[tor-bugs] #22940 [Core Tor/Tor]: prop224: HS revision counter should persist after service reboot

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#22940: prop224: HS revision counter should persist after service reboot
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     Reporter:  asn           |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-hs prop224
Actual Points:                |  Parent ID:  #20657
       Points:  1             |   Reviewer:
      Sponsor:  SponsorR-can  |
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 We currently have the following logic in the HSDir-side when accepting an
 HS descriptor:
 {{{
     if (cache_entry->plaintext_data->revision_counter >=
         desc->plaintext_data->revision_counter) {
       log_info(LD_REND, "Descriptor revision counter in our cache is "
                         "greater or equal than the one we received. "
                         "Rejecting!");
       goto err;
     }
 }}}

 Unfortunately, while HSes keep track of the revision counter in memory,
 they never save it on disk, so if the service reboots the Hs will lose
 track of the rev counter and publish descriptors with small counters that
 will get rejected by the HSDir.

 We have brainstormed the following solutions for this:
 a) Save the latest rev counter in the state file in a form like this:
 `HidServRevCounter <service_pubkey> <rev_counter> <time_period_num>
 b) Set the rev counter to a value like `rounddown(time(NULL))` so that the
 HS always generates correct rev counters even without saving them on
 state.

 However, wrt (b), since the quoted check above rejects equal rev counters
 the HS will have trouble uploading descriptors in the same hour. We could
 backport a fix that changes the check to 0.3.0 and then do this idea but
 that's kind of a PITA.

 We are currently aiming for (a)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22940>
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