fix: ensure schemas are cleared in McapWriter::terminate()

[sofuture]

Changelog

Clear schemas in McapWriter::terminate()

Docs

Fixes: FG-10773

Description

Some internal state was left on McapWriter::terminate() contrary to docs.

[linear]

FG-10773 MCAP C++ API: schemas don't get cleared with the writer's `close()` function

[eloff]

At first glance this appears to be the right fix.

But after a LOT of digging, I don't think it is.

I think schemas_ are meant to persist beyond just the mcap file currently being written. open can be called again on the same writer (e.g. logging to rotating mcap files) and it's fine to re-use the same list of schemas with the same id mappings. A feature, not a bug.

The schemas written in each file are tracked separately in writtenSchemas_, which is cleared in terminate, which makes sense.

But the reported problem is still an issue. And I think it stems from other parts of the writer referencing schemas_ when the should be using writtenSchemas_, thus causing schemas to show up in files they weren't present in, in the summary section.

This happens in close() when repeating the schemas

I think that should reference writtenSchemas_ instead.

@james-rms could you double-check my logic here? I can take over fixing this, but first I want to make sure I understand it correctly.

[jtbandes]

Just want to interject: I don't think we have documented anywhere whether you are even supposed to be able to reuse the same writer for multiple files 😊