[tor-bugs] #20942 [Core Tor/Tor]: Make consensus expiry tolerance for fallbacks lower when the stale consensus bug is fixed

[Tor Bug Tracker & Wiki]

#20942: Make consensus expiry tolerance for fallbacks lower when the stale
consensus bug is fixed
------------------------------+--------------------------------
     Reporter:  teor          |      Owner:
         Type:  enhancement   |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.0.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  fallback
Actual Points:                |  Parent ID:
       Points:  0.1           |   Reviewer:
      Sponsor:                |
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 When #20909 is fixed, make CONSENSUS_EXPIRY_TOLERANCE in
 scripts/maint/updateFallbackDirs.py lower than 24 hours:
 * In update_consensus_networkstatus_fetch_time_impl, directory mirrors
 download consensuses between 2 and 90 minutes after they are created. So
 the tolerance can be as low as -90.
 * In networkstatus_get_reasonably_live_consensus, REASONABLY_LIVE_TIME is
 24 hours after consensus expiry, so the tolerance must not be higher than
 24 hours.
 * Relay clocks are allowed to drift 12 hours ahead, and 24 hours behind.
 So it might be reasonable to have the tolerance be 0, or a slightly
 positive value.

 If 0.2.9 is released without #20909 being fixed, wait until most clients
 are on 0.3.0 before making this change.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20942>
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