Gloas ensure we emit payload attributes SSE event

[eserilev]

Issue Addressed

#8817

we were not emitting payload attribute SSE events for gloas. Since the head snapshot stores the canonical latest execution payload envelope we have all the info we need to emit the event.

[mergify]

Some required checks have failed. Could you please take a look @eserilev? 🙏

[michaelsproul]

This assumption is wrong currently: the snapshot.execution_envelope is also None for empty blocks. Currently we only load the head execution envelope for full blocks, here:

lighthouse/beacon_node/beacon_chain/src/canonical_head.rs

Lines 736 to 748 in 309719d

	let execution_envelope = if new_payload_status == PayloadStatus::Full {
	let envelope = self
	.store
	.get_payload_envelope(&new_view.head_block_root)?
	.map(Arc::new)
	.ok_or(Error::MissingExecutionPayloadEnvelope(
	new_view.head_block_root,
	))?;
	Some(envelope)
	} else {
	None
	};

We could change this invariant so that we always have a payload post-Gloas, by loading/storing the most recent envelope (i.e. the parent's envelope in the case of an empty block).

[eserilev]

thanks for catching that, i've changed the invariant so that we always have a payload post-gloas and added some tests

[mergify]

Some required checks have failed. Could you please take a look @eserilev? 🙏