feat(parachain/availability-distribution): implement get_block_ancestors_in_same_session

[haikoschol]

Issue summary

This function is part of the message handling logic for ActiveLeavesUpdateSignal.

Implementation details

As part of this issue, no caching of runtime information should be implemented. That is covered by #4494, which should be worked on after this issue is completed.

For each newly activated leaf, retrieve the headers of up to leafAncestryLenWithinSession ancestors. Instantiate the runtime for the first ancestor and call ParachainHostSessionIndexForChild() to get the session index of the newly activated leaf. If no ancestors have been found, set this value to 0 instead.

If the list of ancestors is not empty, iterate through the remaining ones and check that they have the same session index as the newly activated leaf. Stop iterating if a different session index is found and discard those ancestors.

package availabilitydistribution

const leafAncestryLenWithinSession = 3

type BlockState interface {
    GetHeader(hash common.Hash) (*types.Header, error)
    GetRuntime(hash common.Hash) (instance runtime.Instance, err error)
}

type AvailabilityDistribution struct {
    blockState BlockState
    // ...
}

func (av *AvailabilityDistribution) getBlockAncestorsInSameSession(
    head common.Hash,
    limit uint,
) (index parachaintypes.SessionIndex, ancestors []common.Hash, err error) {
    runtime, err := av.blockState.GetRuntime(head)
    // ...
}

Other information and links

• Parity node implementation

Acceptance criteria

• New code is 60% covered with unit tests