bug: encoding bug with nested dynamic types

[reecepbcups]

Summary

When encoding a struct that contains dynamic fields (like bytes) for use as a function parameter, the current implementation is missing the necessary offset at the beginning of the encoded data. This causes decoding failures in Solidity contracts.

Decoding would need to also support the proper format

Structure in question

var DataWithIdABI = abi.MustParseStruct(`struct DataWithId { uint64 triggerId; bytes data; }`)

bz := abi.MustEncodeValue(DataWithIdABI, DataWithID{
		TriggerID: trigger_id,
		Data:      output,
	})

Results

Received

		triggerId: 1 (uint64)
		0000000000000000000000000000000000000000000000000000000000000001
		Offset to the dynamic bytes field (64 bytes from struct start)
		0000000000000000000000000000000000000000000000000000000000000040
		Length of bytes array: 9
		0000000000000000000000000000000000000000000000000000000000000009
		Content: "test-data" with padding
		746573742d646174610000000000000000000000000000000000000000000000

Expected (manually generated with solidity console.logBytes())

		Offset to the start of the struct (32 bytes)
		0x0000000000000000000000000000000000000000000000000000000000000020
		triggerId: 1 (uint64)
		0000000000000000000000000000000000000000000000000000000000000001
		Offset to the dynamic bytes field (64 bytes from struct start)
		0000000000000000000000000000000000000000000000000000000000000040
		Length of bytes array: 9
		0000000000000000000000000000000000000000000000000000000000000009
		Content: "test-data" with padding
		746573742d646174610000000000000000000000000000000000000000000000

My current hack

Without this change I have to hack the structure like so.

	bz := abi.MustEncodeValue(DataWithIdABI, DataWithID{
		TriggerID: trigger_id,
		Data:      output,
	})

	offsetBytes, _ := hex.DecodeString("0000000000000000000000000000000000000000000000000000000000000020")
	return append(offsetBytes, bz...)

Solidity func for reference

    function handleSignedData(bytes calldata _data, bytes calldata _signature) external {
       // _serviceManager.validate(_data, _signature);

        DataWithId memory dataWithId = abi.decode(_data, (DataWithId));
        // ....
    }