@Override
  public ImmutablePendingBlock putIfAbsent(
      final Hash hash, final ImmutablePendingBlock pendingBlock) {
    final ImmutablePendingBlock foundBlock = super.putIfAbsent(hash, pendingBlock);
    if (foundBlock == null) {
      removeLowestPriorityBlockWhenCacheFull(pendingBlock.nodeId());
    }
    return foundBlock;
  }
Adds the specified hash in the cache if it is not already associated with a value. Otherwise
returns the current value.

@return the previous value associated with the specified key, or {@code null} if there was no
    mapping for the hash
ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/state/cache/PendingBlockCache.java