[tor-bugs] #25783 [Core Tor/Tor]: Circuit creation loop when primary guards are unreachable

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#25783: Circuit creation loop when primary guards are unreachable
------------------------------+--------------------------------
     Reporter:  asn           |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.4.x-final
    Component:  Core Tor/Tor  |    Version:  Tor: 0.3.0.10
     Severity:  Normal        |   Keywords:  tor-guard
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:  SponsorV-can  |
------------------------------+--------------------------------
 I was offline for a few hours today while Tor was running. At some point I
 went back online but I noticed that Tor was stuck on a circuit creation
 loop which it did not exit until it marked one of its primary guards as
 retriable (which can take lots of time). While in the loop, Tor made one
 circuit per second.

 I spent a good part of today debugging this. I think the issue is that our
 guard algorithm changes the circuit state of circuits that don't use
 primary guards to `CIRCUIT_STATE_GUARD_WAIT` in
 `circuit_build_no_more_hops()`. Then in `circuit_expire_building()` we
 consider those waiting circuits as not `CIRCUIT_STATE_OPEN` and expire
 them quickly with the 2s build timeout. Then we make more, and then expire
 them, ad infinitum, until a primary guards becomes retriable and breaks
 the circle.

 Here is the loop

 ----

 Tor thinks it needs a pre-emptive circuit:
 {{{
 Apr 11 14:47:21.000 [info] circuit_build_times_set_timeout(): Set circuit
 build timeout to 2s (1500.000000ms, 60000.000000ms, Xm: 525, a: 2.177536,
 r: 0.121588) based on 403 circuit times
 Apr 11 14:47:21.000 [info] circuit_predict_and_launch_new(): Have 4 clean
 circs (3 internal), need another exit circ.
 Apr 11 14:47:21.000 [info] origin_circuit_new(): Circuit 139 chose an idle
 timeout of 2967 based on 2875 seconds of predictive building remaining.
 }}}
 Tor picks guard, picks timeouts and connects to it:
 {{{
 Apr 11 14:47:21.000 [warn] No primary guards available. Selected confirmed
 guard ENiGMA ($42B4F52C5B11E4D39855F654955425B0D5A0598B) for circuit. Will
 try other guards before using this circuit.
 Apr 11 14:47:22.000 [warn] Recorded success for confirmed guard ENiGMA
 ($42B4F52C5B11E4D39855F654955425B0D5A0598B)
 Apr 11 14:47:22.000 [info] circuit_build_no_more_hops(): circuit built!
 }}}
 Tor marks the circuit as timeout by calling
 `circuit_build_times_mark_circ_as_measurement_only()` in
 `circuit_expire_building()` and starts making a new predictive circuit
 (loop!):
 {{{
 Apr 11 14:47:23.000 [info] circuit_expire_building(): Deciding to count
 the timeout for circuit 139
 Apr 11 14:47:23.000 [info] circuit_predict_and_launch_new(): Have 4 clean
 circs (3 internal), need another exit circ.
 }}}
 after a minute finally Tor ditches circuit which has been repurposed as
 `CIRCUIT_PURPOSE_C_MEASURE_TIMEOUT`:
 {{{
 Apr 11 14:48:22.000 [info] circuit_expire_building(): Deciding to count
 the timeout for circuit 139
 Apr 11 14:48:22.000 [info] circuit_expire_building(): Abandoning circ 139
 5.9.121.207:443:2179853168 (state 0,3:waiting to see how other guards
 perform, purpose 14, len 3)
 Apr 11 14:48:22.000 [info] pathbias_check_close(): Circuit 139 remote-
 closed without successful use for reason -3. Circuit purpose 14 currently
 0,waiting to see how other guards perform. Len 3.
 }}}

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25783>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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