#!/bin/bash
#
# crontab(1) replacement for hosts without cron (Flatcar Container Linux).
# Each crontab line becomes a systemd --user timer and service for the calling
# user, so jobs keep that user's HOME, groups and working directory.
#
# Install as /opt/bin/crontab (mode 0755).
#
#   crontab FILE | crontab -    replace the crontab
#   crontab -l                  print it
#   crontab -r                  remove it
#
# Anything that cannot be converted faithfully is refused and the installed
# crontab is left as it was.

set -euo pipefail

STATE_DIR="$HOME/.config/crontab-timers"
UNIT_DIR="$HOME/.config/systemd/user"
PREFIX=crontab-job

export XDG_RUNTIME_DIR="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}"

die() { echo "crontab: $*" >&2; exit 1; }

# Convert one numeric cron field: "*", "*/n", "a", "a-b", "a-b/n", or a list.
convert_field() { # value first-value
    local out="" item
    local -a items
    IFS=, read -ra items <<< "$1"
    for item in "${items[@]}"; do
        case "$item" in
            '*') ;;
            '*/'*) item="$2/${item#\*/}" ;;
            *-*) item="${item/-/..}" ;;
        esac
        [[ "$item" =~ ^(\*|[0-9]+((\.\.[0-9]+)?(/[0-9]+)?))$ ]] || return 1
        out+="${out:+,}$item"
    done
    printf '%s' "$out"
}

month_numbers() {
    sed -E 's/jan/1/I; s/feb/2/I; s/mar/3/I; s/apr/4/I; s/may/5/I; s/jun/6/I;
            s/jul/7/I; s/aug/8/I; s/sep/9/I; s/oct/10/I; s/nov/11/I; s/dec/12/I' <<< "$1"
}

# Weekdays are expanded to names, so that 0-2 (Sun..Tue) survives systemd's
# Monday-first week. Prints nothing for "*".
convert_weekdays() {
    local -a names=(Sun Mon Tue Wed Thu Fri Sat) items
    local value item first last day out=""
    [ "$1" = '*' ] && return 0
    value="$(sed -E 's/sun/0/Ig; s/mon/1/Ig; s/tue/2/Ig; s/wed/3/Ig; s/thu/4/Ig; s/fri/5/Ig; s/sat/6/Ig' <<< "$1")"
    IFS=, read -ra items <<< "$value"
    for item in "${items[@]}"; do
        [[ "$item" =~ ^([0-7])(-([0-7]))?$ ]] || return 1
        first="${BASH_REMATCH[1]}"
        last="${BASH_REMATCH[3]:-$first}"
        [ "$first" -le "$last" ] || return 1
        for ((day = first; day <= last; day++)); do
            out+="${out:+,}${names[day % 7]}"
        done
    done
    printf '%s ' "$out"
}

# Prints the OnCalendar= value for the five cron fields.
to_calendar() { # minute hour day-of-month month day-of-week
    local minute hour dom month weekdays
    # cron ORs day-of-month with day-of-week, systemd ANDs them.
    if [ "$3" != '*' ] && [ "$5" != '*' ]; then
        return 1
    fi
    minute="$(convert_field "$1" 0)" || return 1
    hour="$(convert_field "$2" 0)" || return 1
    dom="$(convert_field "$3" 1)" || return 1
    month="$(convert_field "$(month_numbers "$4")" 1)" || return 1
    weekdays="$(convert_weekdays "$5")" || return 1
    printf '%s*-%s-%s %s:%s:00' "$weekdays" "$month" "$dom" "$hour" "$minute"
}

# Turns the crontab in $1 into job scripts and calendars under $2.
parse_crontab() {
    local input="$1" out="$2" line minute hour dom month dow command calendar
    local number=0 line_number=0
    while IFS= read -r line || [ -n "$line" ]; do
        line_number=$((line_number + 1))
        [[ "$line" =~ ^[[:space:]]*(#.*)?$ ]] && continue
        [[ "$line" =~ ^[[:space:]]*[A-Za-z_][A-Za-z0-9_]*[[:space:]]*= ]] \
            && die "line $line_number: environment settings are not supported"
        [[ "$line" =~ ^[[:space:]]*@ ]] \
            && die "line $line_number: @-schedules are not supported, use the five fields"

        read -r minute hour dom month dow command <<< "$line"
        [ -n "$command" ] || die "line $line_number: expected five time fields and a command"
        [[ "${command//\\%/}" == *%* ]] \
            && die "line $line_number: unescaped % (stdin/newline in cron) is not supported"
        command="${command//\\%/%}"

        calendar="$(to_calendar "$minute" "$hour" "$dom" "$month" "$dow")" \
            || die "line $line_number: cannot convert schedule: $minute $hour $dom $month $dow"
        systemd-analyze calendar "$calendar" > /dev/null 2>&1 \
            || die "line $line_number: invalid schedule: $minute $hour $dom $month $dow"

        number=$((number + 1))
        printf '#!/bin/sh\n%s\n' "$command" > "$out/$number.sh"
        printf '%s\n' "$calendar" > "$out/$number.calendar"
    done < "$input"
}

ensure_user_manager() {
    local attempt
    if [ "$(loginctl show-user "$(id -un)" --property=Linger --value 2> /dev/null)" != yes ]; then
        loginctl enable-linger "$(id -un)" 2> /dev/null \
            || sudo -n loginctl enable-linger "$(id -un)" \
            || die "cannot enable linger; run: sudo loginctl enable-linger $(id -un)"
    fi
    for attempt in 1 2 3 4 5 6 7 8 9 10; do
        [ -S "$XDG_RUNTIME_DIR/bus" ] && return 0
        sleep 1
    done
    die "the systemd user manager is not running ($XDG_RUNTIME_DIR/bus is missing)"
}

remove_units() {
    local timer
    for timer in "$UNIT_DIR/$PREFIX"-*.timer; do
        [ -e "$timer" ] || continue
        # Stops the timer only: a job already running is left to finish.
        systemctl --user disable --now "$(basename "$timer")" > /dev/null 2>&1 || true
        rm -f "$timer" "${timer%.timer}.service"
    done
}

write_units() { # number calendar
    cat > "$UNIT_DIR/$PREFIX-$1.service" <<EOF
[Unit]
Description=crontab job $1

[Service]
Type=oneshot
WorkingDirectory=%h
ExecStart=/bin/sh %h/.config/crontab-timers/jobs/$1.sh
EOF
    cat > "$UNIT_DIR/$PREFIX-$1.timer" <<EOF
[Unit]
Description=crontab job $1 schedule

[Timer]
OnCalendar=$2
AccuracySec=1s

[Install]
WantedBy=timers.target
EOF
}

install_crontab() {
    local input="$1" calendar number
    # Not local: the EXIT trap runs after this function has returned.
    work="$(mktemp -d)"
    trap 'rm -rf "$work"' EXIT
    cat -- "$input" > "$work/crontab"
    mkdir "$work/jobs"
    parse_crontab "$work/crontab" "$work/jobs"

    ensure_user_manager
    mkdir -p "$UNIT_DIR" "$STATE_DIR"
    remove_units

    rm -rf "$STATE_DIR/jobs"
    mkdir "$STATE_DIR/jobs"
    for calendar in "$work"/jobs/*.calendar; do
        [ -e "$calendar" ] || continue
        number="$(basename "$calendar" .calendar)"
        cp "$work/jobs/$number.sh" "$STATE_DIR/jobs/$number.sh"
        write_units "$number" "$(cat "$calendar")"
    done
    cp "$work/crontab" "$STATE_DIR/crontab"

    systemctl --user daemon-reload
    for calendar in "$STATE_DIR"/jobs/*.sh; do
        [ -e "$calendar" ] || continue
        systemctl --user enable --now "$PREFIX-$(basename "$calendar" .sh).timer" > /dev/null 2>&1
    done
}

case "${1:-}" in
    -l)
        [ -f "$STATE_DIR/crontab" ] || die "no crontab for $(id -un)"
        cat "$STATE_DIR/crontab"
        ;;
    -r)
        remove_units
        systemctl --user daemon-reload
        rm -rf "$STATE_DIR"
        ;;
    -)
        install_crontab /dev/stdin
        ;;
    '' | -*)
        die "usage: crontab FILE | crontab - | crontab -l | crontab -r"
        ;;
    *)
        [ -r "$1" ] || die "cannot read $1"
        install_crontab "$1"
        ;;
esac