Fix kusama validators getting 0 backing rewards the first session they enter the active set

[alexggh]

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 defaultMIN_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 than25 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

   polkadot-sdk/substrate/frame/session/src/lib.rs

   Line 609 in 02e1a7f

   pub fn rotate_session() {

 <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

   polkadot-sdk/substrate/frame/session/src/lib.rs

   Line 613 in 02e1a7f

   let changed = <QueuedChanged<T>>::get();

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.

polkadot-sdk/substrate/frame/babe/src/lib.rs

Line 655 in 02e1a7f

NextAuthorities::<T>::put(&next_authorities);

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.

[bkchr]

Nice find 👍

Not sure you need to test this on versi. However, the tests should be updated. Especially as there is already one authorities_returns_current_and_next_authority_set ;)

[alexggh]

Not sure you need to test this on versi.

I agree the fix is pretty clear, I just want to make sure there aren't other places that I missed. We don't need to wait with merging this PR, until I get that setup working.

However, the tests should be updated. Especially as there is already one authorities_returns_current_and_next_authority_set ;)

Done.

IMO it would make sense to make the docs of on_new_session more clear.

Did my best hopefully it is clearer now.

[s0me0ne-unkn0wn]

Looks good! I think, although it would be good to test on Versi, it's not strictly required. By the nature of changes, the worst possible scenario is that we're updating something we shouldn't update with the data still correct, so it doesn't make any worse anyway.

[ordian]

👍

[bkchr]

I think, although it would be good to test on Versi, it's not strictly required.

If you want to test this, run a test network locally. Set MIN_GOSSIP_PEERS to 0 and then reproducing this should be fairly simple.

[bkchr]

@alexggh please add some small prdoc explaining the change.

[ordian]

Does this also fix #1819?

[alexggh]

Does this also fix #1819?

It does help, statement-v1 does not require direct connections to your peers in the validator group, so you might get lucky with the random gossiping when you are sparsely connected, but fully lucky, so I guess that's why we would see just some missed statements instead of missing all the statements as it happens with statements-v2.

[alexggh]

If you want to test this, run a test network locally. Set MIN_GOSSIP_PEERS to 0 and then reproducing this should be fairly simple.

Yeah, that's what I ended-up doing, it is not that easy, because ValidatorManager changes the set every session, so it was masking the problem also you need some bigger waiting times and sessions because DHT is updated every 10 minutes.

Managed to reproduce it with zombienet and a modified ValidatorManager and confirmed this patch is fixing the issue.

[alexggh]

@alexggh please add some small prdoc explaining the change.

Done.