[tor-bugs] #18921 [Core Tor/Tor]: Fix IPv6 bridge client directory address selection

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#18921: Fix IPv6 bridge client directory address selection
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 Reporter:  teor                                 |          Owner:
     Type:  defect                               |         Status:
 Priority:  Medium                               |  merge_ready
Component:  Core Tor/Tor                         |      Milestone:  Tor:
 Severity:  Normal                               |  0.2.8.x-final
 Keywords:  must-fix-before-028-rc,              |        Version:  Tor:
  TorCoreTeam201605, TorCoreTeam-                |  0.2.8.1-alpha
  postponed-201604                               |     Resolution:
Parent ID:  #18483                               |  Actual Points:  1 hour
 Reviewer:  isis                                 |         Points:  small
                                                 |        Sponsor:
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Comment (by nickm):

 This is kinda big for an rc/stable patch.  It looks okay and I think we
 need it and I'm okay with taking it anyway, but I'm also nominating it for
 my "I-told-you-so" award in case there is a bug here that none of us
 successfully found.

 Initial Questions:
   * e726459eaf57ffe98c36730af58287229c9531d6: Are we really really sure
 that every possible case of the big complicated function
 directory_fetches_from_authorities() should be replaced with
 !directory_must_use_begindir() ?
   * e155948d0ac54b3102d6efa3d4a689d8a177f0f4: This really really needs
 comments in the new code here.

 Okay, I need to come back and review the rest of this later.  Big branch!

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/18921#comment:9>
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