Add fee payer to protobuf definition

proto/cosmos/tx/v1beta1/tx.proto

@@ -169,4 +169,9 @@ message Fee {
   // gas_limit is the maximum gas that can be used in transaction processing
   // before an out of gas error occurs
   uint64 gas_limit = 2;
+
+  // if unset, the first signer is responsible for paying the fees. If set, the specified account must pay the fees.
+  // the payer must be a tx signer (and thus have signed this field in AuthInfo).
+  // setting this field does *not* change the ordering of required signers for the transaction.
+  string payer = 3;
 }

types/tx/types.go

@@ -67,6 +67,13 @@ func (t *Tx) ValidateBasic() error {
 		)
 	}
 
+	if fee.Payer != "" {
+		_, err := sdk.AccAddressFromBech32(fee.Payer)
+		if err != nil {
+			return sdkerrors.Wrapf(sdkerrors.ErrInvalidAddress, "Invalid fee payer address (%s)", err)
+		}
+	}
+
 	sigs := t.Signatures
 
 	if len(sigs) == 0 {
@@ -84,6 +91,8 @@ func (t *Tx) ValidateBasic() error {
 }
 
 // GetSigners retrieves all the signers of a tx.
+// This includes all unique signers of the messages (in order),
+// as well as the FeePayer (if specified and not already included).
 func (t *Tx) GetSigners() []sdk.AccAddress {
 	var signers []sdk.AccAddress
 	seen := map[string]bool{}
@@ -97,6 +106,17 @@ func (t *Tx) GetSigners() []sdk.AccAddress {
 		}
 	}
 
+	// ensure any specified fee payer is included in the required signers (at the end)
+	feePayer := t.AuthInfo.Fee.Payer
+	if feePayer != "" && !seen[feePayer] {
+		payerAddr, err := sdk.AccAddressFromBech32(feePayer)
+		if err != nil {
+			panic(err)
+		}
+		signers = append(signers, payerAddr)
+		seen[feePayer] = true
+	}
+
 	return signers
 }
 

x/auth/tx/builder.go

@@ -132,6 +132,15 @@ func (w *wrapper) GetFee() sdk.Coins {
 }
 
 func (w *wrapper) FeePayer() sdk.AccAddress {
+	feePayer := w.tx.AuthInfo.Fee.Payer
+	if feePayer != "" {
+		payerAddr, err := sdk.AccAddressFromBech32(feePayer)
+		if err != nil {
+			panic(err)
+		}
+		return payerAddr
+	}
+	// use first signer as default if no payer specified
 	return w.GetSigners()[0]
 }
 
@@ -235,6 +244,17 @@ func (w *wrapper) SetFeeAmount(coins sdk.Coins) {
 	w.authInfoBz = nil
 }
 
+func (w *wrapper) SetFeePayer(feePayer sdk.AccAddress) {
+	if w.tx.AuthInfo.Fee == nil {
+		w.tx.AuthInfo.Fee = &tx.Fee{}
+	}
+
+	w.tx.AuthInfo.Fee.Payer = feePayer.String()
+
+	// set authInfoBz to nil because the cached authInfoBz no longer matches tx.AuthInfo
+	w.authInfoBz = nil
+}
+
 func (w *wrapper) SetSignatures(signatures ...signing.SignatureV2) error {
 	n := len(signatures)
 	signerInfos := make([]*tx.SignerInfo, n)

x/auth/tx/builder_test.go

@@ -239,3 +239,53 @@ func TestBuilderValidateBasic(t *testing.T) {
 	err = txBuilder.ValidateBasic()
 	require.Error(t, err)
 }
+
+func TestBuilderFeePayer(t *testing.T) {
+	// keys and addresses
+	_, _, addr1 := testdata.KeyTestPubAddr()
+	_, _, addr2 := testdata.KeyTestPubAddr()
+	_, _, addr3 := testdata.KeyTestPubAddr()
+
+	// msg and signatures
+	msg1 := testdata.NewTestMsg(addr1, addr2)
+	feeAmount := testdata.NewTestFeeAmount()
+	msgs := []sdk.Msg{msg1}
+
+	cases := map[string]struct {
+		txFeePayer      sdk.AccAddress
+		expectedSigners []sdk.AccAddress
+		expectedPayer   sdk.AccAddress
+	}{
+		"no fee payer specified": {
+			expectedSigners: []sdk.AccAddress{addr1, addr2},
+			expectedPayer:   addr1,
+		},
+		"secondary signer set as fee payer": {
+			txFeePayer:      addr2,
+			expectedSigners: []sdk.AccAddress{addr1, addr2},
+			expectedPayer:   addr2,
+		},
+		"outside signer set as fee payer": {
+			txFeePayer:      addr3,
+			expectedSigners: []sdk.AccAddress{addr1, addr2, addr3},
+			expectedPayer:   addr3,
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			// setup basic tx
+			txBuilder := newBuilder()
+			err := txBuilder.SetMsgs(msgs...)
+			require.NoError(t, err)
+			txBuilder.SetGasLimit(200000)
+			txBuilder.SetFeeAmount(feeAmount)
+
+			// set fee payer
+			txBuilder.SetFeePayer(tc.txFeePayer)
+			// and check it updates fields properly
+			require.Equal(t, tc.expectedSigners, txBuilder.GetSigners())
+			require.Equal(t, tc.expectedPayer, txBuilder.FeePayer())
+		})
+	}
+}