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Fix data_columns sorting when reconstructing blobs #8509

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Fix data_columns sorting when reconstructing blobs#8509
@0xMushow

Description

@0xMushow
0xMushow
opened on Dec 1, 2025

This recent change released in v0.9.0 of the rust-eth-kzg library means that data_columns must be sorted.

Referencing to a similar issue: #7991

The reconstruct_blobs function passes cell_ids to recover_cells_and_compute_kzg_proofs, which requires cell indices to be in ascending order per the spec. Unlike reconstruct_data_columnswhich already sorts:

lighthouse/beacon_node/beacon_chain/src/kzg_utils.rs

Lines 390 to 396 in 90dd5bb

pub fn reconstruct_data_columns<E: EthSpec>(
kzg: &Kzg,
mut data_columns: Vec<Arc<DataColumnSidecar<E>>>,
spec: &ChainSpec,
) -> Result<DataColumnSidecarList<E>, KzgError> {
// Sort data columns by index to ensure ascending order for KZG operations
data_columns.sort_unstable_by_key(|dc| dc.index);

reconstruct_blobs was iterating over data_columns in whatever order they were passed, potentially violating the KZG recovery precondition.

lighthouse/beacon_node/beacon_chain/src/kzg_utils.rs

Lines 334 to 345 in 90dd5bb

for data_column in data_columns {
let cell = data_column
.column
.get(row_index)
.ok_or(format!("Missing data column at row index {row_index}"))
.and_then(|cell| {
ssz_cell_to_crypto_cell::<E>(cell).map_err(|e| format!("{e:?}"))
})?;
cells.push(cell);
cell_ids.push(data_column.index);
}

lighthouse/beacon_node/beacon_chain/src/kzg_utils.rs

Line 350 in 90dd5bb

.recover_cells_and_compute_kzg_proofs(&cell_ids, &cells)

Additional Info

According to the specification, indices passed to recover_cells_and_compute_kzg_proofs must be ordered.

# Check that indices are in ascending order
assert cell_indices == sorted(cell_indices)

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