baseimage-tutorial-framework/tfw/internals/ref_counter/ref_counter.py

from os import remove
from contextlib import contextmanager
from fcntl import flock, LOCK_EX, LOCK_UN


class RefCounter:
    def __init__(self, lockpath):
        self.lockpath = lockpath
        self._lockfile = open(self.lockpath, 'a+')
        with self.locked():
            counter = self._read_counter()
            self._write_counter(counter+1)

    @contextmanager
    def locked(self):
        flock(self._lockfile, LOCK_EX)
        yield
        flock(self._lockfile, LOCK_UN)

    def _read_counter(self):
        self._lockfile.seek(0)
        try:
            counter = int(self._lockfile.read())
        except ValueError:
            counter = 0
        return counter

    def _write_counter(self, counter):
        self._lockfile.seek(0)
        self._lockfile.truncate()
        self._lockfile.write(str(counter))
        self._lockfile.flush()

    def teardown_instance(self):
        with self.locked():
            counter = self._read_counter()
            if counter <= 1:
                remove(self.lockpath)
                self.deallocate()
            else:
                self._write_counter(counter-1)
        self._lockfile.close()

    def deallocate(self):
        pass