Track "default storage implementation" in a single place, to be used by other code

[emersonknapp]

Description

Right now there are a variety of places in the rosbag2 core that encode the explicit string "sqlite3". This is a tight coupling between the core and the default storage implementation.

We should define in a single place what the default storage is, and use that wherever a default is required. Right now, this means finding all places where sqlite3 is mentioned explicitly, outside the sqlite3 storage implementation, and changing them so that it is not needed.

Related Issues

Completion Criteria

• No explicit mention of "sqlite3" string anywhere in rosbag2 core
• Allow empty storage id for record/write - Writer uses default storage implementation when not specified

Implementation Notes / Suggestions

Some options I can think of

• Somehow use the ament_index to register a file from rosbag2_storage_default_plugins, which contains the name of the default plugin. This can be loaded by upstream packages
• Provide an environment variable (via ament_environment_hooks) that user could override. ROSBAG2_DEFAULT_STORAGE maybe. If variable is unset for some reason, still want to have a builtin default to fall back to
• Make rosbag2_storage_default_plugins provide a C++ header with a const string of the default plugin ID, which downstream packages can read and use. Expose this via rosbag2_py as well.

Testing Notes / Suggestions

• All tests should run against the default storage implementation

[james-rms]

@emersonknapp I might be missing something important, but the only places I can see sqlite3 being hardcoded in this repo are:

• at the top of the record verb, here:

  rosbag2/ros2bag/ros2bag/verb/record.py

  Line 38 in 19158f6

  default_writer = 'sqlite3' if 'sqlite3' in writer_choices else \

• All over the help strings / description of ros2bag verbs
• at the top of writer.cpp, here:

  rosbag2/rosbag2_cpp/src/rosbag2_cpp/writer.cpp

  Line 41 in 19158f6

  static constexpr char const * kDefaultStorageID = "sqlite3";

The rest seem to be part of test fixtures or performance evaluation code. Is there somewhere else I should be looking?

[james-rms]

Plan:

1. Expose a get_default_storage_id() function from rosbag2_storage, bind with pybind11
2. Use this to set the default for ros2 bag record -s, and the default for Writer::open(std::string uri)
3. for the tests, do a pass and split them between "sqlite3 specific" and "storage plugin agnostic". Leave the sqlite3-specific tests alone, and parametrize the storage plugin agnostic tests across all default-installed plugins (sqlite3, and soon MCAP).