- 2022-08-10: Initial Draft
- 2023-03-22: Merged
Accepted, packet callback interface implemented
IBC was designed with callbacks between core IBC and IBC applications. IBC apps would send a packet to core IBC. When the result of the packet lifecycle eventually resolved into either an acknowledgement or a timeout, core IBC called a callback on the IBC application so that the IBC application could take action on the basis of the result (e.g. unescrow tokens for ICS-20).
This setup worked well for off-chain users interacting with IBC applications.
We are now seeing the desire for secondary applications (e.g. smart contracts, modules) to call into IBC apps as part of their state machine logic and then do some actions on the basis of the packet result. Or to receive a packet from IBC and do some logic upon receipt.
Example Usecases:
- Send an ICS-20 packet, and if it is successful, then send an ICA-packet to swap tokens on LP and return funds to sender
- Execute some logic upon receipt of token transfer to a smart contract address
This requires a second layer of callbacks. The IBC application already gets the result of the packet from core IBC, but currently there is no standardized way to pass this information on to an actor module/smart contract.
- Actor: an actor is an on-chain module (this may be a hardcoded module in the chain binary or a smart contract) that wishes to execute custom logic whenever IBC receives a packet flow that it has either sent or received. It must be addressable by a string value.
Create a standardized callback interface that actors can implement. IBC applications (or middleware that wraps IBC applications) can now call this callback to route the result of the packet/channel handshake from core IBC to the IBC application to the original actor on the sending chain. IBC applications can route the packet receipt to the destination actor on the receiving chain.
IBC actors may implement the following interface:
type IBCActor interface {
// OnChannelOpen will be called on the IBCActor when the channel opens
// this will happen either on ChanOpenAck or ChanOpenConfirm
OnChannelOpen(ctx sdk.Context, portID, channelID, version string)
// OnChannelClose will be called on the IBCActor if the channel closes
// this will be called on either ChanCloseInit or ChanCloseConfirm and if the channel handshake fails on our end
// NOTE: currently the channel does not automatically close if the counterparty fails the handhshake so actors must be prepared for an OpenInit to never return a callback for the time being
OnChannelClose(ctx sdk.Context, portID, channelID string)
// IBCActor must also implement PacketActor interface
PacketActor
}
// PacketActor is split out into its own separate interface since implementors may choose
// to only support callbacks for packet methods rather than supporting the full IBCActor interface
type PacketActor interface {
// OnRecvPacket will be called on the IBCActor after the IBC Application
// handles the RecvPacket callback if the packet has an IBC Actor as a receiver.
OnRecvPacket(ctx sdk.Context, packet channeltypes.Packet, relayer sdk.AccAddress) error
// OnAcknowledgementPacket will be called on the IBC Actor
// after the IBC Application handles its own OnAcknowledgementPacket callback
OnAcknowledgmentPacket(
ctx sdk.Context,
packet channeltypes.Packet,
ack exported.Acknowledgement,
relayer sdk.AccAddress,
) error
// OnTimeoutPacket will be called on the IBC Actor
// after the IBC Application handles its own OnTimeoutPacket callback
OnTimeoutPacket(
ctx sdk.Context,
packet channeltypes.Packet,
relayer sdk.AccAddress,
) error
}The CallbackPacketData interface will get created to add GetSourceCallbackAddress and GetDestCallbackAddress methods. These may return an address
or they may return the empty string. The address may reference an PacketActor or it may be a regular user address. If the address is not a PacketActor, the actor callback must continue processing (no-op). Any IBC application or middleware that uses these methods must handle these cases. In most cases, the GetSourceCallbackAddress will be the sender address and the GetDestCallbackAddress will be the receiver address. However, these are named generically so that implementors may choose a different contract address for the callback if they choose.
The interface also defines a UserDefinedGasLimit method. Any middleware targeting this interface for callback handling should cap the gas that a callback is allowed to take (especially on AcknowledgePacket and TimeoutPacket) so that a custom callback does not prevent the packet lifecycle from completing. However, since this is a global cap it is likely to be very large. Thus, users may specify a smaller limit to cap the amount of fees a relayer must pay in order to complete the packet lifecycle on the user's behalf.
IBC applications which provide the base packet data type must implement the CallbackPacketData interface to allow PacketActor callbacks.
// Implemented by any packet data type that wants to support PacketActor callbacks
// PacketActor's will be unable to act on any packet data type that does not implement
// this interface.
type CallbackPacketData interface {
// GetSourceCallbackAddress should return the callback address of a packet data on the source chain.
// This may or may not be the sender of the packet. If no source callback address exists for the packet,
// an empty string may be returned.
GetSourceCallbackAddress() string
// GetDestCallbackAddress should return the callback address of a packet data on the destination chain.
// This may or may not be the receiver of the packet. If no dest callback address exists for the packet,
// an empty string may be returned.
GetDestCallbackAddress() string
// UserDefinedGasLimit allows the sender of the packet to define inside the packet data
// a gas limit for how much the ADR-8 callbacks can consume. If defined, this will be passed
// in as the gas limit so that the callback is guaranteed to complete within a specific limit.
// On recvPacket, a gas-overflow will just fail the transaction allowing it to timeout on the sender side.
// On ackPacket and timeoutPacket, a gas-overflow will reject state changes made during callback but still
// commit the transaction. This ensures the packet lifecycle can always complete.
// If the packet data returns 0, the remaining gas limit will be passed in (modulo any chain-defined limit)
// Otherwise, we will set the gas limit passed into the callback to the `min(ctx.GasLimit, UserDefinedGasLimit())`
UserDefinedGasLimit() uint64
}IBC Apps or middleware can then call the IBCActor callbacks like so in their own callbacks:
The OnChanOpenInit handshake callback will need to include an additional field so that the initiating actor can be tracked and called upon during handshake completion.
The actor provided in the OnChanOpenInit callback will be the signer of the MsgChanOpenInit message.
func OnChanOpenInit(
ctx sdk.Context,
order channeltypes.Order,
connectionHops []string,
portID string,
channelID string,
channelCap *capabilitytypes.Capability,
counterparty channeltypes.Counterparty,
version string,
actor string,
) (string, error) {
acc := k.getAccount(ctx, actor)
ibcActor, ok := acc.(IBCActor)
if ok {
k.setActor(ctx, portID, channelID, actor)
}
// continued logic
}
func OnChanOpenAck(
ctx sdk.Context,
portID,
channelID string,
counterpartyChannelID string,
counterpartyVersion string,
) error {
// run any necessary logic first
// negotiate final version
actor := k.getActor(ctx, portID, channelID)
if actor != "" {
ibcActor, _ := acc.(IBCActor)
ibcActor.OnChanOpen(ctx, portID, channelID, version)
}
// the same actor will be used for channel closure
}
func OnChanCloseInit(
ctx sdk.Context,
portID,
channelID,
) error {
// run any necesssary logic first
actor := k.getActor(ctx, portID, channelID)
if actor != "" {
ibcActor, _ := acc.(IBCActor)
ibcActor.OnChanClose(ctx, portID, channelID)
}
// cleanup state
k.deleteActor(ctx, portID, channelID)
}
func OnChanCloseConfirm(
ctx sdk.Context,
portID,
channelID string,
) error {
// run any necesssary logic first
actor := k.getActor(ctx, portID, channelID)
if actor != "" {
ibcActor, _ := acc.(IBCActor)
ibcActor.OnChanClose(ctx, portID, channelID)
}
// cleanup state
k.deleteActor(ctx, portID, channelID)
}NOTE: The handshake calls OnChanOpenTry and OnChanOpenConfirm are explicitly left out as it is still to be determined how the actor of the OnChanOpenTry step should be provided. Initially only the initiating side of the channel handshake may support setting a channel actor, future improvements should allow both sides of the channel handshake to set channel actors.
No packet callback API will need to change.
// Call the IBCActor recvPacket callback after processing the packet
// if the recvPacket callback exists. If the callback returns an error
// then return an error ack to revert all packet data processing.
func OnRecvPacket(
ctx sdk.Context,
packet channeltypes.Packet,
relayer sdk.AccAddress,
) (ack exported.Acknowledgement) {
// run any necesssary logic first
// IBCActor logic will postprocess
// postprocessing should only if the underlying application
// returns a successful ack
// unmarshal packet data into expected interface
var cbPacketData callbackPacketData
unmarshalInterface(packet.GetData(), cbPacketData)
if cbPacketData == nil {
// the packet data does not implement the CallbackPacketData interface
// continue processing (no-op)
return
}
acc := k.getAccount(ctx, cbPacketData.GetDstCallbackAddress())
ibcActor, ok := acc.(IBCActor)
if ok {
// set gas limit for callback
gasLimit := getGasLimit(ctx, cbPacketData)
cbCtx = ctx.WithGasLimit(gasLimit)
err := ibcActor.OnRecvPacket(cbCtx, packet, relayer)
// deduct consumed gas from original context
ctx = ctx.WithGasLimit(ctx.GasMeter().RemainingGas() - cbCtx.GasMeter().GasConsumed())
if err != nil {
// NOTE: by returning an error acknowledgement, it is assumed that the
// base IBC application on the counterparty callback stack will be able
// to properly unmarshal the error acknowledgement. It should not expect
// some custom error acknowledgement. If it does, failed acknowledgements
// will be unsuccessfully processed which can be catastrophic in processing
// refund logic.
//
// If this issue is a serious concern, an ADR 8 implementation can construct its own
// acknowledgement type which wraps the underlying application acknowledgement. This
// would require deployment on both sides of the packet flow, in addition to version
// negotiation to enable the custom acknowledgement type usage.
//
// Future improvmenets should allow for each IBC application in a stack of
// callbacks to provide their own acknowledgement without disrupting the unmarshaling
// of an application above or below it in the stack.
return AcknowledgementError(err)
}
}
return
}
// Call the IBCActor acknowledgementPacket callback after processing the packet
// if the ackPacket callback exists and returns an error
// DO NOT return the error upstream. The acknowledgement must complete for the packet
// lifecycle to end, so the custom callback cannot block completion.
// Instead we emit error events and set the error in state
// so that users and on-chain logic can handle this appropriately
func (im IBCModule) OnAcknowledgementPacket(
ctx sdk.Context,
packet channeltypes.Packet,
acknowledgement []byte,
relayer sdk.AccAddress,
) error {
// application-specific onAcknowledgmentPacket logic
// unmarshal packet data into expected interface
var cbPacketData callbackPacketData
unmarshalInterface(packet.GetData(), cbPacketData)
if cbPacketData == nil {
// the packet data does not implement the CallbackPacketData interface
// continue processing (no-op)
return
}
// unmarshal ack bytes into the acknowledgment interface
var ack exported.Acknowledgement
unmarshal(acknowledgement, ack)
// send acknowledgement to original actor
acc := k.getAccount(ctx, cbPacketData.GetSourceCallbackAddress())
ibcActor, ok := acc.(IBCActor)
if ok {
gasLimit := getGasLimit(ctx, cbPacketData)
handleCallback := func() error {
// create cached context with gas limit
cacheCtx, writeFn := ctx.CacheContext()
cacheCtx = cacheCtx.WithGasLimit(gasLimit)
defer func() {
if e := recover(); e != nil {
log("ran out of gas in callback. reverting callback state")
} else if err == nil {
// only write callback state if we did not panic during execution
// and the error returned is nil
writeFn()
}
}
err := ibcActor.OnAcknowledgementPacket(cacheCtx, packet, ack, relayer)
// deduct consumed gas from original context
ctx = ctx.WithGasLimit(ctx.GasMeter().RemainingGas() - cbCtx.GasMeter().GasConsumed())
return err
}
if err := handleCallback(); err != nil {
setAckCallbackError(ctx, packet, err) // optional
emitAckCallbackErrorEvents(err)
}
}
return nil
}
// Call the IBCActor timeoutPacket callback after processing the packet
// if the timeoutPacket callback exists and returns an error
// DO NOT return the error upstream. The timeout must complete for the packet
// lifecycle to end, so the custom callback cannot block completion.
// Instead we emit error events and set the error in state
// so that users and on-chain logic can handle this appropriately
func (im IBCModule) OnTimeoutPacket(
ctx sdk.Context,
packet channeltypes.Packet,
relayer sdk.AccAddress,
) error {
// application-specific onTimeoutPacket logic
// unmarshal packet data into expected interface
var cbPacketData callbackPacketData
unmarshalInterface(packet.GetData(), cbPacketData)
if cbPacketData == nil {
// the packet data does not implement the CallbackPacketData interface
// continue processing (no-op)
return
}
// call timeout callback on original actor
acc := k.getAccount(ctx, cbPacketData.GetSourceCallbackAddress())
ibcActor, ok := acc.(IBCActor)
if ok {
gasLimit := getGasLimit(ctx, cbPacketData)
handleCallback := func() error {
// create cached context with gas limit
cacheCtx, writeFn := ctx.CacheContext()
cacheCtx = cacheCtx.WithGasLimit(gasLimit)
defer func() {
if e := recover(); e != nil {
log("ran out of gas in callback. reverting callback state")
} else if err == nil {
// only write callback state if we did not panic during execution
// and the error returned is nil
writeFn()
}
}
err := ibcActor.OnTimeoutPacket(ctx, packet, relayer)
// deduct consumed gas from original context
ctx = ctx.WithGasLimit(ctx.GasMeter().RemainingGas() - cbCtx.GasMeter().GasConsumed())
return err
}
if err := handleCallback(); err != nil {
setTimeoutCallbackError(ctx, packet, err) // optional
emitTimeoutCallbackErrorEvents(err)
}
}
return nil
}
func getGasLimit(ctx sdk.Context, cbPacketData CallbackPacketData) uint64 {
// getGasLimit returns the gas limit to pass into the actor callback
// this will be the minimum of the remaining gas limit in the tx
// and the config defined gas limit. The config limit is itself
// the minimum of a user defined gas limit and the chain-defined gas limit
// for actor callbacks
var configLimit uint64
if cbPacketData == 0 {
configLimit = chainDefinedActorCallbackLimit
} else {
configLimit = min(chainDefinedActorCallbackLimit, cbPacketData.UserDefinedGasLimit())
}
return min(ctx.GasMeter().GasRemaining(), configLimit)
}Chains are expected to specify a chainDefinedActorCallbackLimit to ensure that callbacks do not consume an arbitrary amount of gas. Thus, it should always be possible for a relayer to complete the packet lifecycle even if the actor callbacks cannot run successfully.
- IBC Actors can now programatically execute logic that involves sending a packet and then performing some additional logic once the packet lifecycle is complete
- Middleware implementing ADR-8 can be generally used for any application
- Leverages the same callback architecture used between core IBC and IBC applications
- Callbacks may now have unbounded gas consumption since the actor may execute arbitrary logic. Chains implementing this feature should take care to place limitations on how much gas an actor callback can consume.
- Application packets that want to support ADR-8 must additionally have their packet data implement the
CallbackPacketDatainterface and register their implementation on the chain codec