[metrics-bugs] #25767 [Metrics/Consensus Health]: consensus-health should check for clock skew on dir auths

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#25767: consensus-health should check for clock skew on dir auths
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     Reporter:  arma                      |      Owner:  tom
         Type:  enhancement               |     Status:  new
     Priority:  Medium                    |  Milestone:
    Component:  Metrics/Consensus Health  |    Version:
     Severity:  Normal                    |   Keywords:
Actual Points:                            |  Parent ID:
       Points:                            |   Reviewer:
      Sponsor:                            |
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 In #25756 we discovered that it can actually produce a user-visible
 complaint when dizum has its clock wrong by 65 seconds.

 Specifically, when the combination of skew from the dir auth and from the
 relay doing the fetch sum to more than 60 seconds, then if the relay
 decides to update its consensus at just the right time, the relay will get
 a warn message.

 It would be great if consensus-health checked the {{{Date:}}} header on
 dir auth responses, and did something smart if it's not right enough.

 Note that the Date header gets chosen when the dir auth begins to respond
 to the request, so if it takes 25 seconds to finish downloading the
 request, you shouldn't decide that the Date was wrong by 25 seconds.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25767>
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