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Document attestation target clamping as defensive guard #214

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pablodeymo merged 2 commits into from
Mar 13, 2026
Merged

Document attestation target clamping as defensive guard #214

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Document attestation target clamping as a defensive guard not in the …
pablodeymo Mar 13, 2026
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Merge branch 'main' into docs/explain-attestation-target-clamping
pablodeymo Mar 13, 2026
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14 changes: 10 additions & 4 deletions crates/blockchain/src/store.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -693,10 +693,16 @@ pub fn get_attestation_target(store: &Store) -> Checkpoint {
.get_block_header(&target_block_root)
.expect("parent block exists");
}
// Ensure target is at or after the source (latest_justified) to maintain
// the invariant: source.slot <= target.slot. When a block advances
// latest_justified between safe_target updates (interval 2), the walk-back
// above can land on a slot behind the new justified checkpoint.
// Guard: clamp target to latest_justified (not in the spec).
//
// The spec's walk-back has no lower bound, so it can produce attestations
// where target.slot < source.slot (source = latest_justified). These would
// fail is_valid_vote Rule 5 (target.slot > source.slot) and be discarded,
// but producing them wastes work and pollutes the network.
//
// This happens when a block advances latest_justified between safe_target
// updates (interval 2), causing the walk-back to land behind the new
// justified checkpoint.
//
// See https://github.com/blockblaz/zeam/blob/697c293879e922942965cdb1da3c6044187ae00e/pkgs/node/src/forkchoice.zig#L654-L659
let latest_justified = store.latest_justified();
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