src/waitress/trigger.py

##############################################################################
#
# Copyright (c) 2001-2005 Zope Foundation and Contributors.
# All Rights Reserved.
#
# This software is subject to the provisions of the Zope Public License,
# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
# FOR A PARTICULAR PURPOSE
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##############################################################################

import errno
import os
import socket
import threading

from . import wasyncore

# Wake up a call to select() running in the main thread.
#
# This is useful in a context where you are using Medusa's I/O
# subsystem to deliver data, but the data is generated by another
# thread.  Normally, if Medusa is in the middle of a call to
# select(), new output data generated by another thread will have
# to sit until the call to select() either times out or returns.
# If the trigger is 'pulled' by another thread, it should immediately
# generate a READ event on the trigger object, which will force the
# select() invocation to return.
#
# A common use for this facility: letting Medusa manage I/O for a
# large number of connections; but routing each request through a
# thread chosen from a fixed-size thread pool.  When a thread is
# acquired, a transaction is performed, but output data is
# accumulated into buffers that will be emptied more efficiently
# by Medusa. [picture a server that can process database queries
# rapidly, but doesn't want to tie up threads waiting to send data
# to low-bandwidth connections]
#
# The other major feature provided by this class is the ability to
# move work back into the main thread: if you call pull_trigger()
# with a thunk argument, when select() wakes up and receives the
# event it will call your thunk from within that thread.  The main
# purpose of this is to remove the need to wrap thread locks around
# Medusa's data structures, which normally do not need them.  [To see
# why this is true, imagine this scenario: A thread tries to push some
# new data onto a channel's outgoing data queue at the same time that
# the main thread is trying to remove some]


class _triggerbase:
    """OS-independent base class for OS-dependent trigger class."""

    kind = None  # subclass must set to "pipe" or "loopback"; used by repr

    def __init__(self):
        self._closed = False

        # `lock` protects the `thunks` list from being traversed and
        # appended to simultaneously.
        self.lock = threading.Lock()

        # List of no-argument callbacks to invoke when the trigger is
        # pulled.  These run in the thread running the wasyncore mainloop,
        # regardless of which thread pulls the trigger.
        self.thunks = []

    def readable(self):
        return True

    def writable(self):
        return False

    def handle_connect(self):
        pass

    def handle_close(self):
        self.close()

    # Override the wasyncore close() method, because it doesn't know about
    # (so can't close) all the gimmicks we have open.  Subclass must
    # supply a _close() method to do platform-specific closing work.  _close()
    # will be called iff we're not already closed.
    def close(self):
        if not self._closed:
            self._closed = True
            self.del_channel()
            self._close()  # subclass does OS-specific stuff

    def pull_trigger(self, thunk=None):
        if thunk:
            with self.lock:
                self.thunks.append(thunk)
        self._physical_pull()

    def handle_read(self):
        try:
            self.recv(8192)
        except OSError:
            return
        with self.lock:
            for thunk in self.thunks:
                try:
                    thunk()
                except:
                    nil, t, v, tbinfo = wasyncore.compact_traceback()
                    self.log_info(f"exception in trigger thunk: ({t}:{v} {tbinfo})")
            self.thunks = []


if os.name == "posix":

    class trigger(_triggerbase, wasyncore.file_dispatcher):
        kind = "pipe"

        def __init__(self, map):
            _triggerbase.__init__(self)
            r, self.trigger = self._fds = os.pipe()
            wasyncore.file_dispatcher.__init__(self, r, map=map)

        def _close(self):
            for fd in self._fds:
                os.close(fd)
            self._fds = []
            wasyncore.file_dispatcher.close(self)

        def _physical_pull(self):
            os.write(self.trigger, b"x")

else:  # pragma: no cover
    # Windows version; uses just sockets, because a pipe isn't select'able
    # on Windows.

    class trigger(_triggerbase, wasyncore.dispatcher):
        kind = "loopback"

        def __init__(self, map):
            _triggerbase.__init__(self)

            # Get a pair of connected sockets.  The trigger is the 'w'
            # end of the pair, which is connected to 'r'.  'r' is put
            # in the wasyncore socket map.  "pulling the trigger" then
            # means writing something on w, which will wake up r.

            w = socket.socket()
            # Disable buffering -- pulling the trigger sends 1 byte,
            # and we want that sent immediately, to wake up wasyncore's
            # select() ASAP.
            w.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)

            count = 0
            while True:
                count += 1
                # Bind to a local port; for efficiency, let the OS pick
                # a free port for us.
                # Unfortunately, stress tests showed that we may not
                # be able to connect to that port ("Address already in
                # use") despite that the OS picked it.  This appears
                # to be a race bug in the Windows socket implementation.
                # So we loop until a connect() succeeds (almost always
                # on the first try).  See the long thread at
                # http://mail.zope.org/pipermail/zope/2005-July/160433.html
                # for hideous details.
                a = socket.socket()
                a.bind(("127.0.0.1", 0))
                connect_address = a.getsockname()  # assigned (host, port) pair
                a.listen(1)
                try:
                    w.connect(connect_address)
                    break  # success
                except OSError as detail:
                    if getattr(detail, "winerror", None) != errno.WSAEADDRINUSE:
                        # "Address already in use" is the only error
                        # I've seen on two WinXP Pro SP2 boxes, under
                        # Pythons 2.3.5 and 2.4.1.
                        raise
                    # (10048, 'Address already in use')
                    # assert count <= 2 # never triggered in Tim's tests
                    if count >= 10:  # I've never seen it go above 2
                        a.close()
                        w.close()
                        raise RuntimeError("Cannot bind trigger!")
                    # Close `a` and try again.  Note:  I originally put a short
                    # sleep() here, but it didn't appear to help or hurt.
                    a.close()

            r, addr = a.accept()  # r becomes wasyncore's (self.)socket
            a.close()
            self.trigger = w
            wasyncore.dispatcher.__init__(self, r, map=map)

        def _close(self):
            # self.socket is r, and self.trigger is w, from __init__
            self.socket.close()
            self.trigger.close()

        def _physical_pull(self):
            self.trigger.send(b"x")