[tor-bugs] #17903 [Tor]: router_pick_trusteddirserver_impl should distinguish between fallbacks and authorities

[Tor Bug Tracker & Wiki]

#17903: router_pick_trusteddirserver_impl should distinguish between fallbacks and
authorities
-----------------------------+--------------------------------
     Reporter:  teor         |      Owner:
         Type:  enhancement  |     Status:  new
     Priority:  Medium       |  Milestone:  Tor: 0.2.8.x-final
    Component:  Tor          |    Version:
     Severity:  Normal       |   Keywords:
Actual Points:               |  Parent ID:
       Points:               |    Sponsor:
-----------------------------+--------------------------------
 router_pick_directory_server_impl currently prioritises directories in the
 following order:
 * tunneled
   * mirrors
   * overloaded mirrors
   * authorities (trusted)
 * direct
   * mirrors
   * overloaded mirrors
   * authorities (trusted)

 But router_pick_trusteddirserver_impl uses the following order:
 * tunneled
   * fallbacks + authorities
   * overloaded fallbacks + authorities
 * direct
   * fallbacks + authorities
   * overloaded fallbacks + authorities

 I suggest we change it to:
 * tunneled
   * fallbacks
   * overloaded fallbacks
   * authorities
 * direct
   * fallbacks
   * overloaded fallbacks
   * authorities

 We should do this for consistency, and to help with #17847 (unifying these
 two functions).

 (This doesn't make much practical difference, because authorities are
 weighted 1.0, and fallbacks are weighted ~100,000.0, so authorities get
 selected very rarely. However, this would modify the effect of the
 DirAuthorityFallbackRate torrc option.)

 If we do this, let's refactor the code so it's all in one place, probably
 using a macro.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17903>
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