prop224: Expand the overlap period concept to be a full SRV protocol run

Because of #23387, we've realized that there is one scenario that makes
the client unable to reach the service because of a desynch in the time
period used. The scenario is as follows:

  +------------------------------------------------------------------+
  |                                                                  |
  | 00:00      12:00       00:00       12:00       00:00       12:00 |
  | SRV#1      TP#1        SRV#2       TP#2        SRV#3       TP#3  |
  |                                                                  |
  |  $==========|-----------$===========|-----------$===========|    |
  |                                    ^ ^                           |
  |                                    C S                           |
  +------------------------------------------------------------------+

In this scenario the HS has a newer consensus than the client, and the
HS just moved to the next TP but the client is still stuck on the old
one. However, the service is not in any sort of overlap mode so it
doesn't cover the old TP anymore, so the client is unable to fetch a
descriptor.

We've decided to solve this by extending the concept of overlap period
to be permanent so that the service always publishes two descriptors and
aims to cover clients with both older and newer consensuses. See the
spec patch in #23387 for more details.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/or/hs_common.h b/src/or/hs_common.h
index a85e86a0c4..f09bbe94dd 100644
--- a/src/or/hs_common.h
+++ b/src/or/hs_common.h
@@ -195,6 +195,7 @@ void hs_get_subcredential(const ed25519_public_key_t *identity_pk,
                           const ed25519_public_key_t *blinded_pk,
                           uint8_t *subcred_out);
 
+uint64_t hs_get_previous_time_period_num(time_t now);
 uint64_t hs_get_time_period_num(time_t now);
 uint64_t hs_get_next_time_period_num(time_t now);
 time_t hs_get_start_time_of_next_time_period(time_t now);