#!/bin/bash

function printFoundCommonSnapshot() {
    local _ZFS _COMMON_SNAPSHOT_CANDIDATE
    _ZFS="${1:?"printFoundCommonSnapshot(): Missing first parameter ZFS"}"
    _COMMON_SNAPSHOT_CANDIDATE="${2:?"printFoundCommonSnapshot(): Missing second parameter COMMON_SNAPSHOT_CANDIDATE"}"
    readonly _ZFS _COMMON_SNAPSHOT_CANDIDATE

    local _FOUND_COMMON_SNAPSHOT
    _FOUND_COMMON_SNAPSHOT=""

    while read -r _ROW
    do
        if [ "${_ROW}" == "${_ZFS}@${_COMMON_SNAPSHOT_CANDIDATE}" ]; then
            _FOUND_COMMON_SNAPSHOT="${_ROW}"
            break
        fi
    done < <(zfs list -H -o name -S creation -t snapshot "${_ZFS}")

    [ $? -eq 0 ] \
        && echo "${_FOUND_COMMON_SNAPSHOT}" \
        && return 0

    return 1
}

function removeAllSyncSnapshotsExeptTheCommonOne() {
    local _ZFS _RECEIVERHOST _COMMON_SNAPSHOT
    _ZFS="${1:?"removeAllSyncSnapshotsExeptTheCommonOne(): Missing first parameter ZFS"}"
    _RECEIVERHOST="${2:?"removeAllSyncSnapshotsExeptTheCommonOne(): Missing second parameter RECEIVERHOST"}"
    _COMMON_SNAPSHOT="${3:?"removeAllSyncSnapshotsExeptTheCommonOne(): Missing third parameter COMMON_SNAPSHOT"}"
    readonly _ZFS _RECEIVERHOST _COMMON_SNAPSHOT

    while read -r _ROW
    do
        # Skip the common snapshot to keep it.
        # If the common snapshot is not a sync-snapshot all sync-snapshots will be removed.
        if [ "${_ROW}" == "${_COMMON_SNAPSHOT}" ]; then
            continue
        fi
        # Destroy all remaining sync-snapshots of the receiving host
        zfs destroy "${_ROW}"
    done < <(zfs list -H -o name -S creation -t snapshot "${_ZFS}" | grep -E "^${_ZFS}@SYNC_${_RECEIVERHOST}_")
}

function sendResume() {
    local _RESUME_TOKEN
    _RESUME_TOKEN="${1:?"sendResume(): Missing first parameter RESUME_TOKEN"}"
    readonly _RESUME_TOKEN

    zfs send -t "${RESUME_TOKEN}" \
        && return 0

    return 1
}

function isValid() {
    # printf '%s'
    #  - always treats the contents of ${1} as pure plain text.
    # grep -qE: checks RegExp, but quiet
    printf '%s' "${1}" | grep -qE "${2:?"isValid(): Missing REGEXP"}"
}

function isValidOptional() {
    [ -z "${1}" ] || isValid "${1}" "${2}"
}



# Parameter 1: Only alphanumeric characters allowed and [._-]  if not leading (due to: -oProxyCommand=...).
# Parameter 2: Only alphanumeric characters allowed and [.-]   if not leading (due to: -oProxyCommand=...).
# Parameter 3: Only alphanumeric characters allowed and [._:-] if not leading (due to: -oProxyCommand=...), but can be empty.
# Parameter 4: Only alphanumeric characters allowed and [._:-] if not leading (due to: -oProxyCommand=...), but can be empty.
if isValid "${1:?"RECEIVERHOST missing"}" '^[a-zA-Z0-9][a-zA-Z0-9._-]*$' \
    && isValid "${2:?"COMPOSITION missing"}" '^[a-zA-Z0-9][a-zA-Z0-9.-]*$' \
    && isValidOptional "${3}" '^[a-zA-Z0-9][a-zA-Z0-9._:-]*$' \
    && isValidOptional "${4}" '^[a-zA-Z0-9][a-zA-Z0-9._:-]*$'
then
    _RECEIVERHOST="${1}"
    _COMPOSITION="${2}"
    _RECEIVERS_SNAPSHOT="${3}"
    _RESUME_TOKEN="${4}"

    _ZFS="zpool1/persistent/${_COMPOSITION:?"COMPOSITION missing"}"

    # Resume mode
    if [ "${_RECEIVERS_SNAPSHOT}" == "RESUME" ]; then
        sendResume "${_RESUME_TOKEN}"
        exit $?
    fi

    # This common snapshot is the starting-point, if available.
    _COMMON_SNAPSHOT="$(printFoundCommonSnapshot ${_ZFS} ${_RECEIVERS_SNAPSHOT})"

    [ "${_COMMON_SNAPSHOT}" == "" ] \
        && [ "${_RECEIVERS_SNAPSHOT}" != "" ] \
        && echo "Requested snapshot '${_RECEIVERS_SNAPSHOT}' not available" \
        && exit 1

    [ "${_COMMON_SNAPSHOT}" != "" ] \
        && removeAllSyncSnapshotsExeptTheCommonOne "${_ZFS}" "${_RECEIVERHOST}" "${_COMMON_SNAPSHOT}"

    # Now create the first or a further sync-snapshot as end-point.
    _NEW_SNAPSHOT="${_ZFS}@SYNC_${_RECEIVERHOST:?"RECEIVERHOST missing"}_$(date -u "+%Y-%m-%d_%H:%M:%S")"

    [ "${_COMMON_SNAPSHOT}" == "" ] \
        && zfs snapshot "${_NEW_SNAPSHOT}" \
        && zfs send -c -R "${_NEW_SNAPSHOT}" \
        && exit 0

    [ "${_COMMON_SNAPSHOT}" != "" ] \
        && removeAllSyncSnapshotsExeptTheCommonOne "${_ZFS}" "${_RECEIVERHOST}" "${_COMMON_SNAPSHOT}" \
        && zfs snapshot "${_NEW_SNAPSHOT}" \
        && zfs send -c -R -I "${_COMMON_SNAPSHOT}" "${_NEW_SNAPSHOT}" \
        && exit 0

else
    echo "Failure: At least one parameter is invalid" >&2
    exit 1
fi

exit 1