[tor-bugs] #18564 [Tor]: Improve our client HS descriptor fetch logic

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#18564: Improve our client HS descriptor fetch logic
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     Reporter:  dgoulet       |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.9.x-final
    Component:  Tor           |    Version:
     Severity:  Normal        |   Keywords:  tor-hs
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:  SponsorR-can  |
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 (This was discovered because of #15937)

 Considering a client that does 6 connections attempt to a .onion very
 fast, currently we'll do 6 fetches of the descriptor thus on all 6 HSDirs.
 Furthermore, once the descriptor arrives on the client, we do NOT
 terminate the 5 other requests...

 I think we should do the following:

 * Have parallel descriptor fetches because multiple requests improve our
 chances of getting the descriptor faster (maybe). I'm proposing that we
 use one third of the HSDirs set that is 2 HSDirs (one per replica) over
 our 6 in total. (I'm open also to half of the hsdirs, just that one third
 makes it easy to query replicas in a symmetric way.)

  If they all fail, query an other sset or if only one fails, relaunch one
 immediately (so we avoid one HSDir stalling everything) so we always have
 one third of our set being queried.

 * Once the descriptor arrives, terminate the other pending HSDir fetch
 connections.

 * When a .onion request arrives on the SocksPort, check if we have pending
 descriptor fetch(es) and if yes, we should wait! don't launch more! If
 it's lower than our threshold (in this example 2/6), launch new fetches
 until we reach that limit and wait.

 This should also cover the `HSFETCH` control command with a .onion address
 which doesn't require to have a `SERVER=` set.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/18564>
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