[tor-bugs] #19522 [Core Tor/Tor]: HS intro circuit retry logic fails when network interface is down

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#19522: HS intro circuit retry logic fails when network interface is down
------------------------------+--------------------------
     Reporter:  asn           |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.???
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-hs
Actual Points:                |  Parent ID:  #16387
       Points:  1.5           |   Reviewer:
      Sponsor:  SponsorR-can  |
------------------------------+--------------------------
 During investigations in #16387, we found out that mobile HSes won't retry
 their intro point circuits when their network interface is down.

 This is a problem since mobile devices change their IP address all the
 time, and their network interface toggles on and off. Since the retry
 logic fails in this case, the mobile HS ends up rotating intro points
 everytime the interface goes up.

 The problem is this chunk of code in
 `rend_consider_services_intro_points()`:
 {{{
     /* Let's try to rebuild circuit on the nodes we want to retry on. */
     SMARTLIST_FOREACH_BEGIN(retry_nodes, rend_intro_point_t *, intro) {
       r = rend_service_launch_establish_intro(service, intro);
       if (r < 0) {
         log_warn(LD_REND, "Error launching circuit to node %s for service
 %s.",
 safe_str_client(extend_info_describe(intro->extend_info)),
                  safe_str_client(service->service_id));
         /* Unable to launch a circuit to that intro point, remove it from
          * the valid list so we can create a new one. */
         smartlist_remove(service->intro_nodes, intro);
         rend_intro_point_free(intro);
         continue;
       }
       intro->circuit_retries++;
     } SMARTLIST_FOREACH_END(intro);
 }}}

 When our interface is down, `rend_service_launch_establish_intro()` will
 fail immediately since it eventually calls `connection_or_connect()` which
 leads to a `connect()` call with no network.

 As you can see from the code above, when that function fails immediately,
 we remove the intro point no questions asked, with no subsequent retries.

 This is problematic, since the failure there is because of our local
 network, and does not indicate any issues with the intro point, so we
 should not ditch it so easily. Ideally, as long as we know that the error
 is local, we should probably keep on trying that same intro point.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19522>
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