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Add a comment about _LimitedHostnameResolver

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This commit is contained in:
Richard van der Hoff 2020-08-29 00:05:25 +01:00
parent d2ac767de2
commit 8027166dd5
1 changed files with 7 additions and 0 deletions
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7
synapse/app/_base.py
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@ -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