feat: add toindefarray struct tag option for indefinite-length CBOR arrays

[tdnguyenND]

Summary

Add support for encoding Go structs as CBOR indefinite-length arrays using the struct tag option toindefarray.

Usage

type MyStruct struct {
    _      struct{} `cbor:",toindefarray"`
    Data   []byte
    Count  int
}

This encodes as: 9f <field1> <field2> ff (indefinite-length array)
Instead of: 82 <field1> <field2> (definite-length with toarray)

Motivation

CBOR indefinite-length arrays (0x9f ... 0xff) are used in various protocols (e.g. Cardano smart contract datums). Currently there is no way to encode Go structs directly to this format — only definite-length arrays via toarray.

The decoder already handles both definite and indefinite-length arrays for toarray structs, so this change only adds the encoding path.

Changes

• cache.go: Add toIndefArray flag to struct types, hasToIndefArrayOption(), getEncodingStructToIndefArrayType()
• encode.go: Add encodeStructToIndefArray() which writes 0x9f header + fields + 0xff break code, wired into getEncodeFuncInternal()
• toindefarray_test.go: Tests for basic and nested encoding/decoding

Decoding

No decoder changes needed — the existing decoder handles both definite and indefinite-length arrays for toarray structs via getHeadWithIndefiniteLengthFlag().

Tests

All existing tests pass. New tests cover:

• Basic struct encode/decode roundtrip
• Nested indefinite-length structs
• Verified CBOR output format (0x9f header, 0xff break)

[tdnguyenND]

hi @fxamacker can you have a look at this PR?

[fxamacker]

hi @fxamacker can you have a look at this PR?

Hi @tdnguyenND, thanks for opening this PR!

This PR should include the PR checklist form that is automatically added to PRs (see CONTRIBUTING).

I can try to take a look at the request next weekend (April 11 or 12) if I have the specs and more context. Please open an issue to add this feature with link(s) to the Cardano CBOR specification, etc. (CDDL notation would be ideal if available).

Thanks!

[fxamacker]

Thanks again for submitting this PR to add a feature.

For non-security issues, I reply on weekends because this CBOR library has been self-funded for years (weekends & public holidays & deferred Christmases), with my weekdays used for paid work.

I didn’t receive full specs or related CDDL yet, so I reviewed your PR without that context.

I’m inclined not to include the requested feature because:

• It is an edge case to encode a struct (length is always known at runtime) into an indefinite-length array.
• This codec already has a workaround to do this by implementing the cbor.Marshaler interface.

Also, there are security considerations with using indefinite-length values (unrelated to this library).

In general, please consider avoiding indefinite-length if your specifications allow definite-length.

I'm closing this PR, but please feel free to open an issue to discuss adding the feature if I missed anything.