Keep preferring SelectorEventLoop on Windows #669
Merged
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Selector is still preferable when ~all out events are on zmq sockets, which must use the add_reader methods missing from Proactor. Tornado 6.1 puts these events in a background thread when Proactor is used, which we can avoid by using Selector.
cf jupyter/notebook#6052
reverts that part of #564
I don't understand the hang mentioned in #563 - if/where that happens and is caused by this patch, it is presumably multiple event loops with events, but only one running (waiting on async events scheduled on a non-running loop will never finish). That should be traceable and avoidable, though, if it can be reproduced.