pyzmq/zmq/utils/garbage.py

"""Garbage collection thread for representing zmq refcount of Python objects
used in zero-copy sends.
"""

# Copyright (C) PyZMQ Developers
# Distributed under the terms of the Modified BSD License.

import atexit
import struct
import warnings
from collections import namedtuple
from os import getpid
from threading import Event, Lock, Thread

import zmq

gcref = namedtuple('gcref', ['obj', 'event'])


class GarbageCollectorThread(Thread):
    """Thread in which garbage collection actually happens."""

    def __init__(self, gc):
        super().__init__()
        self.gc = gc
        self.daemon = True
        self.pid = getpid()
        self.ready = Event()

    def run(self):
        # detect fork at beginning of the thread
        if getpid is None or getpid() != self.pid:
            self.ready.set()
            return
        try:
            s = self.gc.context.socket(zmq.PULL)
            s.linger = 0
            s.bind(self.gc.url)
        finally:
            self.ready.set()

        while True:
            # detect fork
            if getpid is None or getpid() != self.pid:
                return
            msg = s.recv()
            if msg == b'DIE':
                break
            fmt = 'L' if len(msg) == 4 else 'Q'
            key = struct.unpack(fmt, msg)[0]
            tup = self.gc.refs.pop(key, None)
            if tup and tup.event:
                tup.event.set()
            del tup
        s.close()


class GarbageCollector:
    """PyZMQ Garbage Collector

    Used for representing the reference held by libzmq during zero-copy sends.
    This object holds a dictionary, keyed by Python id,
    of the Python objects whose memory are currently in use by zeromq.

    When zeromq is done with the memory, it sends a message on an inproc PUSH socket
    containing the packed size_t (32 or 64-bit unsigned int),
    which is the key in the dict.
    When the PULL socket in the gc thread receives that message,
    the reference is popped from the dict,
    and any tracker events that should be signaled fire.
    """

    refs = None
    _context = None
    _lock = None
    url = "inproc://pyzmq.gc.01"

    def __init__(self, context=None):
        super().__init__()
        self.refs = {}
        self.pid = None
        self.thread = None
        self._context = context
        self._lock = Lock()
        self._stay_down = False
        self._push = None
        self._push_mutex = None
        atexit.register(self._atexit)

    @property
    def context(self):
        if self._context is None:
            if Thread.__module__.startswith('gevent'):
                # gevent has monkey-patched Thread, use green Context
                from zmq import green

                self._context = green.Context()
            else:
                self._context = zmq.Context()
        return self._context

    @context.setter
    def context(self, ctx):
        if self.is_alive():
            if self.refs:
                warnings.warn(
                    "Replacing gc context while gc is running", RuntimeWarning
                )
            self.stop()
        self._context = ctx

    def _atexit(self):
        """atexit callback

        sets _stay_down flag so that gc doesn't try to start up again in other atexit handlers
        """
        self._stay_down = True
        self.stop()

    def stop(self):
        """stop the garbage-collection thread"""
        if not self.is_alive():
            return
        self._stop()

    def _clear(self):
        """Clear state

        called after stop or when setting up a new subprocess
        """
        self._push = None
        self._push_mutex = None
        self.thread = None
        self.refs.clear()
        self.context = None

    def _stop(self):
        push = self.context.socket(zmq.PUSH)
        push.connect(self.url)
        push.send(b'DIE')
        push.close()
        if self._push:
            self._push.close()
        self.thread.join()
        self.context.term()
        self._clear()

    @property
    def _push_socket(self):
        """The PUSH socket for use in the zmq message destructor callback."""
        if getattr(self, "_stay_down", False):
            raise RuntimeError("zmq gc socket requested during shutdown")
        if not self.is_alive() or self._push is None:
            self._push = self.context.socket(zmq.PUSH)
            self._push.connect(self.url)
        return self._push

    def start(self):
        """Start a new garbage collection thread.

        Creates a new zmq Context used for garbage collection.
        Under most circumstances, this will only be called once per process.
        """
        if self.thread is not None and self.pid != getpid():
            # It's re-starting, must free earlier thread's context
            # since a fork probably broke it
            self._clear()
        self.pid = getpid()
        self.refs = {}
        self.thread = GarbageCollectorThread(self)
        self.thread.start()
        self.thread.ready.wait()

    def is_alive(self):
        """Is the garbage collection thread currently running?

        Includes checks for process shutdown or fork.
        """
        if (
            getpid is None
            or getpid() != self.pid
            or self.thread is None
            or not self.thread.is_alive()
        ):
            return False
        return True

    def store(self, obj, event=None):
        """store an object and (optionally) event for zero-copy"""
        if not self.is_alive():
            if self._stay_down:
                return 0
            # safely start the gc thread
            # use lock and double check,
            # so we don't start multiple threads
            with self._lock:
                if not self.is_alive():
                    self.start()
        tup = gcref(obj, event)
        theid = id(tup)
        self.refs[theid] = tup
        return theid

    def __del__(self):
        if not self.is_alive():
            return
        try:
            self.stop()
        except Exception as e:
            raise (e)


gc = GarbageCollector()