fix(x/tx): add feePayer as signer (#22311) BACKPORT (#24408)

Co-authored-by: Julián Toledano <JulianToledano@users.noreply.github.com>

x/tx/CHANGELOG.md

@@ -37,6 +37,7 @@ Since v0.13.0, x/tx follows Cosmos SDK semver: https://github.com/cosmos/cosmos-
 
 * SDK v0.53.x support.
 * [#23708](https://github.com/cosmos/cosmos-sdk/pull/23708) Add unordered transaction support.
+* [#24408](https://github.com/cosmos/cosmos-sdk/pull/24408) Fix add feePayer as signer.
 * [#24409](https://github.com/cosmos/cosmos-sdk/pull/24409) Fallback to injected resolver for placeholder descriptors.
 
 ## [v0.13.8](https://github.com/cosmos/cosmos-sdk/releases/tag/x/tx/v0.13.8) - 2025-01-28

x/tx/decode/decode.go

@@ -118,6 +118,18 @@ func (d *Decoder) Decode(txBytes []byte) (*DecodedTx, error) {
 		}
 	}
 
+	// If a fee payer is specified in the AuthInfo, it must be added to the list of signers
+	if authInfo.Fee != nil && authInfo.Fee.Payer != "" {
+		feeAddr, err := d.signingCtx.AddressCodec().StringToBytes(authInfo.Fee.Payer)
+		if err != nil {
+			return nil, errorsmod.Wrap(ErrTxDecode, err.Error())
+		}
+
+		if _, seen := seenSigners[string(feeAddr)]; !seen {
+			signers = append(signers, feeAddr)
+		}
+	}
+
 	return &DecodedTx{
 		Messages:                     msgs,
 		Tx:                           theTx,

x/tx/decode/decode_test.go

@@ -50,19 +50,42 @@ func TestDecode(t *testing.T) {
 	require.NoError(t, err)
 
 	testCases := []struct {
-		name  string
-		msg   proto.Message
-		error string
+		name            string
+		msg             proto.Message
+		feePayer        string
+		error           string
+		expectedSigners int
 	}{
 		{
-			name: "happy path",
-			msg:  &bankv1beta1.MsgSend{},
+			name:            "happy path",
+			msg:             &bankv1beta1.MsgSend{},
+			expectedSigners: 1,
 		},
 		{
 			name:  "empty signer option",
 			msg:   &testpb.A{},
 			error: "no cosmos.msg.v1.signer option found for message A; use DefineCustomGetSigners to specify a custom getter: tx parse error",
 		},
+		{
+			name:     "invalid feePayer",
+			msg:      &bankv1beta1.MsgSend{},
+			feePayer: "payer",
+			error:    `encoding/hex: invalid byte: U+0070 'p': tx parse error`,
+		},
+		{
+			name:            "valid feePayer",
+			msg:             &bankv1beta1.MsgSend{},
+			feePayer:        "636f736d6f733168363935356b3836397a72306770383975717034337a373263393033666d35647a366b75306c", // hexadecimal to work with dummyAddressCodec
+			expectedSigners: 2,
+		},
+		{
+			name: "same msg signer and feePayer",
+			msg: &bankv1beta1.MsgSend{
+				FromAddress: "636f736d6f733168363935356b3836397a72306770383975717034337a373263393033666d35647a366b75306c",
+			},
+			feePayer:        "636f736d6f733168363935356b3836397a72306770383975717034337a373263393033666d35647a366b75306c",
+			expectedSigners: 1,
+		},
 	}
 
 	for _, tc := range testCases {
@@ -83,7 +106,7 @@ func TestDecode(t *testing.T) {
 					Fee: &txv1beta1.Fee{
 						Amount:   []*basev1beta1.Coin{{Amount: "100", Denom: "denom"}},
 						GasLimit: 100,
-						Payer:    "payer",
+						Payer:    tc.feePayer,
 						Granter:  "",
 					},
 				},
@@ -98,6 +121,7 @@ func TestDecode(t *testing.T) {
 				return
 			}
 			require.NoError(t, err)
+			require.Equal(t, len(decodeTx.Signers), tc.expectedSigners)
 
 			require.Equal(t,
 				fmt.Sprintf("/%s", tc.msg.ProtoReflect().Descriptor().FullName()),