[tor-bugs] #31561 [Core Tor/Tor]: hs-v3: Service can keep unused intro points in its list

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#31561: hs-v3: Service can keep unused intro points in its list
--------------------------------+--------------------------------
     Reporter:  dgoulet         |      Owner:  dgoulet
         Type:  defect          |     Status:  assigned
     Priority:  Medium          |  Milestone:  Tor: 0.4.2.x-final
    Component:  Core Tor/Tor    |    Version:
     Severity:  Normal          |   Keywords:  tor-hs hv-v3
Actual Points:                  |  Parent ID:  #30200
       Points:  0.2             |   Reviewer:  asn
      Sponsor:  Sponsor27-must  |
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 Tor always selects an extra number of intro points in addition to the
 configured `HiddenServiceNumIntroductionPoints`.

 It launches all of them and the first `NumIntro...` to finish are used
 (considering #31548 is resolved).

 Once the circuit of the remaining intro opens, we notice that we have too
 many and then re-purpose the extra ones.

 However, I've noticed that sometimes establishing an intro circuit
 timeouts during build, basically expiring due to our CBT policy. In that
 case, the circuit is simply close but the intro point remains in the
 service descriptor list.

 This is bad because of #31548, this means an intro point can end up in the
 descriptor even though the service never established any circuits to it...

 We simply need to callback into the HS subsystem when we are expiring an
 HS circuit so appropriate actions can be taken such as in this case,
 removing the IP from the list.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/31561>
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