DHT bootnodes: get randomness from the next epoch descriptor

[dmitry-markin]

Use runtime calls to get epoch randomness only on startup, and later get it from the next epoch descriptor at the first block of every epoch.

Resolves #8377.

[bkchr]

Some edge case that is not covered is when the notification stream skipped blocks (because of major sync). Then you would maybe advertise too long with an invalid key.

[dmitry-markin]

Unfortunately, stop_providing is a local-only action only (Kademlia DHT doesn't have a way to tell target nodes we are not providing the key anymore), so remote nodes will advertise every key until it expires in 48 hours anyway.

[bkchr]

This was not what I wanted to say :) The problem I raised there is when you for example sync across an epoch change, but the node was doing this in "major sync mode". Then you would not get a notification for these blocks.

[dmitry-markin]

Thanks for the explanation, this makes sense. How common is such situation except during the initial sync?

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/14833176530
Failed job name: test-linux-stable

[lexnv]

It looks like an unrelated test to these changes is failing (since we have only modified cumulus/client/bootnodes/src/advertisement.rs). Might be worth rerunning and raising an issue about it? 🤔

──── TRY 1 STDERR:       sc-service-test client::returns_status_for_pruned_blocks
thread 'client::returns_status_for_pruned_blocks' panicked at substrate/client/service/test/src/client/mod.rs:1845:5:
assertion `left == right` failed
  left: InChainWithState
 right: InChainPruned

[skunert]

It looks like an unrelated test to these changes is failing (since we have only modified cumulus/client/bootnodes/src/advertisement.rs). Might be worth rerunning and raising an issue about it? 🤔

──── TRY 1 STDERR:       sc-service-test client::returns_status_for_pruned_blocks
thread 'client::returns_status_for_pruned_blocks' panicked at substrate/client/service/test/src/client/mod.rs:1845:5:
assertion `left == right` failed
  left: InChainWithState
 right: InChainPruned

Yes we can rerun, seems to be known flaky :(

polkadot-sdk/substrate/client/service/test/src/client/mod.rs

Lines 1750 to 1752 in db5ee7d

	#[test]
	// FIXME: https://github.com/paritytech/polkadot-sdk/issues/48
	fn returns_status_for_pruned_blocks() {