In JsonConverter, .Skip() throws, but .TrySkip() never returns false during DeserializeAsync

[joshlang]

Details here: https://stackoverflow.com/questions/63038334/how-do-i-handle-partial-json-in-a-jsonconverter-while-using-deserializeasync-on

Essentially, calling .Skip() in my custom JsonConverter<T> during a DeserializeAsync<T>(stream, ...) throws InvalidOperationException: Cannot skip tokens on partial JSON. Either get the whole payload and create a Utf8JsonReader instance where isFinalBlock is true or call TrySkip.

Calling TrySkip never returns false, though. So... why would Skip fail?

Anyway - this may be a case of me not knowing how to write a JsonConverter properly. However, apparently it doesn't occur in 3.1 and I was encouraged to open an issue. So here it is :D

[joshlang]

Looking through source, I see "Skip" is not even attempted if a non-final block is passed in. I was expecting the logic to be basically if (!TrySkip()) { throw new Explosion(); }. Looks like I was wrong.

[layomia]

In the existing JsonConverter model, custom converters do not have to worry about handling partial data, as the serializer passes all the data for the current JSON scope. Skip/TrySkip logic is unnecessary. cc @tdykstra we should consider adding a section about this in the docs.

The "read ahead" logic that the serializer performs to make this possible does have some perf implications. There has been some discussion around a new model to, among other benefits, enable more performant async-handling when custom converters are used - #1562. Relevant parts of the new model have been implemented internally in serializer, but exposing this publicly is not currently on the roadmap for System.Text.Json.