Monday, 1 February 2016

Using flock in bash without invoking a subshell

flock -c can call external commands but not bash functions. Consequently users mess about with file descriptors, and often making a mess of it.

Inspired by Ivan's post http://blog.famzah.net/2013/07/31/using-flock-in-bash-without-invoking-a-subshell I've written a flock wrapper for bash, that uses flock underneath but allows flock -c to work for bash functions.

It can be called just like the regular flock command, with the benefit that the -c invocation is supported for bash functions; so you can use it like this:

flock -o /tmp/process my_thing "$@"

and I strongly recommend the -o option so that the file descriptor used for the lock is not passed to any sub-processes, which could be problematic if long live sub-processes (e.g. re-spawned daemons) keep it open.

It will pass through and invoke the regular flock if the command isn't a bash function, or if you aren't trying to execute a function.

Sadly, it doesn't recognize bash built-in's.

But the good news is, you can use flock -o ...file... on a shell function inside your shell script without having to worry about file descriptors.

# Helper function (in order to preserve $@ in the caller)
# If this isn't used to call a shell function then return 0
# otherwise return $? as the argumment number which represents
# the command/function to be called
_is_standard_flock() {
  local args=$#
  # find the first argument that doesn't begin with a -
  while test $# != 0
  do case "$1" in
     -*) shift ; continue ;;
     *) break ;;
     esac
  done

  # if it is numeric and there are not additional arguments 
  test $# = 1 -a -n "$1" -a -z "${1//[0-9]}" && return 0
  shift
  # (skipping -c if present)
  # or the following argument is also not a shell function then use the original flock
  if test "$1" = "-c"
  then declare -F "$2" >/dev/null || return 0
  else declare -F "$1" >/dev/null || return 0
  fi

  # we can't have shifted many args if this is a legitimate use of flock
  # so we will be in range of the exit code
  return $(( args - $# + 1))
}

# Help function to determine if -o or --close was given in the flock arguments
_wants_close() {
  test "${*/#--close/}" != "$*" && return # will also match bogus arguments like --closed
  # remove any -- options
  set -- "${@/#--*/}"
  # look for options with o
  test "${*/#-*o/}" != "$*" && return
  return 1
}

flock() {
  if _is_standard_flock "$@"
  then : # do outside the if-clause so bash can optimise exec where possible
  else # save the exit code (offset) as $1
       set -- $? "$@"
       # ${!$1} is the lock file 
       # ${@:$(($1 + 1))} might be -c
       test "${@:$(($1 + 1)):1}" = "-c" && set -- "${@:1:$1}" "${@:$(($1 + 2))}"
       if _wants_close "${*:2:$(( $1 - 1))}"
       then { set -- "$1" "$_" "${@:2}" ; command flock "${@:3:$(( $1 - 2))}" $2 && eval '"${@:$(( $1 + 2))}"' "$2>&-" ; set -- $? $2 ; command flock -u $2 ; return $1 ; } {_}<"${!1}"
       else { set -- "$1" "$_" "${@:2}" ; command flock "${@:3:$(( $1 - 2))}" $2 &&       "${@:$(( $1 + 2))}"          ; set -- $? $2 ; command flock -u $2 ; return $1 ; } {_}<"${!1}"
       fi
  fi
  command flock "$@"
}

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