leader: do not perform blockstore check in next_leader_slot

[AshwinSekar]

Problem

next_leader_slot will skip over any group of 4 leader slots if there is a shred present for the first leader slot. This can lead to accidentally skipping leader slots where only the first block has been produced in cases of reset to an earlier slot.

Summary of Changes

Instead remove the blockstore check entirely. In poh_recorder, if we've ticked past our last slot, recompute the next leader slot's tick heights to be used instead. This builds on #2416 which performs the blockstore check in poh_recorder

[jstarry]

I don't think the problem you stated is true, is it? next_leader_slot uses a flat_map to iterate over each of their leader slots, not just the first slot of each leader slot span

[AshwinSekar]

I don't think the problem you stated is true, is it? next_leader_slot uses a flat_map to iterate over each of their leader slots, not just the first slot of each leader slot span

You're correct, we evaluate each slot, not just the first one.
I suppose even in a case like this, If we've only produced the 2nd leader block:

1 - 2 (have shreds) - 3 - 4

And then reset to slot 0, next_leader_slot will return Some((1, 1)). However we will still produce slot 3 and 4 after we've ticked through 2 (even if we don't reset to 1), since reached_leader_slot does not care if you've ticked past last_leader_tick_height.

It seems then that this change is not needed.

Here's the code in question:

agave/ledger/src/leader_schedule_cache.rs

Lines 128 to 156 in 83bb29e

	let mut schedule = (epoch..=max_epoch)
	.map(|epoch| self.get_epoch_schedule_else_compute(epoch, bank))
	.while_some()
	.zip(epoch..)
	.flat_map(|(leader_schedule, k)| {
	let offset = if k == epoch { start_index as usize } else { 0 };
	let num_slots = bank.get_slots_in_epoch(k) as usize;
	let first_slot = bank.epoch_schedule().get_first_slot_in_epoch(k);
	leader_schedule
	.get_indices(pubkey, offset)
	.take_while(move |i| *i < num_slots)
	.map(move |i| i as Slot + first_slot)
	})
	.skip_while(|slot| {
	// Skip slots we already have shreds for
	blockstore
	.map(|bs| bs.has_existing_shreds_for_slot(*slot))
	.unwrap_or(false)
	});
	let first_slot = schedule.next()?;
	let max_slot = first_slot.saturating_add(max_slot_range);
	let last_slot = schedule
	.take_while(|slot| *slot < max_slot)
	.zip(first_slot + 1..)
	.take_while(|(a, b)| a == b)
	.map(|(s, _)| s)
	.last()
	.unwrap_or(first_slot);
	Some((first_slot, last_slot))