[tor-bugs] #18816 [Tor]: We still wait 120 seconds for cert fetches from missing dir mirrors

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#18816: We still wait 120 seconds for cert fetches from missing dir mirrors
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     Reporter:  arma    |      Owner:
         Type:  defect  |     Status:  new
     Priority:  Medium  |  Milestone:  Tor: 0.2.???
    Component:  Tor     |    Version:
     Severity:  Normal  |   Keywords:  029-proposed
Actual Points:          |  Parent ID:
       Points:          |   Reviewer:
      Sponsor:          |
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 In #4483 and prop210 we set up an elaborate download schedule for
 consistently reaching fallbackdirs when fetching the consensus, so we
 don't end up just sitting there for 120 seconds while a tcp connection
 waits (and eventually the SocksTimeout parameter is reached and we move
 on).

 But we didn't do any similar thing with fetching the key certs. I just had
 my bootstrap go smoothly through the #4483 features (with the fixes from
 #18809) and then it stalled for 2 minutes trying to fetch the certs from a
 fallbackdir that's offline.

 Sure enough, in authority_certs_fetch_missing() I see
 {{{
       /* XXX - do we want certs from authorities or mirrors? - teor */
       directory_get_from_dirserver(DIR_PURPOSE_FETCH_CERTIFICATE, 0,
                                    resource, PDS_RETRY_IF_NO_SERVERS,
                                    DL_WANT_ANY_DIRSERVER);
 }}}

 So teor noticed this one too.

 I think in 0.2.8, if we leave the fallbackdir stuff in (meaning we merge
 #18809 or equivalent into 0.2.8), we could bandage this one by changing
 DL_WANT_ANY_DIRSERVER to DL_WANT_AUTHORITY, and then it wouldn't be much
 worse than it is now (in terms of performance -- we would indeed lose the
 ability to bootstrap from scratch when the authorities are unavailable).

 Longer term (0.2.9 and later), I think we should explore a) having
 directory_get_from_dirserver() notice that there are tls conns established
 to dir mirrors that we just recently used (and prefer them), or b) trying
 to explicitly remember the dir mirror that gave us the consensus and re-
 use it, and/or c) designing a piggy-back mechanism so we can ask for "the
 certs that go with this consensus" when we're fetching a consensus and we
 know we will want the certs for it too (thus saving a round-trip).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/18816>
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