[tor-bugs] #15557 [Tor]: Improve relaunch logic for failed rendezvous circuits

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#15557: Improve relaunch logic for failed rendezvous circuits
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 Reporter:  asn                       |          Owner:
     Type:  defect                    |         Status:  new
 Priority:  normal                    |      Milestone:  Tor: 0.2.7.x-final
Component:  Tor                       |        Version:
 Keywords:  SponsorR SponsorU tor-hs  |  Actual Points:
Parent ID:  #15463                    |         Points:
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 When a hidden service fails its rendezvous circuits (maybe because it's
 too overworked, or maybe its guard is overwhelmed) it currently retries up
 to MAX_REND_FAILURES times (this used to be 8 but #11447 will change it to
 1).

 When the client notices the failure, it will also retry every 2 seconds or
 so, subject to normal `circuit_expire_building()` expiry.

 In the future, to reduce the computational costs of hidden services, we
 could push the retry logic solely to the clients. At that point the client
 should ensure that the rendezvous circuit retries enough time to be
 correct.

 Furthermore, currently, when the client notices a rend circ failure, it
 will establish a new rendezvous point and send a new INTRODUCE1 cell to
 the IP. Maybe this can be optimized, and have the client keep the same
 rendezvous point for a while and just send more introductions. After a few
 more failed introductions, the client should switch rendezvous point as
 well.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15557>
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