[tor-bugs] #9972 [Tor]: Failed to find node for hop 0 of our path. Discarding this circuit.

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#9972: Failed to find node for hop 0 of our path. Discarding this circuit.
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     Reporter:  mr-4    |      Owner:  nickm
         Type:  defect  |     Status:  needs_revision
     Priority:  normal  |  Milestone:  Tor: 0.2.6.x-final
    Component:  Tor     |    Version:  Tor: 0.2.4.17-rc
   Resolution:          |   Keywords:  tor-client 025-triaged
Actual Points:          |  Parent ID:
       Points:          |
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Comment (by asn):

 I did some debugging using mr-4's config. Here is at least part of the
 problem:

 If we start with no state, we have 0 entry guards. When we call
 `choose_random_entry_impl()` with `EntryNodes {US}` set, Tor is supposed
 to use `entry_guards_set_from_config()` to add all entry nodes from the US
 to the `entry_guards` list.
 Then the `entry_guards` list is suppose to be filtered, so that in the end
 we pick a single entry guard.

 However, it seems that `entry_guards_set_from_config()` is called quite
 early in the bootstrap process, and when it calls
 `routerset_get_all_nodes()` (which is supposed to add all those US entry
 guards) it gets to:
 {{{
   } else {
     /* We need to iterate over the routerlist to get all the ones of the
      * right kind. */
     smartlist_t *nodes = nodelist_get_list();
     SMARTLIST_FOREACH(nodes, const node_t *, node, {
 }}}
 but so early in the bootstrap process we don't have a consensus and
 `nodelist_get_list()` just returns an empty smartlist. So no looping
 happens, and no entry guards are added to our `entry_guards` smartlist.

 To test this hypothesis, I added:
 {{{
   if (smartlist_len(entry_guards) > 1)
     should_add_entry_nodes = 0;
 }}}
 in the end of `entry_guards_set_from_config()`, instead of zeroing
 `should_add_entry_nodes` in the beginning. I just did this to test what
 would happen if we keep on trying to add entry nodes till we succeed to
 add at least one. With the above change, Tor managed to bootstrap
 normally!

 We should think of a smarter way to realize when we
 `should_add_entry_nodes` should be toggled.
 `entry_nodes_should_be_added()` is not smart enough.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/9972#comment:24>
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