fix: allow OnRecvPacket to return ack bytes of nil

This bypasses the synchronous call to PacketExecuted and allows
the module to call it at a later time.

CHANGELOG.md

@@ -206,6 +206,7 @@ invalid or incomplete requests.
 * (x/genutil) [\#5938](https://github.com/cosmos/cosmos-sdk/pull/5938) Fix `InitializeNodeValidatorFiles` error handling.
 * (x/staking) [\#5949](https://github.com/cosmos/cosmos-sdk/pull/5949) Skip staking `HistoricalInfoKey` in simulations as headers are not exported.
 * (client) [\#5964](https://github.com/cosmos/cosmos-sdk/issues/5964) `--trust-node` is now false by default - for real. Users must ensure it is set to true if they don't want to enable the verifier.
+* (x/ibc) [\#7200](https://github.com/cosmos/cosmos-sdk/pull/7200) an `OnRecvPacket` callback can declare that it will do its own `PacketExecuted` if the callback returns an acknowledgment of `[]byte(nil)`.
 
 ### State Machine Breaking
 

x/ibc/handler.go

@@ -192,9 +192,12 @@ func NewHandler(k keeper.Keeper) sdk.Handler {
 				return nil, sdkerrors.Wrap(err, "receive packet callback failed")
 			}
 
-			// Set packet acknowledgement
-			if err = k.ChannelKeeper.PacketExecuted(ctx, cap, msg.Packet, ack); err != nil {
-				return nil, err
+			// If the caller returned a nil ack, it intends to do its own PacketExecuted.
+			if ack != nil {
+				// Set packet acknowledgement
+				if err = k.ChannelKeeper.PacketExecuted(ctx, cap, msg.Packet, ack); err != nil {
+					return nil, err
+				}
 			}
 
 			return res, nil