// incomplete reads may still go undetected.
  if (block->data.size() < block_size_) {
    Key fmax = std::make_pair(key.first, std::numeric_limits<size_t>::max());
    auto fcmp = block_map_.upper_bound(fmax);
    if (fcmp != block_map_.begin() && key < (--fcmp)->first) {
      return TF_SetStatus(status, TF_INTERNAL,
                          "Block cache contents are inconsistent.");
    }
  }

  Trim();

    calls++;
    char c = (filename == "a") ? 'a' : (filename == "b") ? 'b' : 'x';
    if (offset > 0) {
      // The first block is lower case and all subsequent blocks are upper case.
      c = toupper(c);
    }
    memset(buffer, c, n);
    TF_SetStatus(status, TF_OK, "");
    return n;
  };
  // This cache has space for 4 blocks; we'll read from two files.
  const size_t n = 3;