public CompletableFuture<FastSyncState> downloadPivotBlockHeader() {
    if (isStarted.compareAndSet(false, true)) {
      LOG.info("Retrieve a pivot block that can be confirmed by at least {} peers.", peersToQuery);
      confirmBlock(pivotBlockNumber.get());
    }

    return result;
  }
This task attempts to find a non-controversial pivot block by confirming the pivot block number
with a minimum number of peers. If a particular pivot block cannot be confirmed, the pivot block
number is pushed back and the confirmation process repeats. If a maximum number of retries is
reached, the task fails with a {@code FastSyncException} containing {@code
FastSyncError.PIVOT_BLOCK_HEADER_MISMATCH}.
ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/fastsync/PivotBlockRetriever.java