[tor-bugs] #23100 [Core Tor/Tor]: Circuit Build Timeout needs to count hidden service circuits

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#23100: Circuit Build Timeout needs to count hidden service circuits
-------------------------+-------------------------------------------------
     Reporter:           |      Owner:
  mikeperry              |
         Type:           |     Status:  new
  enhancement            |
     Priority:  Medium   |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core     |    Version:  Tor: 0.2.7
  Tor/Tor                |   Keywords:  tor-hs, path-bias, guard-discovery,
     Severity:  Normal   |  needs-proposal, mike-can, prop247, tor-guard
Actual Points:           |  Parent ID:  #9001
       Points:           |   Reviewer:
      Sponsor:           |
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 The Circuit Build Timeout code currently ignores all circuits with a
 desired_path_len other than 3. This causes it not to count hidden service
 circuits of longer path lengths.

 When we change the path selection for Proposal247/#9001, we are going to
 want to count those circuits, because we want the circuit build timeout to
 prune 20% of Prop247 circuits, too. If we omit those circuits from the CBT
 calculation, we risk timing out either too few, too many, or even *all* of
 them.

 The simplest way to fix this is to change
 circuit_timeout_want_to_count_circ() to decide to count every circuit once
 it has completed 3 hops, even if it plans to build more.

 We may also want to alter the timeout application in
 circuit_expire_building(), depending on the prop247 implementation we
 choose. In some versions of the proposal, we can end up creating what are
 technically 5 hop paths (though these topologies were not very popular in
 Wilmington).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23100>
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