Field override is failing

[rjl493456442]

The issue occurs when I try to replace **big.Int with **hexutil.Big.

//go:generate go run github.com/fjl/gencodec -type OverrideAccount -field-override overrideMarshaling -out gen_override_json.go

// OverrideAccount indicates the overriding fields of account during the
// execution of a message call.
//
// Note, state and stateDiff can't be specified at the same time.
// - If state is set, state lookup will be performed in this state set first;
// - If statDiff is set, state lookup will be performed in the diff set first;
type OverrideAccount struct {
    Nonce     *uint64                      `json:"nonce"`
    Code      *[]byte                      `json:"code"`
    Balance   **big.Int                    `json:"balance"`
    State     *map[common.Hash]common.Hash `json:"state"`
    StateDiff *map[common.Hash]common.Hash `json:"stateDiff"`
}

// nolint
type overrideMarshaling struct {
    Nonce   *hexutil.Uint64
    Code    *hexutil.Bytes
    Balance **hexutil.Big
}

Error message is

/usr/local/go/bin/go generate -run go run github.com/fjl/gencodec -type OverrideAccount -field-override overrideMarshaling -out gen_override_json.go -: invalid field override: type **github.com/ethereum/go-ethereum/common/hexutil.Big is not convertible to **math/big.Int exit status 1 overrides.go:28: running "go": exit status 1

[fjl]

It doesn't support double-pointer conversion. We could add this, but I'm wondering why you need double-pointer in the first place?

[fjl]

Double pointer conversion is not hard to add if you really do need it though.

First you would have to allow it in this function here: https://github.com/fjl/gencodec/blob/f9840df7b83e876ee46553727f7523cebdc7e90b/types_util.go#L128

Then it needs to be handled in convert(): https://github.com/fjl/gencodec/blob/f9840df7b83e876ee46553727f7523cebdc7e90b/generate.go#L256

The output code for convert(**A, **B) where *A is trivially convertible to *B would be something like this:

if in.a == nil {
    out.b = nil
} else 
    v := (*B)(*in.a)
    out.b = &v
}

[rjl493456442]

The reason I want double pointer conversion is the field is a double pointer.

e.g. **big.Int, an nil *big.Int means the balance is not overrided; while this override object needs to have JSONRPC marshaller and in which the *big.Int is encoded as a *hexutil.Big.

[rjl493456442]

But it's no hurry at all! I can even make the change in gencodec tool by myself. It's not decided yet whether the whole conversion is needed or not.

Good to know how to support it.

[fjl]

e.g. *big.Int, an nil big.Int means the balance is not overrided;

What I find strange about this, is that usually it is enough to have a single pointer in this case, and *big.Int is already pointer. So in your OverrideAccount, if you set the Balance to a non-nil *big.Int, it can be considered overridden. You would only need the double pointer if you want to allow overriding with nil.

Same with slice/map values. Since they can be nil, it is usually OK to use them without pointer in this 'override' scenario.