Commits on Mar 18, 2024

1. Fix kusama validators getting 0 backing rewards the first session the… …

   …y enter the active set (#3722)
   
   There is a problem in the way we update `authorithy-discovery` next keys
   and because of that nodes that enter the active set would be noticed at
   the start of the session they become active, instead of the start of the
   previous session as it was intended. This is problematic because:
   
   1. The node itself advertises its addresses on the DHT only when it
   notices it should become active on around ~10m loop, so in this case it
   would notice after it becomes active.
   2. The other nodes won't be able to detect the new nodes addresses at
   the beginning of the session, so it won't added them to the reserved
   set.
   
   With 1 + 2, we end-up in a situation where the the new node won't be
   able to properly connect to its peers because it won't be in its peers
   reserved set. Now, the nodes accept by default`MIN_GOSSIP_PEERS: usize =
   25` connections to nodes that are not in the reserved set, but given
   Kusama size(> 1000 nodes) you could easily have more than`25` new nodes
   entering the active set or simply the nodes don't have slots anymore
   because, they already have connections to peers not in the active set.
   
   In the end what the node would notice is 0 backing rewards because it
   wasn't directly connected to the peers in its backing group.
   
   ## Root-cause
   
   The flow is like this:
   1. At BAD_SESSION - 1, in `rotate_session` new nodes are added to
   QueuedKeys
   https://github.com/paritytech/polkadot-sdk/blob/02e1a7f476d7d7c67153e975ab9a1bdc02ffea12/substrate/frame/session/src/lib.rs#L609
   ```
    <QueuedKeys<T>>::put(queued_amalgamated.clone());
   <QueuedChanged<T>>::put(next_changed);
   ```
   2. AuthorityDiscovery::on_new_session is called with `changed` being the
   value of `<QueuedChanged<T>>:` at BAD_SESSION - **2** because it was
   saved before being updated
   https://github.com/paritytech/polkadot-sdk/blob/02e1a7f476d7d7c67153e975ab9a1bdc02ffea12/substrate/frame/session/src/lib.rs#L613
   3. At BAD_SESSION - 1, `AuthorityDiscovery::on_new_session` doesn't
   updated its next_keys because `changed` was false.
   4. For the entire durations of `BAD_SESSION - 1` everyone calling
   runtime api `authorities`(should return past, present and future
   authorities) won't discover the nodes that should become active .
   5. At the beginning of BAD_SESSION, all nodes discover the new nodes are
   authorities, but it is already too late because reserved_nodes are
   updated only at the beginning of the session by the `gossip-support`.
   See above why this bad.
   
   ## Fix
   Update next keys with the queued_validators at every session, not matter
   the value of `changed` this is the same way babe pallet correctly does
   it.
   https://github.com/paritytech/polkadot-sdk/blob/02e1a7f476d7d7c67153e975ab9a1bdc02ffea12/substrate/frame/babe/src/lib.rs#L655
   
   ## Notes
   
   - The issue doesn't reproduce with proof-authorities changes like
   `versi` because `changed` would always be true and `AuthorityDiscovery`
   correctly updates its next_keys every time.
   - Confirmed at session `37651` on kusama that this is exactly what it
   happens by looking at blocks with polkadot.js.
   
   ## TODO
   - [ ] Move versi on proof of stake and properly test before and after
   fix to confirm there is no other issue.
   
   ---------
   
   Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>
   Co-authored-by: Bastian Köcher <git@kchr.de>
   (cherry picked from commit 8d0cd4f)
   Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>

   

   alexggh and bkchr committed Mar 18, 2024
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