niyue commented on a change in pull request #11486:
URL: https://github.com/apache/arrow/pull/11486#discussion_r732839467
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File path: cpp/src/arrow/ipc/message.cc
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@@ -279,8 +279,37 @@ std::string FormatMessageType(MessageType type) {
return "unknown";
}
+Status ReadFieldsSubset(int64_t offset, int32_t metadata_length,
+ io::RandomAccessFile* file,
+ const FieldsLoaderFunction& fields_loader,
+ const std::shared_ptr<Buffer>& metadata, int64_t
required_size,
+ std::shared_ptr<Buffer>& body) {
+ const flatbuf::Message* message = nullptr;
+ uint8_t continuation_metadata_size = sizeof(int32_t) + sizeof(int32_t);
+ // skip 8 bytes (32-bit continuation indicator + 32-bit little-endian length
prefix)
+ RETURN_NOT_OK(internal::VerifyMessage(metadata->data() +
continuation_metadata_size,
+ metadata->size() -
continuation_metadata_size,
+ &message));
+
+ auto batch = message->header_as_RecordBatch();
+ auto io_recorded_random_access_file =
std::unique_ptr<IoRecordedRandomAccessFile>(
+ new IoRecordedRandomAccessFile(required_size));
+ RETURN_NOT_OK(fields_loader(batch, io_recorded_random_access_file.get()));
+ auto io_ops = io_recorded_random_access_file->GetRecordedIoOps();
+ for (auto const& io_op : io_ops) {
+ auto read_result = file->ReadAt(offset + metadata_length + io_op.first,
io_op.second,
+ body->mutable_data() + io_op.first);
Review comment:
The recorded read IO operations are replayed here to really reading data
from the file into `body`.
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