#!/usr/bin/env bash
# Snapshot ZFS filesystems and stream each snapshot to a remote ZFS host.

set -Eeuo pipefail

usage() {
    cat <<'EOF'
Usage: zfs-move-over-ssh.sh [options] SOURCE_POOL REMOTE_HOST DESTINATION_ROOT

Create one snapshot for every filesystem in SOURCE_POOL and send each one over
SSH. Filesystems retain their relative paths below DESTINATION_ROOT.

Arguments:
    SOURCE_POOL       Source ZFS pool, for example: tank
  REMOTE_HOST       SSH host, optionally user@host
  DESTINATION_ROOT  Existing or new destination ZFS filesystem, for example: backup/data

Options:
  -n, --dry-run              Print the ZFS and SSH commands without running them.
    -i, --identity-file FILE   SSH private key to use.
  -p, --ssh-port PORT        SSH port to use.
  --destroy-source-snapshots Destroy each local migration snapshot after its successful receive.
  -h, --help                 Show this help.

SSH must use key or agent authentication; password prompts are disabled. The
remote SSH account must be able to run `sudo -n zfs receive` without a password
prompt. This script does not destroy source filesystems or data.
EOF
}

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

DRY_RUN=false
DESTROY_SOURCE_SNAPSHOTS=false
SSH_OPTIONS=(-o BatchMode=yes)

while (($#)); do
    case "$1" in
        -n|--dry-run)
            DRY_RUN=true
            ;;
        -i|--identity-file)
            (($# >= 2)) || die "$1 requires a file path"
            SSH_OPTIONS+=(-i "$2")
            shift
            ;;
        -p|--ssh-port)
            (($# >= 2)) || die "$1 requires a port"
            SSH_OPTIONS+=(-p "$2")
            shift
            ;;
        --destroy-source-snapshots)
            DESTROY_SOURCE_SNAPSHOTS=true
            ;;
        -h|--help)
            usage
            exit 0
            ;;
        --)
            shift
            break
            ;;
        -*)
            die "unknown option: $1"
            ;;
        *)
            break
            ;;
    esac
    shift
done

(($# == 3)) || {
    usage >&2
    exit 2
}

SOURCE_POOL="$1"
REMOTE_HOST="$2"
DESTINATION_ROOT="${3%/}"
SNAPSHOT_NAME="move-$(date +%Y%m%d-%H%M%S)"

[[ "$REMOTE_HOST" != *@ ]] || die "REMOTE_HOST is missing a hostname; use user@server-name or server-name"

command -v zfs >/dev/null || die "zfs command was not found"
command -v zpool >/dev/null || die "zpool command was not found"
command -v ssh >/dev/null || die "ssh command was not found"
zpool list -H -o name "$SOURCE_POOL" >/dev/null || die "source pool does not exist: $SOURCE_POOL"

if ! $DRY_RUN; then
    ssh "${SSH_OPTIONS[@]}" "$REMOTE_HOST" true \
        || die "cannot authenticate to $REMOTE_HOST with SSH keys or an SSH agent"
    ssh "${SSH_OPTIONS[@]}" "$REMOTE_HOST" 'sudo -n zfs list -H -o name -t filesystem >/dev/null' \
        || die "$REMOTE_HOST accepts SSH, but cannot run 'sudo -n zfs'; configure passwordless sudo for zfs"
fi

map_destination() {
    local filesystem="$1"
    local relative_path="${filesystem#"$SOURCE_POOL"}"
    printf '%s%s\n' "$DESTINATION_ROOT" "$relative_path"
}

run() {
    if $DRY_RUN; then
        printf '+' >&2
        printf ' %q' "$@" >&2
        printf '\n' >&2
    else
        "$@"
    fi
}

mapfile -t FILESYSTEMS < <(zfs list -H -o name -t filesystem -r "$SOURCE_POOL")
((${#FILESYSTEMS[@]} > 0)) || die "no filesystems found in $SOURCE_POOL"

printf 'Migration snapshot: %s\n' "$SNAPSHOT_NAME"
for filesystem in "${FILESYSTEMS[@]}"; do
    destination="$(map_destination "$filesystem")"
    snapshot="${filesystem}@${SNAPSHOT_NAME}"

    printf 'Transferring %s to %s:%s\n' "$filesystem" "$REMOTE_HOST" "$destination"
    run zfs snapshot "$snapshot"

    if $DRY_RUN; then
        printf '+ zfs send -p %q | ssh' "$snapshot" >&2
        printf ' %q' "${SSH_OPTIONS[@]}" "$REMOTE_HOST" "sudo -n zfs receive -uF '$destination'" >&2
        printf '\n' >&2
    else
        zfs send -p "$snapshot" | ssh "${SSH_OPTIONS[@]}" "$REMOTE_HOST" "sudo -n zfs receive -uF '$destination'"
    fi

    if $DESTROY_SOURCE_SNAPSHOTS; then
        run zfs destroy "$snapshot"
    fi
done

printf 'Completed. Local source filesystems were not destroyed.\n'