QRecordSchema: close schemaLatch in Close

[serprex]

goroutines were leaking waiting on schemaLatch,
if provider hit error then schemaLatch could be left indefinitely open

[serprex]

Some options: have Schema method take context.Context, have Schema return error

[iskakaushik]

Some options: have Schema method take context.Context, have Schema return error

This option is definitely better.

[serprex]

So I went this route because I think context will make the error a bit flaky. Between both context error which is probably tied with stream error, but also because we should also handle context being alive while stream is closed. In that case the error should be stream.Err() so we'd still want this code on top of selecting over both stream.schemaLatch & ctx.Done()

This way makes flow/pua/stream_adapter.go work unchanged, ie stream transforms don't have an error path

If stream is closed without error without schema it's a strange scenario. Seems best in that case to return 0, nil, whereas with error return we'd get back nil & go ahead with no schema. Guess we could make a separate errors.New("NoSchema")

So this seemed like the simplest path. But could have Schema() still return error, which would just be stream.Err(), so that it isn't so easy to call Schema without error handling