[tor-bugs] #7479 [Core Tor/Tor]: Replace more linked lists with queue.h implementations

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#7479: Replace more linked lists with queue.h implementations
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 Reporter:  nickm                             |          Owner:
     Type:  defect                            |         Status:  new
 Priority:  Low                               |      Milestone:  Tor:
                                              |  unspecified
Component:  Core Tor/Tor                      |        Version:
 Severity:  Normal                            |     Resolution:
 Keywords:  tor-relay refactoring easy intro  |  Actual Points:
Parent ID:                                    |         Points:
 Reviewer:                                    |        Sponsor:
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Comment (by arma):

 Replying to [comment:1 nickm]:
 >   * crypt_path_t (weird circular list. I forget why it is circular.)

 It is almost entirely so we can have easy access to "the last hop in the
 circuit" just by looking at cpath->prev.

 There are a couple of places where we use the reverse links, e.g.
 {{{
   if (circ->cpath &&
       circ->cpath->prev->state != CPATH_STATE_OPEN &&
       circ->cpath->prev->prev->state == CPATH_STATE_OPEN) {
     failed_at_last_hop = 1;
   }
 }}}
 but those could probably be refactored or just turned into a more brute
 force walk over the forward list, if we needed.

 Though, check out this use in circuit_package_relay_cell() where it is
 honestly more convenient to be able to walk backwards on the list:
 {{{
     thishop = layer_hint;
     /* moving from farthest to nearest hop */
     do {
       tor_assert(thishop);
       log_debug(LD_OR,"crypting a layer of the relay cell.");
       if (relay_crypt_one_payload(thishop->f_crypto, cell->payload, 1) <
 0) {
         return -1;
       }

       thishop = thishop->prev;
     } while (thishop != TO_ORIGIN_CIRCUIT(circ)->cpath->prev);
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/7479#comment:2>
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