[tor-bugs] #28780 [Core Tor/Tor]: circpadding: Add machine flag for not closing circuit if machine is active

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#28780: circpadding: Add machine flag for not closing circuit if machine is active
-------------------------------------------------+-------------------------
 Reporter:  asn                                  |          Owner:  (none)
     Type:  defect                               |         Status:
                                                 |  needs_review
 Priority:  Very High                            |      Milestone:  Tor:
                                                 |  0.4.1.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  wtf-pad, tor-relay, tor-cell,        |  Actual Points:  6
  padding, 041-proposed, network-team-           |
  roadmap-2019-Q1Q2                              |
Parent ID:  #28634                               |         Points:  5
 Reviewer:  asn                                  |        Sponsor:
                                                 |  Sponsor2
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Changes (by mikeperry):

 * status:  needs_information => needs_review


Comment:

 Ok, in addition to the pathbias fix and log improvements above, I pushed
 two more additional commits to the PR:

 1.
 https://github.com/torproject/tor/pull/966/commits/54bc9f59c0b52f75de76872db7fc872dc4f8f7f4
 - Check for liveness while holding open padding circuits.
 2.
 https://github.com/torproject/tor/pull/966/commits/d03035c8a68f1c0201167d106de703a9ebf2f64a
 - Refactor and clarify hold open logic.

 With these two commits, it should be much easier to verify that it is not
 possible for circpad to hold open a circuit if more than
 CIRCPAD_DELAY_INFINITE==UNIT32_MAX microseconds (or about 1.25 hours) pass
 with no circuit cell delivery events happening. I have not written tests
 for this yet, but if we like this approach, I can.

 I am open to adding additional checks to
 circuit_expire_old_circuits_clientside(), but I want to temperature check
 how people felt about this handbrake-style lifespan check in the first
 place, because that's the type of thing I'd add to
 circuit_expire_old_circuit_clientside() first.

 I can still be persuaded to eliminate circuit_mark_for_close() and make it
 super clear for callers that they must pick between a new error-close
 version and a differently-named normal-close version, and have each
 version assert if they are called with reason codes that should be used
 with the other version, but that change will be invasive and I am not sure
 that will actually save us from circuit-leak errors (which will actually
 arise from circuit_expire_old_circuits_clientside()).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28780#comment:35>
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