[tor-bugs] #19879 [Core Tor/Tor]: Derive FILTERED_GUARDS / USABLE_FILTERED_GUARDS from SAMPLED_GUARDS per new guard plan

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#19879: Derive FILTERED_GUARDS / USABLE_FILTERED_GUARDS from SAMPLED_GUARDS per new
guard plan
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     Reporter:  andrea        |      Owner:
         Type:  task          |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.9.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:  #19877
       Points:  3             |   Reviewer:
      Sponsor:  SponsorU      |
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 New guard plan summarized at https://lists.torproject.org/pipermail/tor-
 dev/2016-July/011234.html

 3) Derive FILTERED_GUARDS / USABLE_FILTERED_GUARDS from SAMPLED_GUARDS
  - The filtering function can be called at any point (e.g. ON_CONSENSUS)
  - Write unittests ensuring correctness of filtering
  - See populate_live_entry_guards()
  - Thoughtworks function: filter_set() / filter_sampled()

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19879>
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