[tor-bugs] #16389 [Tor]: Redesign the HS client descriptor cache

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#16389: Redesign the HS client descriptor cache
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 Reporter:  dgoulet           |          Owner:
     Type:  defect            |         Status:  new
 Priority:  major             |      Milestone:  Tor: 0.2.7.x-final
Component:  Tor               |        Version:
 Keywords:  SponsorR, tor-hs  |  Actual Points:
Parent ID:  #16381            |         Points:
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 Bug #16381 vs #14219 demonstarted an important issue with the current
 design of the HS client descriptor cache.

 In a nutshell, we can't rely on the timestamp to order descriptors because
 the timestamp is rounded down to the nearest hour thus any change in that
 time period will never be seen by the client (#16381). Furthermore, we
 can't rely on comparing the descriptor content because we have two
 replicas with the same content but have different descriptor ID (#14219).

 One solution proposed by arma (and +1 by special) is to have an intro
 point failure cache. Every time we encounter this "we keep state about the
 previous HS desc by keeping a copy of it, and then seeing if the new one
 is different".

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/16389>
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