code: CompletableFuture<Long> getBackpointerOffset(DirectSegmentAccess segment, long offset, Duration timeout) {
          UUID key = getBackpointerAttributeKey(offset);
          return segment.getAttributes(Collections.singleton(key), false, timeout)
                        .thenApply(attributes -> {
                            long result = attributes.getOrDefault(key, Attributes.NULL_ATTRIBUTE_VALUE);
                            return result == Attributes.NULL_ATTRIBUTE_VALUE ? -1 : result;
                        });
      }
docs: Looks up a Backpointer offset.
  
  @param segment A DirectSegmentAccess providing access to the Segment to search in.
  @param offset  The offset to find a backpointer from.
  @param timeout Timeout for the operation.
  @return A CompletableFuture that, when completed, will contain the backpointer offset, or -1 if no such pointer exists.