Sync lag gate: allow block proposals

[zemse]

Context

Original issue #689
Spec PR #708

What is wrong

When SYNC_LAG_THRESHOLD consecutive validators do not propose a block maliciously, it tricks the network into thinking that it is sync lagged. The next NETWORK_STALL_THRESHOLD - SYNC_LAG_THRESHOLD honest validators do not author the blocks even when they can until the NETWORK_STALL_THRESHOLD is crossed.

This issue can be reproduced with ethlambda when 5 consecutive validators maliciously skip authoring blocks.

Found this issue while working on sync lag in Ream ReamLabs/ream#1385 (comment).

Proposed fix

If the validator is proposer for current slot, they should propose block regardless of sync lagged or not. Only skip attestations when sync lagged. Proposing from a stale view does not cause a lot of pollution (as compared to attestations) because it is very rare that a validator becomes a proposer.

leanSpec/src/lean_spec/node/validator/service.py

Lines 125 to 126 in eca701e

	if self._is_synced_for_duties(slot, "block"):
	await self._maybe_produce_block(slot)