Writing Arrow IPC format panic within encode_dictionary() method due to access out of bound fields vector

[artyyouth]

It panic at this line trying to access index 0 while &field.fields is a 0 length vector.

I haven't dived too deep to figure out what should be fixed, but turns out it panics when I have a List<Utf8> as Array item (btw: schema definition is Field::new("labels", DataType::List(Box::new(Field::new("labels", DataType::Utf8, true))))

I suspect the above match array.data_type().to_physical_type() will always match on List, and when it recursively invokes encode_dictionary() method with the nested IpcField, it will panic eventually because the innermost IpcField holds an empty fields vector.

Seems like a bug? Not sure if it is similar to this one #830

[artyyouth]

Turns out it's a typo in my code messed up the schema...sorry for reporting this wrong issue...

[jorgecarleitao]

Hey, thanks and no worries! I think we should still not panic and offer a better error message, so I would say we can keep this open and return a OutOfSpec error when this happens with a good description. What do you think?

[artyyouth]

Actually I noticed I can still repro this out of bound error since previously I commented out all the List fields in my schema...

Anyway I probably will try to dig deeper in the weekend to see if I can have a minimal viable repro, thanks for following up, and feel free to reopen this issue if you like. I will also keep my findings posted here. Thanks! :)