Failing test zombienet-cumulus-0005-migrate_solo_to_para

[skunert]

zombienet-cumulus-0005-migrate_solo_to_para has been failing consistently.

File:

polkadot-sdk/cumulus/zombienet/tests/0005-migrate_solo_to_para.toml

Line 23 in 4059282

name = "dave"

Expectation

This test spawns two parachains with the same para id but different genesis, dave and eve. Node dave will produce blocks and after a few blocks output custom head_data through the PVF. This head_data contains the header that matches node eves parachain, which should now start producing blocks.

Reality

Dave starts producing blocks, but after the custom head data is submitted, eve does not start producing blocks. Instead dave continues to produce blocks.

Investigation

The culprit seems to be #4880, before that PR the test was passing consistently. The question is why the custom head data presented by dave is not set as included head.
In the logs the following messages appear a lot:

2025-01-09 17:56:06.013 DEBUG tokio-runtime-worker parachain::collator-protocol: Collation having parent head data hash 0x9fd4…10d8 is blocked from seconding. Waiting on its parent to be validated. candidate_hash=0xe666001ec78a5546c0ee6ad7723ebcb34ef3e3245fc752c5ed9bfa386c17f214 relay_parent=0x045189b697186a3ea5f0ae67aba091a1ceb7c760590b6036a5ddca6d0f593a5a traceID=306252055748700830037795645451537398963

Apparently some seconding limit was reached and the block in question not included. Modifying the lookahead collator here to build always only a single block fixes the issue and makes the test pass:

polkadot-sdk/cumulus/client/consensus/aura/src/collators/lookahead.rs

Line 360 in 4059282

for n_built in 0..2 {

cc @tdimitrov any ideas what is happening here?

[tdimitrov]

With the collation fairness (the PR you linked) we limit the number of collations a collator can provide up to the number of claims for its para id in the claim queue. So the collator is building two blocks per relay parent no matter what and they are built on top of each other(right?). In this case sooner or later a collation will be dropped and the rest of the collations will be unbackable, because of the missing candidate.

DQ but solo chain (Dave) means just a single collator node producing blocks on its own. Is this correct? If yes - there are no validators to throttle it and that's why it is producing blocks.

I haven't run the test locally but I'm a bit puzzled why Eve doesn't produce blocks at all. I think it should produce at least one or two blocks depending on the claim queue and async backing parameter values.

I also believe the test will work fine with --experimental-use-slot-based flag. Is this acceptable?