[tor-bugs] #23817 [Core Tor/Tor]: Tor re-tries directory mirrors that it knows are missing microdescriptors

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#23817: Tor re-tries directory mirrors that it knows are missing microdescriptors
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 Reporter:  teor                        |          Owner:  (none)
     Type:  defect                      |         Status:  new
 Priority:  Medium                      |      Milestone:  Tor:
                                        |  0.3.3.x-final
Component:  Core Tor/Tor                |        Version:
 Severity:  Normal                      |     Resolution:
 Keywords:  tor-guard, tor-hs, prop224  |  Actual Points:
Parent ID:  #21969                      |         Points:
 Reviewer:                              |        Sponsor:
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Comment (by asn):

 Here is a draft of a suggested technical solution here which involves a
 cache tracking failed md downloads:

 1) Introduce an `md_failure_cache` cache which tracks failed or missing
 mds downloads per relays. That is a cache with entries as follows: "We
 failed to fetch md <hash> from relays X, Y, Z".

   engineering XXX: how to figure out missing mds from received batches

 2) When we want to fetch a batch of mds from relay X, check
 `md_failure_cache` to see if any of the batched mds is included in the
 cache for relay X. if that's the case, add relay X to the excluded list of
 nodes for dirservers, so that our directory subsystems skips relay X and
 moves to the next one in line.

          engineering XXX: is there an exclude list for dirserver
 downloads?

 3) When we succeed in fetching an md, remove its entries from the cache
 (so that we retry in the future from  relay X which is probably our top
 primary guard).

 4) Finally, to address #23863, when we detect that we are missing mds from
 our primary guards, order an immediate download of those missing mds, so
 that the previous steps get done.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23817#comment:4>
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