Rip out the DNS lookup limiter (#10190)

As I've written in various places in the past (#7113, #9865) I'm pretty sure this is doing nothing useful at all.

changelog.d/10190.misc

@@ -0,0 +1 @@
+Remove redundant DNS lookup limiter.

synapse/app/_base.py

@@ -38,7 +38,6 @@ from synapse.crypto import context_factory
 from synapse.logging.context import PreserveLoggingContext
 from synapse.metrics.background_process_metrics import wrap_as_background_process
 from synapse.metrics.jemalloc import setup_jemalloc_stats
-from synapse.util.async_helpers import Linearizer
 from synapse.util.daemonize import daemonize_process
 from synapse.util.rlimit import change_resource_limit
 from synapse.util.versionstring import get_version_string
@@ -112,8 +111,6 @@ def start_reactor(
         run_command (Callable[]): callable that actually runs the reactor
     """
 
-    install_dns_limiter(reactor)
-
     def run():
         logger.info("Running")
         setup_jemalloc_stats()
@@ -398,107 +395,6 @@ def setup_sdnotify(hs):
     )
 
 
-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
-
-    XXX: I'm confused by this. reactor.nameResolver does not use twisted.names unless
-    you explicitly install twisted.names as the resolver; rather it uses a GAIResolver
-    backed by the reactor's default threadpool (which is limited to 10 threads). So
-    (a) I don't understand why twisted ticket 9620 is relevant, and (b) I don't
-    understand why we would run out of FDs if we did too many lookups at once.
-    -- richvdh 2020/08/29
-    """
-    new_resolver = _LimitedHostnameResolver(
-        reactor.nameResolver, max_dns_requests_in_flight
-    )
-
-    reactor.installNameResolver(new_resolver)
-
-
-class _LimitedHostnameResolver:
-    """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",
-    ):
-        # We need this function to return `resolutionReceiver` so we do all the
-        # actual logic involving deferreds in a separate function.
-
-        # even though this is happening within the depths of twisted, we need to drop
-        # our logcontext before starting _resolve, otherwise: (a) _resolve will drop
-        # the logcontext if it returns an incomplete deferred; (b) _resolve will
-        # call the resolutionReceiver *with* a logcontext, which it won't be expecting.
-        with PreserveLoggingContext():
-            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:
-    """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()
-
-
 sdnotify_sockaddr = os.getenv("NOTIFY_SOCKET")