Method Read

cpp/src/arrow/io/caching.cc:202–232

Read the given range from the cache, blocking if needed. Cannot read a range that spans cache entries.

  // Read the given range from the cache, blocking if needed. Cannot read a range
  // that spans cache entries.
  virtual Result<std::shared_ptr<Buffer>> Read(ReadRange range) {
    if (range.length == 0) {
      static const uint8_t byte = 0;
      return std::make_shared<Buffer>(&byte, 0);
    }

    const auto it = std::lower_bound(
        entries.begin(), entries.end(), range,
        [](const RangeCacheEntry& entry, const ReadRange& range) {
          return entry.range.offset + entry.range.length < range.offset + range.length;
        });
    if (it != entries.end() && it->range.Contains(range)) {
      auto fut = MaybeRead(&*it);
      ARROW_ASSIGN_OR_RAISE(auto buf, fut.result());
      if (options.lazy && options.prefetch_limit > 0) {
        int64_t num_prefetched = 0;
        for (auto next_it = it + 1;
             next_it != entries.end() && num_prefetched < options.prefetch_limit;
             ++next_it) {
          if (!next_it->future.is_valid()) {
            next_it->future =
                file->ReadAsync(ctx, next_it->range.offset, next_it->range.length,
                                /*allow_short_read=*/false);
          }
          ++num_prefetched;
        }
      }
      return SliceBuffer(std::move(buf), range.offset - it->range.offset, range.length);
    }
    return Status::Invalid("ReadRangeCache did not find matching cache entry");
  }

  virtual Future<> Wait() {
    std::vector<Future<>> futures;