collator-protocol: Handle unknown validator heads #5538
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There is a race condition when a validator sends its heads to the collator, but the collator doesn't yet know these heads. Before it is aware of these heads by importing the block(s), any collation registered on the collator is not announced to the validators. The collations aren't advertised, because the collator doesn't know yet that these heads of the validator are descendants of the collations relay parent. The solution is to store these unknown heads of the validators and to handle them when the collator updates its own view.
bkchr
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Looks good to me and good catch.
Did you catch any scenarios when this make a difference?
Right now we have two paths when we advertise a collation:
- When we create the collation.
- When peer sends its view update.
We can't really have a collation if we don't know about the leaf, so I would have expected we try at least once to advertise the collation.
But, then the scenario you describe can happen at the next relay chain leaf, but we should've already advertised the collation, wouldn't we ? And advertise collation seems to have logic to not advertise a collation more than once to the same peer.
Yes. When I was building this, I was running constantly into this issue. For this to work I was building on an older relay chain block and the collation was most of the time always announced internally in this small window before the collator had imported these new blocks. |
polkadot/node/network/collator-protocol/src/collator_side/tests/prospective_parachains.rs
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polkadot/node/network/collator-protocol/src/collator_side/tests/prospective_parachains.rs
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| // Announce relevant collations to these peers. | ||
| for (peer_id, peer_version) in &peers { | ||
| advertise_collation( |
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I understand the race condition, but I still don't get why the collations are not advertised when calling distribute_collation adter we built on a known relay parent.
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This is quite strange for local testing, but in practice it could happen that the collation is not advertised for other reasons. The collator has to be really unlucky.
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The CI pipeline was cancelled due to failure one of the required jobs. |
…ocol-view-mismatch'
There is a race condition when a validator sends its heads to the collator, but the collator doesn't yet know these heads. Before it is aware of these heads by importing the block(s), any collation registered on the collator is not announced to the validators. The collations aren't advertised, because the collator doesn't know yet that these heads of the validator are descendants of the collations relay parent. The solution is to store these unknown heads of the validators and to handle them when the collator updates its own view.
There is a race condition when a validator sends its heads to the collator, but the collator doesn't yet know these heads. Before it is aware of these heads by importing the block(s), any collation registered on the collator is not announced to the validators. The collations aren't advertised, because the collator doesn't know yet that these heads of the validator are descendants of the collations relay parent.
The solution is to store these unknown heads of the validators and to handle them when the collator updates its own view.