#!/bin/bash
# Copyright (C) 2019 Ian Kelling
# SPDX-License-Identifier: AGPL-3.0-or-later

append() {
  cat >> "$1"
}
log-once() {
  local cbase c log line i out file o tmp mvout mverr
  cbase=/var/local/cron-errors
  [[ $EUID == 0 ]] || cbase=$HOME/cron-errors
  local help="Usage: some-command-that-outputs-on-error |& log-once [OPTION]... LOG_NAME

Main use case: in cronjobs where STDIN represents an error,
but we only want to output that to STDOUT if we've seen this type of
error ERRORS(default 3) number of times in a row, then we don't
want to output anything again until we've seen a success: no standard input.

Logs STDIN to /var/local/cron-errors/LOG_NAME\$error_count or
$HOME/cron-errors if not root, and keeps state in the same directory.

-ERRORS:  ERRORS is the number of errors to accumulate before outputing the error


You can emulate how cronjobs work by doing this for example:

    cmdname |& log-once | ifne mail -s 'cmdname failed' root@localhost

I could imagine a similar command that considers its non-option args to
be an error, but I haven't written it yet.
"
  errors=3
  mverr=0
  while true; do
    if [[ $1 == --help ]]; then
      echo "$help"
      return
    elif [[ $1 == -[0-9]* ]]; then
      errors=${1#-}
      shift
    elif [[ $1 == -- ]]; then
      shift
      break
    else
      break
    fi
  done
  log_name=$1
  # todo, make option & make them overridable based on command line or env variable
  [[ -d $cbase ]] || mkdir -p $cbase
  c=$cbase/$log_name
  log=false
  while read -r line; do
    output+=( "$line" )
    # If we find something that is not just a newline:
    if [[ $line ]]; then
      log=true
      break
    fi
  done
  glob="$c[0-9]*"
  # file is error file indicating previous error
  tmp=($glob); file="${tmp[0]}"
  if [[ $file == "$glob" ]]; then
    file=
  fi
  if $log; then
    out=append
    if [[ $file ]]; then
      i="${file#$c}"
      if (( i < errors )); then
        new_file=$c$((i+1))
        mvout=$(mv $file $new_file 2>&1) || mverr=$?
        if (( mverr )); then
          if [[ $mvout == *": No such file or directory" ]]; then
            # We've very likely hit a race condition, where another
            # log-once did the mv before us.
            return 0
          else
            echo "log-once: error on mv $file $new_file"
            return $mverr
          fi
        fi
        file=$new_file
        if [[ $file == $c$errors ]]; then
          out="tee -a"
        fi
      fi
    else
      file=${c}1
      if (( errors == 1 )); then
        out="tee -a"
      fi
    fi
    $out $file <<<"log-once: $(date -R)"
    if [[ $2 ]]; then
      $out $file <<<"$*"
    else
      for o in "${output[@]}"; do
        $out $file <<<"$o"
      done
      $out $file
    fi
    return 0
  fi
  if [[ $file ]]; then
    rm -f $file
    if [[ $file == $c$errors ]]; then
      echo "log-once: $(date -R): success after failure for $c"
    fi
  fi
  return 0
}
log-once "$@"