feature: add support for handling CBOR enumerated alternative data items #508

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1. agaffney commented on Mar 18, 2024

   

   agaffney

   on Mar 18, 2024

   Last edited by agaffney

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   A reasonable way to approach this might be a flag in TagOptions that will pass the entire tag CBOR to the custom type's Unmarshal function rather than just the tag content CBOR. This would allow the custom type to do a generic decode of the tag to extract the tag number without breaking any existing interfaces.

   EDIT: my understanding of how this works was wrong
2. fxamacker commented on Mar 20, 2024

   

   fxamacker

   on Mar 20, 2024

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   @agaffney thanks for opening this issue and suggesting an approach!

   To confirm my understanding, the suggested approach doesn't change API or default behavior. It adds a new opt-in flag that makes codec provide both CBOR tag number and content (instead of just content) to custom type's Unmarshal function.

   This sounds promising, I'll try to take a look this month at existing tag-related code to confirm if this is feasible.

   👍React with 👍1Reacted by Aurora Gaffney
3. fxamacker

   changed the title [-]feature: method for handling CBOR alternatives[/-] [+]feature: add support for handling CBOR enumerated alternative data items[/+] on Mar 20, 2024
4. fxamacker

   added

   enhancementNew feature or requestNew feature or request

   on Mar 20, 2024
5. fxamacker commented on Mar 25, 2024

   

   fxamacker

   on Mar 25, 2024

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   @agaffney Thanks again for opening this issue! I took a closer look at the current tag-related code.

   Currently, both CBOR tag number and its content are passed to the custom type's UnmarshalCBOR() if custom type was registered with TagSet. However, only unique type is allowed to register with TagSet because TagSet is also used for encoding and we can't encode the same type with different tag numbers.

   I'd like more info. Can you share some example code for your use case? For example,

   • does your custom type implement Unmarshaler?
   • are you decoding to interface{}?
   • etc.
6. agaffney commented on Mar 25, 2024

   

   agaffney

   on Mar 25, 2024

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   @fxamacker I'm decoding into interface{} and my custom type implements Unmarshaler.

   You say that both the CBOR tag number and its content are passed to the custom type's UnmarshalCBOR() function, but that's not my experience in practice. The decode seems to be asymmetrical with encode with custom marshal/unmarshal functions with TagSet in that regard. I've got an example custom tag type implemented here, and I have to ignore the tag part on unmarshal and explicitly add it on the marshal.

   https://github.com/blinklabs-io/gouroboros/blob/ef3e13e5b5e2fc84bbf47305ab6e2a25b556b0a6/cbor/tags.go#L92-L110
7. fxamacker commented on Mar 25, 2024

   

   fxamacker

   on Mar 25, 2024

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   @agaffney

   I'm decoding into interface{} and my custom type implements Unmarshaler.

   I see, thanks!

   You say that both the CBOR tag number and its content are passed to the custom type's UnmarshalCBOR() function, but that's not my experience in practice. The decode seems to be asymmetrical with encode with custom marshal/unmarshal functions with TagSet in that regard. I've got an example custom tag type implemented here, and I have to ignore the tag part on unmarshal and explicitly add it on the marshal.

   Hmm, in the following code example, UnmarshalCBOR() receives entire encoded CBOR tag (both number and content).

   https://go.dev/play/p/-zSz6NcTiWL

   type enum struct {
   	data []byte
   }
   
   func (e *enum) UnmarshalCBOR(data []byte) error {
           // 👉 data contains entire CBOR tag (both number and content)
   	e.data = append(e.data[:0], data...)
   	return nil
   }
   
   func main() {
   
   	data, _ := hex.DecodeString("d87a42ff00") // 122(h'ff00')
   
   	tags := cbor.NewTagSet()
   	_ = tags.Add(
   		cbor.TagOptions{EncTag: cbor.EncTagRequired, DecTag: cbor.DecTagRequired},
   		reflect.TypeOf(enum{}),
   		122,
   	)
   
   	dm, _ := cbor.DecOptions{}.DecModeWithTags(tags)
   
   	var v interface{}
   	err := dm.Unmarshal(data, &v)
   	if err != nil {
   		fmt.Printf("failed to unmarshal data 0x%x: %s\n", data, err)
   	} else {
   		fmt.Printf("enum.data: %x\n", v.(enum).data)
   	}
   
   	// Output:
   	// enum.data: d87a42ff00
   }
8. agaffney commented on Mar 26, 2024

   

   agaffney

   on Mar 26, 2024

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   @fxamacker it seems there may be more magic than I had thought. In your example, I can take data in enum.UnmarshalCBOR() and unmarshal it into both cbor.Tag and []byte without any special handling. I guess I had originally implemented it to only decode the tag content and then assumed that it must be that way because otherwise how could it be working?
9. agaffney commented on Mar 26, 2024

   

   agaffney

   on Mar 26, 2024

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   I can now confirm that I can capture that tag number, but what I can't do is register multiple tag numbers with the same object.

   👍React with 👍1Reacted by Faye Amacker