Can't handle optional nested submessages when parsing MCAP file

Description

If I generate an MCAP log with a channel whose type has multiple layers of nested sub-messages (in this schema, MessageType.fetchers[0].channel.schema.root_table.name is a potentially valid reference), but don’t specify one of the sub-fields, foxglove studio barfs complaining about an undefined reference.

In this, case we have the below logfile: foo.mcap.gz

Which just contains a single channel/schema and a single message on that channel. The schema for the channel is:

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The contents of the actual message are:

{"fetchers": [{"count": 123}], "pid": 12345}

Foxglove Studio pops up an error saying Error initializing player: Cannot read properties of undefined (reading 'schema') (the schema here is a reference to the schema sub-object in my type–if I change the name in my type, it changes the name in the error message). It appears that foxglove attempts to read the fetchers[0].channel.schema, but channel is undefined. Interestingly, the logic appears to handle on a single layer of missing sub-messages (so if channels were itself just a flat object with no sub-messages, then not populating it seems to work fine–I assume that https://github.com/foxglove/studio/blob/a20cf7ef51cae38d143a2d76e3e048c84c04a0d7/packages/mcap-support/src/parseJsonSchema.ts is somehow iterating over all the sub-types in a message when attempting to read things).

Also, empty JSON messages ({}) don’t seem to be allowed, even if the schema permits it.

Expected Behavior

The logfile is successfully read and I can view the data that is populated.

Actual Behavior

I get an error and can’t view any data 😦

[jtbandes]

Awesome, thanks for all the references. Feel free to also provide any feedback on the https://github.com/foxglove/mcap repo or in our Slack group if you run into issues with the readers/writers or the spec. It’s still pretty new and we’re happy to have others trying it out!

[jameskuszmaul-brt]

If it does look like an AOS<->foxglove integration works well, I will probably try to get everything relevant open-sourced so that we have a clean visualization answer for both the FRC team and anyone else that may end up wanting to use AOS.