test(fc): head selection by attestation weight, not fork depth

[tcoratger]

Context

LMD-GHOST selects the head based on attestation weight, not fork depth. A shorter fork with more attestation support should win over a deeper fork with less support.

This is a fundamental property of LMD-GHOST that distinguishes it from longest-chain rules. Every client must implement this correctly.

The existing fork choice head tests cover competing forks, but they use chain depth (more blocks = more weight implicitly, since each block's proposer attests). This test explicitly separates depth from weight.

What to test

Write a fork choice filler that:

1. Creates a common prefix at slot 1
2. Fork A: extends 5 blocks deep (slots 2–6) with 1 attestation each
3. Fork B: extends only 2 blocks (slots 2–3) but with 4 attestations per block
4. Verifies LMD-GHOST selects Fork B (heavier), not Fork A (deeper)

Setup with 6 validators

• Fork A: blocks at slots 2, 3, 4, 5, 6 — each block has 1 validator attesting
• Fork B: blocks at slots 2, 3 — but validators 0, 1, 2, 3 all attest to slot 3
• Fork B has more total weight on its head (4 attestations) vs Fork A (1 attestation on its head)

Key assertions

• `StoreChecks(head_root_label="fork_b_3")` — head is on the shorter but heavier fork
• After removing attestations from Fork B validators, head could flip to Fork A

Where to add the test

Add to: `tests/consensus/devnet/fc/test_fork_choice_head.py`

Code skeleton

def test_head_selection_by_weight_not_depth(
    fork_choice_test: ForkChoiceTestFiller,
) -> None:
    """Shorter fork with more attestation weight wins over deeper fork."""
    fork_choice_test(
        num_validators=6,
        steps=[
            # Common prefix
            BlockStep(block=BlockSpec(slot=Slot(1), label="common")),
            # Fork A: 5 blocks deep, minimal attestations
            BlockStep(
                block=BlockSpec(slot=Slot(2), parent_label="common", label="a_2"),
            ),
            BlockStep(block=BlockSpec(slot=Slot(3), parent_label="a_2", label="a_3")),
            BlockStep(block=BlockSpec(slot=Slot(4), parent_label="a_3", label="a_4")),
            BlockStep(block=BlockSpec(slot=Slot(5), parent_label="a_4", label="a_5")),
            BlockStep(block=BlockSpec(slot=Slot(6), parent_label="a_5", label="a_6")),
            # Fork B: only 2 blocks, but heavy attestation support
            BlockStep(
                block=BlockSpec(slot=Slot(2), parent_label="common", label="b_2"),
            ),
            BlockStep(
                block=BlockSpec(slot=Slot(3), parent_label="b_2", label="b_3"),
            ),
            # 4 out of 6 validators attest to fork B's head
            *[
                AttestationStep(
                    attestation=GossipAttestationSpec(
                        validator_id=ValidatorIndex(i),
                        slot=Slot(3),
                        target_slot=Slot(3),
                        target_root_label="b_3",
                    ),
                )
                for i in range(4)
            ],
            # Head should be fork B (heavier), not fork A (deeper)
            # TODO: tick to accept attestations, then check head
        ],
    )

How to run

uv run fill --fork=devnet --clean -n auto -k test_head_selection_by_weight

References

• `Store._compute_lmd_ghost_head`: `src/lean_spec/subspecs/forkchoice/store.py`
• `Store.compute_block_weights`: same file
• LMD-GHOST: Latest Message Driven Greediest Heaviest Observed SubTree

[menawar]

hi @tcoratger i will love to work on this