[tor-bugs] #29990 [Core Tor/Tor]: test_circuitpadding_circuitsetup_machine() fails when monotonic time difference is zero

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#29990: test_circuitpadding_circuitsetup_machine() fails when monotonic time
difference is zero
-------------------------+-------------------------------------------------
     Reporter:  teor     |      Owner:  (none)
         Type:  defect   |     Status:  new
     Priority:  Medium   |  Milestone:  Tor: 0.4.1.x-final
    Component:  Core     |    Version:  Tor: unspecified
  Tor/Tor                |   Keywords:  wtf-pad, tor-relay, tor-cell,
     Severity:  Normal   |  padding, 041-must
Actual Points:           |  Parent ID:  #29500
       Points:  2        |   Reviewer:
      Sponsor:           |
-------------------------+-------------------------------------------------
 I can reliably get the following failure on my macOS VM when its wall
 clock time is out of sync with the host time.

 Failures like this also intermittently happen when the underlying API is
 low-resolution, or not actually monotonic (for example, Windows).

 {{{
 circuitpadding/circuitpadding_circuitsetup_machine: [forking]
   FAIL ../src/test/test_circuitpadding.c:1900:
 assert(client_side->padding_info[0]->padding_scheduled_at_usec OP_NE 0): 0
 vs 0
   [circuitpadding_circuitsetup_machine FAILED]
 }}}

 I've worked around the issue in #29500 by disabling these tests, but we
 should fix them long-term.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29990>
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