[tor-bugs] #30031 [Core Tor/Tor]: Test circuitpadding with zero monotonic time deltas, and fix any bugs

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#30031: Test circuitpadding with zero monotonic time deltas, and fix any bugs
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     Reporter:  teor          |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: unspecified
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  wtf-pad
Actual Points:                |  Parent ID:
       Points:  2             |   Reviewer:
      Sponsor:                |
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 Any pre-ratchet monotime increment can be zero or negative, because the
 Windows API and gettimeofday() don't provide monotonic source times.

 So after Tor applies the ratchet, any number of calls to the monotonic
 time functions may return the same value.

 We have seen this happen when:
 * Windows sleeps (see
 https://trac.torproject.org/projects/tor/ticket/23696#comment:13 )
 * macOS has (VM?) time sync issues

 We know it can also happen when:
 * Tor uses gettimeofday() and a ratchet to emulate monotime, and the wall
 clock time goes backwards

 But these circumstances are rare:
 1. unit tests like test_util_monotonic_time() pass on Linux, macOS, and
 Windows CI
 2. Sleeping Windows boxes don't care about padding timing accuracy
 3. macOS VMs are rare
 4. Systems without monotime functions are rare, and clock changes on those
 systems are also rare

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/30031>
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