[metrics-bugs] #27050 [Metrics/Onionoo]: Reverse DNS lookups are still slow

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#27050: Reverse DNS lookups are still slow
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     Reporter:  irl              |      Owner:  metrics-team
         Type:  defect           |     Status:  new
     Priority:  Medium           |  Milestone:
    Component:  Metrics/Onionoo  |    Version:
     Severity:  Normal           |   Keywords:
Actual Points:                   |  Parent ID:
       Points:                   |   Reviewer:
      Sponsor:                   |
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 It would appear that only new relays are looked up. Any relay that already
 exists with a `NodeStatus` is not looked up. I believe this is due to
 incorrectly recording the last lookup time even where no lookup has
 actually occurred.

 The lack of a reverse DNS record and the failure of a lookup (including
 running out of time before even getting to it) all count as success and a
 lookup time will be recorded. This prevents the lookup being attempted
 again for another 12 hours.

 I hadn't spotted this when testing locally as I'd always set
 `RDNS_LOOKUP_MAX_AGE_MILLIS` to `0L` to ensure that every relay was looked
 up every time.

 This has happened this way as I had tried to maintain backwards-bug-
 compatibility with the old field that returned an IP address in cases
 where the lookup did not find an error but also did not find a domain. The
 implementation wasn't strict enough in its bug compatibility because it
 returned an IP address also in error conditions.

 I wonder if it is worth spending time to fix this here, or instead
 removing the `host_name` field (which is optional in the spec, and we did
 deprecate on July 16th) and so not requiring the returning of IP addresses
 at all.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/27050>
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