fix: sort cell indices before calling recoverCellsAndKzgProofs (#8450)

For context please view
https://discord.com/channels/593655374469660673/1229813597501395028/1419794786121945141
.

The latest spec requires cell indices to be sorted in
[`recover_cells_and_kzg_proofs
`](https://github.com/jtraglia/consensus-specs/blob/340d23358a0cdb4f4b547226dadd36d1cf775f20/specs/fulu/polynomial-commitments-sampling.md?plain=1#L805)
so we need to sort them before passing it to kzg.

Author: ensi321

packages/beacon-node/src/util/blobs.ts

@@ -84,7 +84,10 @@ export async function dataColumnMatrixRecovery(
     return Array.from(partialSidecars.values());
   }
 
-  const firstDataColumn = partialSidecars.values().next().value;
+  // Sort data columns by index in ascending order before passing for kzg operations
+  const partialSidecarsSorted = Array.from(partialSidecars.values()).sort((a, b) => a.index - b.index);
+
+  const firstDataColumn = partialSidecarsSorted[0];
   if (firstDataColumn == null) {
     // should not happen because we check the size of the cache before this
     throw new Error("No data column found in cache to recover from");
@@ -101,8 +104,8 @@ export async function dataColumnMatrixRecovery(
     blobProofs.map((_, blobIndex) => {
       const cellIndices: number[] = [];
       const cells: Uint8Array[] = [];
-      for (const [columnIndex, dataColumn] of partialSidecars.entries()) {
-        cellIndices.push(columnIndex);
+      for (const dataColumn of partialSidecarsSorted) {
+        cellIndices.push(dataColumn.index);
         cells.push(dataColumn.column[blobIndex]);
       }
       // recovered cells and proofs are of the same row/blob, their length should be NUMBER_OF_COLUMNS

packages/beacon-node/test/spec/general/kzg.ts

@@ -169,6 +169,11 @@ type RecoverCellsAndKzgProofsInput = {
   cells: string[];
 };
 function recoverCellsAndKzgProofs(input: RecoverCellsAndKzgProofsInput): [string[], string[]] | null {
+  const isSorted = input.cell_indices.every((val, i, arr) => i === 0 || arr[i - 1] < val);
+  // If cell indices are not in ascending order, they are deemed invalid and cannot pass it to kzg
+  // Though in practice, we sort them before passing to kzg, here we follow the spec test case expectation
+  // See https://github.com/ChainSafe/lodestar/pull/8450/files#r2372830108 for context
+  if (!isSorted) return null;
   const cellIndices = input.cell_indices.map(BigInt);
   const cells = input.cells.map(fromHexString);
   try {