Issue Addressed

Part of:

• Block lookup sync technical debt #5549

The job of single_block_component_processed is to decide what action to do in response to a block components processing result. The current code is correct, but IMO it is hard to audit to ensure that:

• The branching logic results in some action
• The lookup is not dropped un-intentionally

Proposed Changes

This PR changes the current match into returning a single Action value

enum Action {
    Retry,
    ParentUnknown,
    Drop,
}

This style achieves the goals:

• Ensures branching logic results in exactly one action
• Ensures the lookup is not dropped un-intentionally
• Allows to convert lookup to a mutable reference as the match does not call into some self. method (not impl in this PR). We can switch from the current pattern of remove -> insert to retry / noop to drop, to get_mut -> noop to retry / remove to drop.

I'd recommend looking at the final file for a better read
https://github.com/dapplion/lighthouse/blob/540f87325f132b5065379117d92fba1e308c8a02/beacon_node/network/src/sync/block_lookups/mod.rs#L798

Next steps

There are 3 other functions that could adopt this pattern:

• single_lookup_response
• parent_block_processed
• parent_chain_processed

But if this PR is well received I would prefer to tackle each latter and separately