chore(example): minor improvement for grc20

examples/gno.land/p/demo/tokens/grc20/token.gno

@@ -81,15 +81,17 @@ func (tok Token) RenderHome() string {
 
 // SpendAllowance decreases the allowance of the specified owner and spender.
 func (led *PrivateLedger) SpendAllowance(owner, spender address, amount int64) error {
-	if !owner.IsValid() {
-		return ErrInvalidAddress
-	}
-	if !spender.IsValid() {
+	if !owner.IsValid() || !spender.IsValid() {
 		return ErrInvalidAddress
 	}
+
 	if amount < 0 {
 		return ErrInvalidAmount
 	}
+	// do nothing
+	if amount == 0 {
+		return nil
+	}
 
 	currentAllowance := led.allowance(owner, spender)
 	if currentAllowance < amount {
@@ -162,30 +164,25 @@ func (led *PrivateLedger) TransferFrom(owner, spender, to address, amount int64)
 	if amount < 0 {
 		return ErrInvalidAmount
 	}
+
+	if !owner.IsValid() || !to.IsValid() {
+		return ErrInvalidAddress
+	}
+
 	if led.balanceOf(owner) < amount {
 		return ErrInsufficientBalance
 	}
 
-	// allowance must be sufficient
-	currentAllowance := led.allowance(owner, spender)
-	if currentAllowance < amount {
-		return ErrInsufficientAllowance
+	// The check above guarantees that Transfer will succeed, ensuring
+	// atomicity for the subsequent operations.
+	if err := led.SpendAllowance(owner, spender, amount); err != nil {
+		return err
 	}
 
 	if err := led.Transfer(owner, to, amount); err != nil {
 		return err
 	}
 
-	// decrease the allowance only when transfer is successful
-	key := allowanceKey(owner, spender)
-	newAllowance := overflow.Sub64p(currentAllowance, amount)
-
-	if newAllowance == 0 {
-		led.allowances.Remove(key)
-	} else {
-		led.allowances.Set(key, newAllowance)
-	}
-
 	return nil
 }
 

examples/gno.land/p/demo/tokens/grc20/token_test.gno

@@ -98,6 +98,7 @@ func TestTransferFromAtomicity(t *testing.T) {
 		spender = testutils.TestAddress("spender")
 
 		invalidRecipient = address("")
+		recipient        = testutils.TestAddress("to")
 	)
 
 	token, admin := NewToken("Test", "TEST", 6)
@@ -121,6 +122,15 @@ func TestTransferFromAtomicity(t *testing.T) {
 	remainingAllowance := token.Allowance(owner, spender)
 	uassert.Equal(t, remainingAllowance, initialAllowance,
 		"allowance should not be reduced when transfer fails")
+
+	// transfer all tokens
+	admin.Transfer(owner, recipient, 100)
+	remainingBalance := token.BalanceOf(owner)
+	uassert.Equal(t, remainingBalance, int64(0),
+		"balance should be zero")
+
+	err = admin.TransferFrom(owner, spender, recipient, transferAmount)
+	uassert.Error(t, err, "transfer should fail due to insufficient balance")
 }
 
 func TestMintUntilOverflow(t *testing.T) {