fix: skip emission of unpopulated memo field in ics20 (#2651)

## Description

By setting `EmitDefaults` to false, we will not include an empty memo field in the marshaled json bytes

closes: #2645 

---

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checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

- [x] Targeted PR against correct branch (see [CONTRIBUTING.md](https://github.com/cosmos/ibc-go/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [ ] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/main/docs/docs/building-modules/10-structure.md).
- [ ] Wrote unit and integration [tests](https://github.com/cosmos/ibc-go/blob/master/CONTRIBUTING.md#testing)
- [ ] Updated relevant documentation (`docs/`) or specification (`x/<module>/spec/`)
- [ ] Added relevant `godoc` [comments](https://blog.golang.org/godoc-documenting-go-code).
- [ ] Added a relevant changelog entry to the `Unreleased` section in `CHANGELOG.md`
- [ ] Re-reviewed `Files changed` in the Github PR explorer
- [ ] Review `Codecov Report` in the comment section below once CI passes

CHANGELOG.md

@@ -70,6 +70,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### State Machine Breaking
 
+* (apps/transfer) [\#2651](https://github.com/cosmos/ibc-go/pull/2651) Introduce `mustProtoMarshalJSON` for ics20 packet data marshalling which will skip emission (marshalling) of the memo field if unpopulated (empty). 
 * (27-interchain-accounts) [\#2580](https://github.com/cosmos/ibc-go/issues/2580) Removing port prefix requirement from the ICA host channel handshake
 * (transfer) [\#2377](https://github.com/cosmos/ibc-go/pull/2377) Adding `sequence` to `MsgTransferResponse`.
 * (light-clients/07-tendermint) [\#2554](https://github.com/cosmos/ibc-go/pull/2554) Forbid negative values for `TrustingPeriod`, `UnbondingPeriod` and `MaxClockDrift` (as specified in ICS-07).

modules/apps/transfer/types/codec.go

@@ -1,10 +1,14 @@
 package types
 
 import (
+	"bytes"
+
 	"github.com/cosmos/cosmos-sdk/codec"
 	codectypes "github.com/cosmos/cosmos-sdk/codec/types"
 	sdk "github.com/cosmos/cosmos-sdk/types"
 	"github.com/cosmos/cosmos-sdk/types/msgservice"
+	"github.com/gogo/protobuf/jsonpb"
+	"github.com/gogo/protobuf/proto"
 )
 
 // RegisterLegacyAminoCodec registers the necessary x/ibc transfer interfaces and concrete types
@@ -39,3 +43,32 @@ func init() {
 	RegisterLegacyAminoCodec(amino)
 	amino.Seal()
 }
+
+// mustProtoMarshalJSON provides an auxiliary function to return Proto3 JSON encoded
+// bytes of a message.
+// NOTE: Copied from https://github.com/cosmos/cosmos-sdk/blob/971c542453e0972ef1dfc5a80159ad5049c7211c/codec/json.go
+// and modified in order to allow `EmitDefaults` to be set to false for ics20 packet marshalling.
+// This allows for the introduction of the memo field to be backwards compatible.
+func mustProtoMarshalJSON(msg proto.Message) []byte {
+	anyResolver := codectypes.NewInterfaceRegistry()
+
+	// EmitDefaults is set to false to prevent marshalling of unpopulated fields (memo)
+	// OrigName and the anyResovler match the fields the original SDK function would expect
+	// in order to minimize changes.
+
+	// OrigName is true since there is no particular reason to use camel case
+	// The any resolver is empty, but provided anyways.
+	jm := &jsonpb.Marshaler{OrigName: true, EmitDefaults: false, AnyResolver: anyResolver}
+
+	err := codectypes.UnpackInterfaces(msg, codectypes.ProtoJSONPacker{JSONPBMarshaler: jm})
+	if err != nil {
+		panic(err)
+	}
+
+	buf := new(bytes.Buffer)
+	if err := jm.Marshal(buf, msg); err != nil {
+		panic(err)
+	}
+
+	return buf.Bytes()
+}

modules/apps/transfer/types/codec_test.go

@@ -0,0 +1,25 @@
+package types_test
+
+import (
+	"strings"
+
+	sdk "github.com/cosmos/cosmos-sdk/types"
+
+	"github.com/cosmos/ibc-go/v6/modules/apps/transfer/types"
+)
+
+// TestMustMarshalProtoJSON tests that the memo field is only emitted (marshalled) if it is populated
+func (suite *TypesTestSuite) TestMustMarshalProtoJSON() {
+	memo := "memo"
+	packetData := types.NewFungibleTokenPacketData(sdk.DefaultBondDenom, "1", suite.chainA.SenderAccount.GetAddress().String(), suite.chainB.SenderAccount.GetAddress().String(), memo)
+
+	bz := packetData.GetBytes()
+	exists := strings.Contains(string(bz), memo)
+	suite.Require().True(exists)
+
+	packetData.Memo = ""
+
+	bz = packetData.GetBytes()
+	exists = strings.Contains(string(bz), memo)
+	suite.Require().False(exists)
+}

modules/apps/transfer/types/packet.go

@@ -58,5 +58,5 @@ func (ftpd FungibleTokenPacketData) ValidateBasic() error {
 
 // GetBytes is a helper for serialising
 func (ftpd FungibleTokenPacketData) GetBytes() []byte {
-	return sdk.MustSortJSON(ModuleCdc.MustMarshalJSON(&ftpd))
+	return sdk.MustSortJSON(mustProtoMarshalJSON(&ftpd))
 }