#!/bin/sh

# Planner for periodic backups.
#
# busybox crond is deliberately not used here: when the container runs under a
# non-root uid (a very common k8s securityContext) crond loads no crontab at
# all and never says why - it just keeps printing "wakeup dt=60" forever while
# no backup is ever made. This loop does the same job, works under any uid and
# fails loudly on a bad schedule.

# -f matters: the schedule is full of '*' and word splitting would otherwise
# expand them against the files in the working directory.
set -euf

SCHEDULE_RAW="${CRONTAB:-0 * * * *}"
POLL_INTERVAL="${POLL_INTERVAL:-20}"

log()  { echo "[scheduler] $*"; }
die()  { echo "[scheduler] FATAL: $*" >&2; exit 1; }

# A value coming from .env, a configmap or a helm chart may carry quotes or a
# trailing CR; both silently break the schedule, so strip them up front.
SCHEDULE=$(printf '%s' "$SCHEDULE_RAW" | tr -d '\r' \
  | sed -e 's/^"\(.*\)"$/\1/' -e "s/^'\(.*\)'\$/\1/")

MONTH_NAMES="jan feb mar apr may jun jul aug sep oct nov dec"
DOW_NAMES="sun mon tue wed thu fri sat"

# Resolve a three-letter month/weekday name to its number, echo the input back
# when it is not a name.
resolve_name() { # $1=token $2=names $3=base
  _tok=$(echo "$1" | tr '[:upper:]' '[:lower:]')
  _n=0
  for _w in $2; do
    if [ "$_tok" = "$_w" ]; then
      echo $((_n + $3))
      return 0
    fi
    _n=$((_n + 1))
  done
  echo "$1"
}

# Drop a leading zero so that "07" is not read as an invalid octal number.
strip_zero() {
  _v=${1#0}
  [ -n "$_v" ] || _v=0
  echo "$_v"
}

# Expand one crontab field into the explicit list of values it matches.
# Supports *, N, a-b, a,b, */n, a-b/n and N/n.
expand_field() { # $1=spec $2=lo $3=hi $4=field name $5=names $6=names base
  _spec=$1; _lo=$2; _hi=$3; _name=$4; _names=$5; _base=$6
  _out=''
  _oifs=$IFS
  IFS=,
  for _part in $_spec; do
    IFS=$_oifs

    _step=1
    case $_part in
      */*)
        _step=${_part##*/}
        _part=${_part%%/*}
        ;;
    esac
    case $_step in
      ''|*[!0-9]*) die "$_name: bad step in '$_spec'" ;;
    esac
    _step=$(strip_zero "$_step")
    [ "$_step" -ge 1 ] || die "$_name: step must be >= 1 in '$_spec'"

    case $_part in
      '*')
        _s=$_lo; _e=$_hi
        ;;
      *-*)
        _s=$(resolve_name "${_part%%-*}" "$_names" "$_base")
        _e=$(resolve_name "${_part##*-}" "$_names" "$_base")
        ;;
      *)
        _s=$(resolve_name "$_part" "$_names" "$_base")
        # "N/step" means "from N to the end of the range", like vixie cron.
        if [ "$_step" -gt 1 ]; then _e=$_hi; else _e=$_s; fi
        ;;
    esac

    for _v in "$_s" "$_e"; do
      case $_v in
        ''|*[!0-9]*) die "$_name: '$_part' is not a number or a known name" ;;
      esac
    done
    _s=$(strip_zero "$_s")
    _e=$(strip_zero "$_e")
    [ "$_s" -ge "$_lo" ] && [ "$_e" -le "$_hi" ] \
      || die "$_name: '$_part' is outside ${_lo}-${_hi}"
    [ "$_s" -le "$_e" ] || die "$_name: reversed range '$_part'"

    _i=$_s
    while [ "$_i" -le "$_e" ]; do
      _out="$_out $_i"
      _i=$((_i + _step))
    done

    IFS=,
  done
  IFS=$_oifs
  [ -n "$_out" ] || die "$_name: empty field"
  echo "$_out"
}

in_list() { # $1=list $2=value
  case " $1 " in
    *" $2 "*) return 0 ;;
  esac
  return 1
}

# Word splitting is the point here, and globbing is off (set -f).
# shellcheck disable=SC2086
set -- $SCHEDULE
[ $# -eq 5 ] || die "CRONTAB must have 5 fields, got $#: '$SCHEDULE_RAW'"
SPEC_MIN=$1 SPEC_HOUR=$2 SPEC_DOM=$3 SPEC_MON=$4 SPEC_DOW=$5

LIST_MIN=$(expand_field  "$SPEC_MIN"  0 59 minute  '' 0)
LIST_HOUR=$(expand_field "$SPEC_HOUR" 0 23 hour    '' 0)
LIST_DOM=$(expand_field  "$SPEC_DOM"  1 31 day     '' 0)
LIST_MON=$(expand_field  "$SPEC_MON"  1 12 month   "$MONTH_NAMES" 1)
LIST_DOW=$(expand_field  "$SPEC_DOW"  0  7 weekday "$DOW_NAMES"   0)
# Sunday is both 0 and 7.
if in_list "$LIST_DOW" 7 && ! in_list "$LIST_DOW" 0; then
  LIST_DOW="$LIST_DOW 0"
fi

# crontab(5): when both day-of-month and day-of-week are restricted, a run
# happens if either of them matches, not both.
if [ "$SPEC_DOM" != '*' ] && [ "$SPEC_DOW" != '*' ]; then
  DAY_MATCH=or
else
  DAY_MATCH=and
fi

log "schedule: $SCHEDULE (TZ=${TZ:-UTC}, now $(date '+%F %T %z'))"
log "expanded: min=[${LIST_MIN# }] hour=[${LIST_HOUR# }] dom=[${LIST_DOM# }] mon=[${LIST_MON# }] dow=[${LIST_DOW# }] day-match=$DAY_MATCH"

matches_now() { # $1=minute $2=hour $3=dom $4=month $5=dow
  in_list "$LIST_MIN"  "$1" || return 1
  in_list "$LIST_HOUR" "$2" || return 1
  in_list "$LIST_MON"  "$4" || return 1

  if [ "$DAY_MATCH" = or ]; then
    in_list "$LIST_DOM" "$3" || in_list "$LIST_DOW" "$5" || return 1
  else
    in_list "$LIST_DOM" "$3" || return 1
    in_list "$LIST_DOW" "$5" || return 1
  fi
  return 0
}

TERMINATE=0
trap 'TERMINATE=1' TERM INT

# Initial backup: do not abort the container if it fails, the schedule matters
# more than this one run.
/scripts/backup.sh || log "initial backup failed, continuing to the schedule"

# The startup backup counts as this minute's run, otherwise a schedule that
# matches the minute the container came up would immediately back up twice.
LAST_RUN=$(date '+%Y-%m-%dT%H:%M')
while [ "$TERMINATE" -eq 0 ]; do
  # Polling instead of sleeping to the minute boundary: a backup can take
  # minutes and the clock can jump, LAST_RUN is what keeps a minute from
  # firing twice.
  # shellcheck disable=SC2046
  set -- $(date '+%Y-%m-%dT%H:%M %M %H %d %m %w')
  KEY=$1
  MIN=$(strip_zero "$2"); HOUR=$(strip_zero "$3")
  DOM=$(strip_zero "$4"); MON=$(strip_zero "$5"); DOW=$(strip_zero "$6")

  if [ "$KEY" != "$LAST_RUN" ] && matches_now "$MIN" "$HOUR" "$DOM" "$MON" "$DOW"; then
    LAST_RUN=$KEY
    log "triggered at $KEY"
    /scripts/backup.sh || log "backup failed, waiting for the next run"
  fi

  # Backgrounded so that SIGTERM is handled right away instead of after the
  # whole interval.
  sleep "$POLL_INTERVAL" &
  wait $! || true
done

log "terminating"