[tor-bugs] #18398 [Tor]: Check fallback directory performance before including them in the list

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#18398: Check fallback directory performance before including them in the list
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     Reporter:  teor         |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  Medium       |  Milestone:  Tor: 0.2.8.x-final
    Component:  Tor          |    Version:
     Severity:  Normal       |   Keywords:  easy
Actual Points:               |  Parent ID:
       Points:  small        |    Sponsor:
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 One fallback directory is quite slow to provide consensuses (~30s).
 atagar suggested in #18177 that we check fallbacks' speeds before we add
 them to the list (and provided code!)

 We need to add this to the updateFallbackDirs.py script before we generate
 the next fallback list.

 (One drawback of checking times is that we'll get slightly different
 results from different connections. This shouldn't be an issue if we
 download one at a time, and re-time failed connections.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/18398>
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