brew/Library/Homebrew/utils/lock.sh

# create a lock using `flock(2)`. A name is required as first argument.
# the lock will be automatically unlocked when the shell process quits.
# Noted due to the fixed FD, a shell process can only create one lock.
# HOMEBREW_PREFIX is set by extend/ENV/super.rb
# shellcheck disable=SC2154
lock() {
  local name="$1"
  local lock_dir="${HOMEBREW_PREFIX}/var/homebrew/locks"
  local lock_file="${lock_dir}/${name}"
  [[ -d "${lock_dir}" ]] || mkdir -p "${lock_dir}"
  if [[ ! -w "${lock_dir}" ]]
  then
    odie <<EOS
Can't create ${name} lock in ${lock_dir}!
Fix permissions by running:
  sudo chown -R \$(whoami) ${HOMEBREW_PREFIX}/var/homebrew
EOS
  fi
  # 200 is the file descriptor used in the lock.
  # This FD should be used exclusively for lock purpose.
  # Any value except 0(stdin), 1(stdout) and 2(stderr) can do the job.
  # Noted, FD is unique per process but it will be shared to subprocess.
  # It is recommended to choose a large number to avoid conflicting with
  # other FD opened by the script.
  #
  # close FD first, this is required if parent process holds a different lock.
  exec 200>&-
  # open the lock file to FD, so the shell process can hold the lock.
  exec 200>"${lock_file}"
  if ! _create_lock 200 "${name}"
  then
    odie <<EOS
Another active Homebrew ${name} process is already in progress.
Please wait for it to finish or terminate it to continue.
EOS
  fi
}

_create_lock() {
  local lock_fd="$1"
  local name="$2"
  local ruby="/usr/bin/ruby"
  local python="/usr/bin/python"
  [[ -x "${ruby}" ]] || ruby="$(type -P ruby)"
  [[ -x "${python}" ]] || python="$(type -P python)"

  if [[ -x "${ruby}" ]] && "${ruby}" -e "exit(RUBY_VERSION >= '1.8.7')"
  then
    "${ruby}" -e "File.new(${lock_fd}).flock(File::LOCK_EX | File::LOCK_NB) || exit(1)"
  elif [[ -x "${python}" ]]
  then
    "${python}" -c "import fcntl; fcntl.flock(${lock_fd}, fcntl.LOCK_EX | fcntl.LOCK_NB)"
  elif [[ -x "$(type -P flock)" ]]
  then
    flock -n "${lock_fd}"
  else
    onoe "Cannot create ${name} lock, please avoid running Homebrew in parallel."
  fi
}