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This was previously broken (#13252) by f46369b8, which ported the fdReady function from `select` to `poll` and in so doing dropping support for timeouts. Unfortunately, while `select` tells us the amount of time not slept (on Linux anyways; it turns out this is implementation dependent), `poll` does not give us this luxury. Consequently, we manually need to track time slept in this case. Unfortunately, portably measuring time is hard. Ideally we would use `clock_gettime` with the monotonic clock here, but sadly this isn't supported on most versions of Darwin. Consequently, we instead use `gettimeofday`, running the risk of system time changes messing us up. Test Plan: Validate Reviewers: simonmar, austin, hvr Reviewed By: simonmar Subscribers: rwbarton, thomie GHC Trac Issues: #13252 Differential Revision: https://phabricator.haskell.org/D3473
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