fix: null guard for tx fee amounts (backport #10327)

CHANGELOG.md

@@ -44,6 +44,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Improvements
 
+* [\#10327](https://github.com/cosmos/cosmos-sdk/pull/10327) Add null guard for possible nil `Amount` in tx fee `Coins`
 * (store) [\#10026](https://github.com/cosmos/cosmos-sdk/pull/10026) Improve CacheKVStore datastructures / algorithms, to no longer take O(N^2) time when interleaving iterators and insertions.
 * (store) [\#10040](https://github.com/cosmos/cosmos-sdk/pull/10040) Bump IAVL to v0.17.1 which includes performance improvements on a batch load.
 * [\#10211](https://github.com/cosmos/cosmos-sdk/pull/10211) Backport of the mechanism to reject redundant IBC transactions from [ibc-go \#235](https://github.com/cosmos/ibc-go/pull/235).

types/coin.go

@@ -129,6 +129,11 @@ func (coin Coin) IsNegative() bool {
 	return coin.Amount.Sign() == -1
 }
 
+// IsNil returns true if the coin amount is nil and false otherwise.
+func (coin Coin) IsNil() bool {
+	return coin.Amount.i == nil
+}
+
 //-----------------------------------------------------------------------------
 // Coins
 
@@ -543,6 +548,19 @@ func (coins Coins) IsAnyNegative() bool {
 	return false
 }
 
+// IsAnyNil returns true if there is at least one coin whose amount
+// is nil; returns false otherwise. It returns false if the coin set
+// is empty too.
+func (coins Coins) IsAnyNil() bool {
+	for _, coin := range coins {
+		if coin.IsNil() {
+			return true
+		}
+	}
+
+	return false
+}
+
 // negative returns a set of coins with all amount negative.
 //
 // TODO: Remove once unsigned integers are used.

types/coin_test.go

@@ -237,6 +237,20 @@ func (s *coinTestSuite) TestCoinIsZero() {
 	s.Require().False(res)
 }
 
+func (s *coinTestSuite) TestCoinIsNil() {
+	coin := sdk.Coin{}
+	res := coin.IsNil()
+	s.Require().True(res)
+
+	coin = sdk.Coin{Denom: "uatom"}
+	res = coin.IsNil()
+	s.Require().True(res)
+
+	coin = sdk.NewInt64Coin(testDenom1, 1)
+	res = coin.IsNil()
+	s.Require().False(res)
+}
+
 func (s *coinTestSuite) TestFilteredZeroCoins() {
 	cases := []struct {
 		name     string
@@ -902,6 +916,19 @@ func (s *coinTestSuite) TestCoinsIsAnyGT() {
 	}
 }
 
+func (s *coinTestSuite) TestCoinsIsAnyNil() {
+	twoAtom := sdk.NewInt64Coin("atom", 2)
+	fiveAtom := sdk.NewInt64Coin("atom", 5)
+	threeEth := sdk.NewInt64Coin("eth", 3)
+	nilAtom := sdk.Coin{Denom: "atom"}
+
+	s.Require().True(sdk.Coins{twoAtom, fiveAtom, threeEth, nilAtom}.IsAnyNil())
+	s.Require().True(sdk.Coins{twoAtom, nilAtom, fiveAtom, threeEth}.IsAnyNil())
+	s.Require().True(sdk.Coins{nilAtom, twoAtom, fiveAtom, threeEth}.IsAnyNil())
+	s.Require().False(sdk.Coins{twoAtom, fiveAtom, threeEth}.IsAnyNil())
+
+}
+
 func (s *coinTestSuite) TestMarshalJSONCoins() {
 	cdc := codec.NewLegacyAmino()
 	sdk.RegisterLegacyAminoCodec(cdc)

types/tx/types.go

@@ -72,6 +72,13 @@ func (t *Tx) ValidateBasic() error {
 		)
 	}
 
+	if fee.Amount.IsAnyNil() {
+		return sdkerrors.Wrapf(
+			sdkerrors.ErrInsufficientFee,
+			"invalid fee provided: null",
+		)
+	}
+
 	if fee.Amount.IsAnyNegative() {
 		return sdkerrors.Wrapf(
 			sdkerrors.ErrInsufficientFee,