[tor-bugs] #29034 [Core Tor/Tor]: circuit: Cleanup an HS circuit when it is being re-purposed

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#29034: circuit: Cleanup an HS circuit when it is being re-purposed
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     Reporter:  dgoulet       |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  High          |  Milestone:  Tor: 0.4.0.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-hs, 035-backport
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 Mike found out that when an IP/RP circuit fails to build in the right
 amount of time (for instance through `circuit_expire_building()`), it is
 re-purposed to become a measurement circuit.

 The issue is that those HS circuits are set in the HS circuitmap and have
 an `hs_ident` or `rend_data` set to them that should really not linger in
 the circuit object if the circuit is not an HS one anymore.

 Offenders: `circuit_build_times_mark_circ_as_measurement_only()` and
 `pathbias_send_usable_probe()`.

 Solution:

 `circuit_change_purpose()` is probably the right place to make a callback
 within the HS subsystem specific to cleaning up a circuit for a purpose
 change. I think we need a new function that specifically does that and not
 use `hs_circ_cleanup()` since it won't remove the ident.

 Lingering circuits in the HS circuitmap is bad and this bug could probably
 explain some of the issues we had with clients unable to establish
 connections because the IP auth key wouldn't match the one in the circuit
 ident.

 I strongly believe this should be backported up to 0.3.5 at the very
 least.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29034>
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