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RUST-856 Fix race between server selection and server monitoring #460

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Sep 13, 2021
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RUST-856 Fix race between server selection and server monitoring #460

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8 changes: 5 additions & 3 deletions src/sdam/monitor.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -122,13 +122,15 @@ impl HeartbeatMonitor {
None => break,
};

// subscribe to check requests before performing the check in case one comes in
// after the check completes
let mut topology_check_requests_subscriber =
topology.subscribe_to_topology_check_requests();

if self.check_server(&topology, &server).await {
topology.notify_topology_changed();
}

let mut topology_check_requests_subscriber =
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My theory is that the following sequence happens:

  • check completes
  • monitor notifies that topology changed
  • operation sleeping in server selection gets notification and quickly fails server selection, requesting another check
  • monitor starts subscribing to request checks, missing the previous one
  • monitor sleeps until heartbeatFrequencyMS is hit

This fixes that by ensuring any requests that come in after the check will be recorded too, hopefully fixing this case.

topology.subscribe_to_topology_check_requests();

// drop strong reference to topology before going back to sleep in case it drops off
// in between checks.
drop(topology);
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