[tor-bugs] #12595 [Tor]: Think of better data structures for guard nodes

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#12595: Think of better data structures for guard nodes
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     Reporter:  asn     |      Owner:
         Type:  defect  |     Status:  new
     Priority:  normal  |  Milestone:  Tor: 0.2.6.x-final
    Component:  Tor     |    Version:
   Resolution:          |   Keywords:  tor-guard
Actual Points:          |  Parent ID:
       Points:          |
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Comment (by asn):

 One of the possible designs discussed during the dev meeting, is to have
 two guard lists. One list is the `entry_guards` list, and the other is the
 `directory_guards` list.

 The idea is that if your primary entry guard is not a directory cache,
 then you use the `directory_guards` list to get a directory guard. Or
 maybe you just use the `directory_guards` list for all your directory
 needs anyway. Also, in the future when #12597 becomes totally deployed, we
 will be able to move to a single list, where all nodes will be both
 directory guards and entry guards.

 Some more ideas that were thrown around during the dev meeting, and would
 make guard selection smoother:
 - `All guards should be directory caches` which will happen as part of
 #12597.
 - `All guards should be considered fast/stable`, '''or''' `Relays can be
 guards iff they are fast/stable`. This will make it more likely that
 guards on the top of our list can be used for our purposes, since we will
 not have to skip guards till we find a stable guard for stable circuits.
 However, because of the analysis in #12204, this is not very likely to
 happen anyway.

 Also, it's worth noting that designing data structures with the assumption
 that we are only going to have 1 primary guard is probably easier than
 designing data structures that can accommodate N primary guards.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/12595#comment:1>
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