[tor-bugs] #13718 [Tor]: Reachability Tests aren't conducted if there are no exit nodes

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#13718: Reachability Tests aren't conducted if there are no exit nodes
-------------------------+-------------------------------------------------
     Reporter:  tom      |      Owner:  teor
         Type:  defect   |     Status:  assigned
     Priority:  normal   |  Milestone:  Tor: 0.2.6.x-final
    Component:  Tor      |    Version:  Tor: 0.2.6.1-alpha
   Resolution:           |   Keywords:  tor-relay test-network lorax
Actual Points:           |  chutney 026-deferrable
       Points:           |  Parent ID:
-------------------------+-------------------------------------------------

Comment (by teor):

 #13839 can be resolved using `TestingMinExitFlagThreshold 0`, which causes
 authorities to ignore advertised / measured bandwidth when assigning the
 Exit flag.

 A minor fix is required to `router_is_active()` to ignore measured
 bandwidth when assigning the Active flag (which is required for the Exit
 flag).

 After this change:

 An exit node with `AssumeReachable 0` now makes it into the consensus
 after around 30-40 seconds. Clients then use this updated consensus from
 NN-NN seconds (the old consensus must expire first). The network uses
 `TestingMinExitFlagThreshold 0` on the authorities, rather than
 `TestingDirAuthVoteExit` (from #13161).

 This means that the exit node correctly: (20s-30s)
 * determines that no Exits are available in the consensus
 * continues to bootstrap with internal paths only
 * successfully self-tests reachability with an internal path
 * posts its descriptor to the authorities

 And the authorities correctly: (30s-40s)
 * assign the Exit flag
 * distribute an updated consensus containing the Exit node

 And the client correctly: (40s-60s)
 * requests the newest consensus
   * see #13963 for a fix that reduces this request time from 3 minutes to
 half the consensus interval, in networks with a low consensus interval.
 * determines that Exits are now available in the new consensus
 * starts building external paths

 I'll attach authority, relay and client torrcs that exhibit this behaviour
 with the patches listed above.

 Now I'll review my changes and group them into commits with appropriate
 changes/* files. This could take me a few days.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13718#comment:32>
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