Collator proto revamp: Picking an advertisement to fetch doesn't take into account the lookahead value

[tdimitrov]

When launching fetch requests we examine pending candidate fetches/seconded candidates and based on the claim queue we make a list of assignments (ParaIds) which we need to make a fetch for. However when building the assignment list we don't take into account the position of the assignment in the claim queue.

Due to this we can end up in the following situation:

lookahead = 3
claim queue state:

block_state=[
	ClaimInfo { hash: Some(0x7c9b8a6d207fda4d22ce0f0c641111a3e5ce73a65efd7427d2250d8e602d43b7), claim: Some(1000), claim_queue_len: 3, claimed: Seconded(0x66ee09778b68a2541c1de649be0d54afb562ec0b83b3ac93d5ab96b5c50ae7a3) }, 	// 64
	ClaimInfo { hash: Some(0xfbdf9156264ac83f9e910b2f82a14357c81f16654dc826619233434b77dcbd0a), claim: Some(1000), claim_queue_len: 3, claimed: Pending(Some(0x3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e)) }, //65
	ClaimInfo { hash: Some(0x7c870a0cb9e0531d437598cea565be97ff1cc6125a329a50f46f58d4289d9a41), claim: Some(1000), claim_queue_len: 3, claimed: Pending(Some(0x7b2c6c2353cff5eed11ea58ed4aa40e7b683124c228656c77db877dd8e6b008a)) }] //66
future_blocks=[
	ClaimInfo { hash: None, claim: Some(1000), claim_queue_len: 1, claimed: Pending(Some(0x3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e3e)) }, 
	ClaimInfo { hash: None, claim: Some(1000), claim_queue_len: 1, claimed: Free }

In this case we have got one free position in the claim queue (in future blocks) and we need to make a fetch for it. However when we pick the best advertisement to fetch we don't take into account it's relay parent.

For example in the case above, we can't fetch a candidate with RP 0x7c9b8a6d207fda4d22ce0f0c641111a3e5ce73a65efd7427d2250d8e602d43b7 (first entry) because the free claim is not within its view. However pick_best_advertisement does exactly that and as a result we fail to claim a slot for it (here) and we skip the slot.

[tdimitrov]

Possible soluition: make the claim in pick_best_advertisement. That way if we see the claim fails, we can continue looking for candidates.

[eskimor]

Is this theory or has this been witnessed in testing? If witnessed in testing, in what scenario?

[tdimitrov]

Witnessed in testing, under some load. It's not causing any serious issues, but it's nice to have it fixed.