push()
{
    $(ARGS_NE mnt src dst_dir)

    now=$(date +%F.%H%M%S) || die
    snap_dir=$mnt/snapshot.$now
    prev_dir=$mnt/snapshot.prev

    local BTRFS_RECEIVE_DESTINATION_PATH="$dst_dir"
    push_helper true "$snap_dir" "$prev_dir" "$src" local_btrfs_receiver
}

push_simple()
{
    $(ARGS_NE mnt src dst_dir)
    local BTRFS_RECEIVE_DESTINATION_PATH="$dst_dir"
    push_helper false "$mnt" "$src" local_btrfs_receiver
}

sex()
{
    (set -x; "$@")
}

local_btrfs_receiver()
{
    btrfs receive "$BTRFS_RECEIVE_DESTINATION_PATH"
}

shellescape()
{
    if [ "$BASH_VERSION" ]; then
        printf %q "$1"
    else
        bash -c 'printf %q "$1"' bash "$1"
    fi
}

remote_btrfs_receiver()
{
    ssh "$BTRFS_RECEIVE_DESTINATION_HOST" -- "btrfs receive $(shellescape "$BTRFS_RECEIVE_DESTINATION_PATH")"
}

push_helper()
{
    $(ARGS     keep_as_prev snap_dir prev_dir src dst_pipe)
    $(NONEMPTY keep_as_prev snap_dir          src dst_pipe)

    local full_dest rw_dest

    btrfs subvolume snapshot -r "$src" "$snap_dir" || die

    if [ "$prev_dir" -a -d "$prev_dir" ]; then
        btrfs send -p "$prev_dir" "$snap_dir"
    else
        btrfs send "$snap_dir"
    fi | "$dst_pipe" || die

    if [ "$dst_pipe" = local_btrfs_receiver ]; then
        local dst="$BTRFS_RECEIVE_DESTINATION_PATH"
        full_dest=$dst/$(basename "$snap_dir")
        rw_dest=$full_dest.rw
        btrfs subvolume snapshot "$full_dest" "$rw_dest"  || die
        btrfs_replace_default_subvolume_with "$rw_dest"
    fi

    if $keep_as_prev && [ "$prev_dir" ]
    then
        # keep the pushed snapshot in order to reuse it on subsequent pushes.
        with_dir "$prev_dir"  btrfs subvolume delete  || die
        sex mv "$snap_dir" "$prev_dir"                || die
    else
        btrfs subvolume delete "$snap_dir"
    fi
}

btrfs_mountpoint()
{
    $(ARGS_NE dir)
    btrfs filesystem show -m "$dir" >/dev/null 2>&1
}

btrfs_get_mountpoint()
{
    $(ARGS_NE dir)
    while [ "$dir" -a "$dir" != '.' ]; do
        if btrfs_mountpoint "$dir"
        then printf '%s\n' "$dir"
             return
        fi
        dir=$(dirname "$dir")
    done
    false
}

btrfs_show_default_path()
{
    $(ARGS_NE mp)
    local path
    mp=$(btrfs_get_mountpoint "$mp") || die # TODO: fix caller?
    btrfs_mountpoint "$mp" || die "not a mountpoint: $mp"
    path=$(btrfs subvolume get-default "$mp"/|sed -n -e 's/.* path //p')
    if [ "$path" ]; then
        printf '%s\n' "$mp/$path"
    else
        printf '%s\n' "$mp"
    fi
}

btrfs_show_default_id()
{
    $(ARGS_NE mp)
    local id
    mp=$(btrfs_get_mountpoint "$mp") || die # TODO: fix caller?
    btrfs_mountpoint "$mp" || die "not a mountpoint: $mp"
    id=$(btrfs subvolume get-default "$mp"/|sed -n -e 's/^ID \([^ ]*\) .*/\1/p')
    [ "$id" ] || return
    echo $id
}

btrfs_replace_default_subvolume_with()
{
    $(ARGS_NE new_default)
    local old_default old_default_id new_default_id
    old_default_id=$(btrfs_show_default_id "$new_default")       || die
    new_default_id=$(btrfs_show_subvolume_id "$new_default")     || die

    [ "$new_default_id" = "$old_default_id" ] && return

    if [ "$old_default_id" != 5 ]; then
        old_default=$(btrfs_show_default_path "$new_default")    || die
    else
        old_default=
    fi

    btrfs subvolume set-default "$new_default_id" "$new_default" || die

    if [ "$old_default" ]; then
        btrfs subvolume delete "$old_default"
        sex mv "$new_default" "$old_default"
    fi
}

btrfs_show_subvolume_id()
{
    $(ARGS_NE path)
    local result
    result=$(btrfs subvolume show "$path" | sed -n -e 's/^[ \t]*Subvolume ID:[ \t]*//p; s/.*is toplevel subvolume/5/p')
    if [ "$result" ]
    then printf '%s\n' "$result"
    else false
    fi
}

with_dir()
{
    $(ARGS_NE d)
    shift
    [ -d "$d" ] || return 0
    "$@" "$d"
}