Limit in flight DNS requests

This is to work around a bug in twisted where a large number of
concurrent DNS requests cause it to tight loop forever.

c.f. https://twistedmatrix.com/trac/ticket/9620#ticket
This commit is contained in:
Erik Johnston 2019-04-09 17:03:35 +01:00
parent bfc8fdf1fc
commit a0fc256d65

View file

@ -22,13 +22,14 @@ import traceback
import psutil
from daemonize import Daemonize
from twisted.internet import error, reactor
from twisted.internet import defer, error, reactor
from twisted.protocols.tls import TLSMemoryBIOFactory
import synapse
from synapse.app import check_bind_error
from synapse.crypto import context_factory
from synapse.util import PreserveLoggingContext
from synapse.util.async_helpers import Linearizer
from synapse.util.rlimit import change_resource_limit
from synapse.util.versionstring import get_version_string
@ -99,6 +100,8 @@ def start_reactor(
logger (logging.Logger): logger instance to pass to Daemonize
"""
install_dns_limiter(reactor)
def run():
# make sure that we run the reactor with the sentinel log context,
# otherwise other PreserveLoggingContext instances will get confused
@ -312,3 +315,81 @@ def setup_sentry(hs):
name = hs.config.worker_name if hs.config.worker_name else "master"
scope.set_tag("worker_app", app)
scope.set_tag("worker_name", name)
def install_dns_limiter(reactor, max_dns_requests_in_flight=100):
"""Replaces the resolver with one that limits the number of in flight DNS
requests.
This is to workaround https://twistedmatrix.com/trac/ticket/9620, where we
can run out of file descriptors and infinite loop if we attempt to do too
many DNS queries at once
"""
new_resolver = _LimitedHostnameResolver(
reactor.nameResolver, max_dns_requests_in_flight,
)
reactor.installNameResolver(new_resolver)
class _LimitedHostnameResolver(object):
"""Wraps a IHostnameResolver, limiting the number of in-flight DNS lookups.
"""
def __init__(self, resolver, max_dns_requests_in_flight):
self._resolver = resolver
self._limiter = Linearizer(
name="dns_client_limiter", max_count=max_dns_requests_in_flight,
)
def resolveHostName(self, resolutionReceiver, hostName, portNumber=0,
addressTypes=None, transportSemantics='TCP'):
# Note this is happening deep within the reactor, so we don't need to
# worry about log contexts.
# We need this function to return `resolutionReceiver` so we do all the
# actual logic involving deferreds in a separate function.
self._resolve(
resolutionReceiver, hostName, portNumber,
addressTypes, transportSemantics,
)
return resolutionReceiver
@defer.inlineCallbacks
def _resolve(self, resolutionReceiver, hostName, portNumber=0,
addressTypes=None, transportSemantics='TCP'):
with (yield self._limiter.queue(())):
# resolveHostName doesn't return a Deferred, so we need to hook into
# the receiver interface to get told when resolution has finished.
deferred = defer.Deferred()
receiver = _DeferredResolutionReceiver(resolutionReceiver, deferred)
self._resolver.resolveHostName(
receiver, hostName, portNumber,
addressTypes, transportSemantics,
)
yield deferred
class _DeferredResolutionReceiver(object):
"""Wraps a IResolutionReceiver and simply resolves the given deferred when
resolution is complete
"""
def __init__(self, receiver, deferred):
self._receiver = receiver
self._deferred = deferred
def resolutionBegan(self, resolutionInProgress):
self._receiver.resolutionBegan(resolutionInProgress)
def addressResolved(self, address):
self._receiver.addressResolved(address)
def resolutionComplete(self):
self._deferred.callback(())
self._receiver.resolutionComplete()