piqi loops forever on self-recursive type definitions

[yfyf]

This is certainly some super-corner-case unhappy-path behaviour, but if you have a spec like:

[~/sandbox]λ cat foo.piqi 
.alias [
    .name foo
    .type foo
]

then piqi convert foo.piqi will loop forever. Ideally it would throw up with an error.

[yfyf]

Also, no level of --debug seems to produce output either.

[alavrik]

Thanks for reminding about it. This is actually a TODO. It triggers a tight infinite loop that doesn't have any debug logging in it. We also need to check for infinite types with records and variants. I was planning to cover all of it at once one day.

[yfyf]

I realised that a slight complication is that you also need to check for cycles of any length if you want to fix this properly: i.e. a chain of aliases a -> b -> c -> a should also be impossible. Furthermore you can have nasty things like a record with a field which points back to the record itself, which is a valid construct if the field is optional. Such definitions are OK in protobuf, but not in Erlang though...

[yfyf]

Although in Erlang you could workaround by desugaring the record to it's tuple representation and then manually specifying the field to be of this form. Or just not specifying the field type at all. Though you will have to give up Dialyzer for these for sure.

[alavrik]

Implemented in https://github.com/alavrik/piqi/commit/b7fbb9179854c4742094bb1da49a14d13fe3fbca

@yfyf this is exactly what I meant by infinite types. Obviously this includes looped alias chains as well. All of this should be covered now.

Also, record fields pointing back to the record has always been supported in piqi-erlang. This is actually very simple. piqic-erlang generates -type r() :: #r{} for each record -record(r, ...) and whenever record the record is referenced, type r() is used instead of #r{}.

[yfyf]

Wow, that was quick!

Confirmed to work:

[~/sandbox]λ piqi convert foo.piqi 
foo.piqi:1:8: alias "foo" forms a loop

Thanks!

P.S. Neat hack with the type aliasing for records, did not realise that could work.

[yfyf]

The loop-checker seems to be too restrictive, this is IMHO a fine definition:

$ cat sample.piqi 
.variant [
    .name where
    .option [ .name rec .type where-list ]
    .option [ .type uint ]
]
.list [ .name where-list .type where ]

$ piqi convert sample.piqi 
sample.piqi:6:7: list "where-list" forms a loop

[alavrik]

Fixed. Thanks!