Possible problem with the `nil` ack

[aeryz]

Hello, we are using IbcReceiveResponse::without_ack to return a nil ack. But we are hitting this error. Our initial investigation shows us that nil ack returns non-nil ack with empty data. That's why we are hitting this error.

As far as I understand, this feature is meant to return a nil ack and cause WriteAcknowledgement to be deferred. Is there something that we are missing or is this a bug?

[webmaster128]

Which version of wasmd and wasmvm do you use? Optional acks are a feature only supported in CosmWasm 2 chains.

In general this is a two step process:

1. CosmWasm 2: Make acks optional. This is preparation for 2. but there can also be protocols that never ever write acks.
2. CosmWasm 2.1: Allow writing asynchronous acks for packets received packets.

[PoisonPhang]

// You can edit this code!
// Click here and start typing.
package main

import "fmt"

type Ack interface {
    Value() []byte
}

type ContractAck []byte

var _ Ack = ContractAck{}

func (ack ContractAck) Value() []byte {
    return ack
}

func main() {
    var x []byte
    y := ContractAck(x)
    var z Ack
    z = y
    var b Ack
    b = nil
    var c Ack
    c = ContractAck(nil)

    if x == nil {
        fmt.Println("nil x")
    }
    if y == nil {
        fmt.Println("nil y")
    }
    if z == nil {
        fmt.Println("nil z")
    }
    if z.Value() == nil {
        fmt.Println("nil z.value")
    }
    if b == nil {
        fmt.Println("nil b")
    } else if b.Value() == nil {
        fmt.Println("nil b.value")
    }
    if c == nil {
        fmt.Println("nil c")
    } else if c.Value() == nil {
        fmt.Println("nil c.value")
    }

}

nil x
nil y
nil z.value
nil b
nil c.value

[PoisonPhang]

https://github.com/cosmos/ibc-go/blob/083422914ad465d2d7d829a9696b3b929c87e28f/modules/core/keeper/msg_server.go#L495-L511

https://github.com/CosmWasm/wasmd/blob/21b048d54e395ff9168e5c3037356a73797500ba/x/wasm/keeper/relay.go#L173

[webmaster128]

WTF – so there is a Go type system bug in wasmd?

[aeryz]

We had it working by changing the ibc-go code to this:

    // Set packet acknowledgement only if the acknowledgement is not nil.
    // NOTE: IBC applications modules may call the WriteAcknowledgement asynchronously if the
    // acknowledgement is nil.
    if ack != nil && ack.Acknowledgement() != nil {
        if err := k.ChannelKeeper.WriteAcknowledgement(ctx, capability, msg.Packet, ack); err != nil {
            return nil, err
        }
    }

So it seems like although the wrapped bytes is nil, the interface is not nil

[hussein-aitlahcen]

So, the issue is that interfaces in Go are a bit weird:

• if you non initialize or assing nil to an interface, the interface is nil
• if you assign an implementation of an interface to the interface, even if the value you cast is nil, the interface won't be nil

Hence, there is an implicit newtype wrapping (extra pointer indirection) when casting, even if the value you cast is nil (so the value isn't directly used as the interface, but a new pointer is created)

[chipshort]

I think I already stumbled upon this while implementing WriteAcknowledgement for 2.1: https://github.com/CosmWasm/wasmd/pull/1876/files#diff-8eeb0bf4af45c3a552b61aed85469953f39a51bb5a534c0f4d52256bbfce37eb In the PR, you can see I had to make some changes in OnRecvPacket to catch this special case.

[webmaster128]

As this is clearly a bug in wasmd, I am moving it over there