[tor-bugs] #17520 [Tor]: Relax the rend cache failure cleanup timer

[Tor Bug Tracker & Wiki]

#17520: Relax the rend cache failure cleanup timer
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     Reporter:  dgoulet  |      Owner:
         Type:  defect   |     Status:  new
     Priority:  Medium   |  Milestone:  Tor: 0.2.8.x-final
    Component:  Tor      |    Version:
     Severity:  Normal   |   Keywords:
Actual Points:           |  Parent ID:
       Points:           |    Sponsor:  SponsorR
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 `rend_cache_failure_clean()` is called every second and the reason is that
 we want to make the client wait as little as possible so we try our best
 to cleanup the failure cache.

 This is not ideal CPU wise since the cache could technically grow to the
 number of possible introduction point in the network thus making it a
 larger loop every second.

 Let's relax the cleanup timer here to 5 minutes (expiry time of an entry)
 and at each lookup, if the entry did expire, clean it and return that "we
 do not have an entry". This will not address the size of the cache that
 can grows but that's fine since we can handle that in the OOM. Also, a
 cache that has the maximum number of entries is a valid use case.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17520>
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