[tor-bugs] #28868 [Core Tor/sbws]: best_priority() can starve the worker threads of good relays

["Tor Bug Tracker & Wiki" <blackhole@xxxxxxxxxxxxxx>]

#28868: best_priority() can starve the worker threads of good relays
-------------------------------+-------------------------------
     Reporter:  teor           |      Owner:  (none)
         Type:  defect         |     Status:  new
     Priority:  Medium         |  Milestone:  sbws: 1.0.x-final
    Component:  Core Tor/sbws  |    Version:  sbws: 1.0.2
     Severity:  Normal         |   Keywords:
Actual Points:                 |  Parent ID:  #28663
       Points:                 |   Reviewer:
      Sponsor:                 |
-------------------------------+-------------------------------
 `best_priority()` tries to measure unmeasured and failing relays first.

 But if `fraction_relays` or `min_relays` always fail, those relays will
 always end up first in the priority queue. (More precisely, those relays
 will end up first in the priority queue, until the results of the good
 relays time out.)

 Thinking about starvation is complicated, because of the
 `freshness_reduction_factor` on some errors.

 Here's a very simple algorithm that avoids starving good relays for failed
 relays:
 1. Count the number of times that sbws has attempted to get a result from
 each relay.
 2. Test the relays with the lowest number of attempts first. (Don't check
 if the attempt succeeded or failed.)

 For this priority rule to work, every time a relay is queued, it must get
 a result. Here's how we can make that happen"
 * Modify `result_putter_error()` to store an error result to the queue.
 * Make sure timeouts store an error result to the queue.
 * Add a unit test and integration test that makes sure every queued relay
 has a result.

 Here's an alternative that might be simpler to implement:
 * before a relay is queued using `pool.apply_async()` in
 `run_speedtest()`, store a `ResultAttempt` to the queue
 * only count `ResultAttempt`s when prioritising relays

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28868>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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