Feature request: rosbag2 reader that automatically detects if the bagfile is compressed or not

[Rayman]

Description

At the moment there are two readers:

1. rosbag2_cpp::Reader
2. rosbag2_compression::SequentialCompressionReader

If I use the normal reader on a compressed message bagfile I get this error:

'rmw_serialize: invalid data size, at ./src/rmw_node.cpp:1910'

If I use the compression reader on an uncompressed bagfile I get this

  what():  SequentialCompressionReader should not be initialized with NONE compression mode.

So that means I need to pick which reader based on the compression used in the bagfile.
But the bagfile contains the compression used, so there could be a reader that figures this out automatically.

Related Issues

Completion Criteria

A single reader interface that can read both compressed and uncompressed rosbags.

Implementation Notes / Suggestions

Testing Notes / Suggestions

[MichaelOrlov]

Hi @Rayman, There is the only one robust way to detect whether the bag is compressed or not is to look into the metadata.yaml file and see what is inside the compression_format field.
And we have already do this in the ReaderWriterFactory::make_reader(storage_options) api

rosbag2/rosbag2_transport/src/rosbag2_transport/reader_writer_factory.cpp

Lines 25 to 45 in e01e1ef

	namespace rosbag2_transport
	{
	std::unique_ptr<rosbag2_cpp::Reader> ReaderWriterFactory::make_reader(
	const rosbag2_storage::StorageOptions & storage_options)
	{
	rosbag2_storage::MetadataIo metadata_io;
	std::unique_ptr<rosbag2_cpp::reader_interfaces::BaseReaderInterface> reader_impl;
	if (metadata_io.metadata_file_exists(storage_options.uri)) {
	auto metadata = metadata_io.read_metadata(storage_options.uri);
	if (!metadata.compression_format.empty()) {
	reader_impl = std::make_unique<rosbag2_compression::SequentialCompressionReader>();
	}
	}
	if (!reader_impl) {
	reader_impl = std::make_unique<rosbag2_cpp::readers::SequentialReader>();
	}
	return std::make_unique<rosbag2_cpp::Reader>(std::move(reader_impl));
	}

I would encourage you to use this factory method as well.

[Rayman]

Ah nice, I was looking for something like that.

Is it a good idea to update the tutorials to make use of this API? I can open a PR for that

https://docs.ros.org/en/rolling/Tutorials/Advanced/Recording-A-Bag-From-Your-Own-Node-CPP.html
https://docs.ros.org/en/iron/Tutorials/Advanced/Reading-From-A-Bag-File-CPP.html

[MichaelOrlov]

Hmm, for recording, we also have a factory method for creating the Writer. However, it relies on the recorder options.
IMO, it will overcomplicate the tutorial.
Although for the https://docs.ros.org/en/iron/Tutorials/Advanced/Reading-From-A-Bag-File-CPP.html it does make sense to show how to use factory method. I would appreciate a PR for that.

[Rayman]

I've updated the documentation, so I'll close this issue

[MichaelOrlov]

@Rayman Thanks for updating the documentation.
Now I think we need to consider moving the ReaderWriterFactory to the rosbag2_cpp layer.

[Rayman]

Yes, it makes sense to have single Reader class that can be used in all circumstances (like in ROS1). And there could be an UncompressedReader and a CompressedReader underneath to do the actual work.