo "$0 assuming this host was just for receiving and latest snap is freshest"
        freshest_snap=$last_snap
        stale=true
        stale-file
        continue
    fi


    # if there is a last_received, we can assume stale or fresh if we are newer/older
    if [[ $last_received ]]; then
        svp_cgen=$(btrfs sub show $svp | sed -rn 's,^\s*Gen at creation:\s+([0-9]+).*,\1,p')
        if [[ $svp_cgen -ge $last_received_cgen ]]; then
            stale=false
        else
            echo "$svp stale: it's gen at creation, $svp_cgen, is earlier than the last received snapshot, $last_received's gen at creation: $last_received_cgen"
            freshest_snap=$last_received
            stale=true
        fi
        stale-file
        continue
    fi

    # fallback to using last_snap as the freshest
    freshest_snap=$last_snap
    stale=true
    # fresh if $svp has $last_snap as a snapshot,
    if btrfs sub show $svp 2>/dev/null | sed '0,/^\s*Snapshot(s):/d;s/^\s*//' | \
            grep -xF btrbk/$last_snap &>/dev/null; then
        stale=false
    else # or else $svp is a snapshot of $last_snap. we use a uuid
        # comparison, which if I remember from the docs, is a bit more
        # robust, perhaps to renames.
        last_snap_uuid=$(btrfs sub show $last_snap| awk '$1 == "UUID:" {print $2}')
        if btrfs sub show $svp| grep "^\s*Parent UUID:\s*$last_snap_uuid$" &>/dev/null; then
            stale=false
        fi
    fi

    stale-file

done
exit $ret