[tor-bugs] #28897 [Core Tor/sbws]: Stop running twice destination usability tests

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#28897: Stop running twice destination usability tests
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 Reporter:  juga           |          Owner:  juga
     Type:  defect         |         Status:  needs_review
 Priority:  Medium         |      Milestone:  sbws: 1.0.x-final
Component:  Core Tor/sbws  |        Version:  sbws: 1.0.2
 Severity:  Normal         |     Resolution:
 Keywords:                 |  Actual Points:
Parent ID:  #28663         |         Points:
 Reviewer:                 |        Sponsor:
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Changes (by juga):

 * status:  assigned => needs_review


Comment:

 I think this is what is making sbws stalls.
 A backtrace [0] in the moment it's stalled shows two threads trying to get
 the  next destination.
 It locks
 https://github.com/torproject/sbws/blob/ee64d76df54ceb3a3c9e1e2a797fd70d68bb0035/sbws/lib/destination.py#L248,
 then while True, then if it enters in _perform_usability_test, will lock
 again, then call _is_usable, which call connect_destination_over_circuit
 which locks again. If it fails then it sleep.
 Many times connect_to_destination_over_circuit fails several times in a
 row, cause it's not reliable to do it through Tor and random relays.
 If _usable_dests is not overwritten every time that a destination is
 chosen, then a lock is only needed in connect_destination_over_circuit.
 It would be better to refactor all this code to trace it and debug it
 easier.
 As a temporal solution, the "usability" can be tracked out of the class
 and without locks.
 For now i'd just disable checking for usability, and prioritize the
 refactoring ticket as soon as 1.0 is done.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28897#comment:1>
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