[tor-bugs] #26129 [Core Tor/Stem]: Stem should invalidate exit_policy cache for DNS_USELESS event

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#26129: Stem should invalidate exit_policy cache for DNS_USELESS event
-------------------------------+------------------------------
     Reporter:  dmr            |      Owner:  atagar
         Type:  defect         |     Status:  new
     Priority:  Low            |  Milestone:
    Component:  Core Tor/Stem  |    Version:
     Severity:  Minor          |   Keywords:  controller, easy
Actual Points:                 |  Parent ID:
       Points:                 |   Reviewer:
      Sponsor:                 |
-------------------------------+------------------------------
 As originally noted in #25423, a number of ControlPort events indicate
 that tor has changed its exit policy; stem should use these to invalidate
 the `exit_policy` cache.
 All events mentioned in #25423 appear to be implemented except for the
 [[https://gitweb.torproject.org/torspec.git/tree/control-
 spec.txt?id=436d08b49fb84aa62d7bc96013002a0c27534bbb#n2634|`DNS_USELESS`
 event]].

 (So this ticket is for that.)

 ==== Severity/priority
 This event should be very rare, and there are probably a number of other
 things consumers would want to do if this event occurred, that are out of
 scope for this ticket. Overall I believe this is best categorized as
 Low/Minor.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26129>
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