Add a comment about _LimitedHostnameResolver

Author: richvdh

synapse/app/_base.py

@@ -334,6 +334,13 @@ def install_dns_limiter(reactor, max_dns_requests_in_flight=100):
     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