refactor(grc20): re-use SpendAllowance in TransferFrom

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

@@ -71,11 +71,11 @@ func TestTeller(t *testing.T) {
 	checkBalances(1000, 0, 100)
 	checkAllowances(300, 0, 0, 0, 0, 0)
 
-	urequire.Error(t, ledger.SpendAllowance(alice, bob, 2000000))
+	urequire.Error(t, ledger.spendAllowance(alice, bob, 2000000))
 	checkBalances(1000, 0, 100)
 	checkAllowances(300, 0, 0, 0, 0, 0)
 
-	urequire.NoError(t, ledger.SpendAllowance(alice, bob, 100))
+	urequire.NoError(t, ledger.spendAllowance(alice, bob, 100))
 	checkBalances(1000, 0, 100)
 	checkAllowances(200, 0, 0, 0, 0, 0)
 }

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

@@ -79,35 +79,6 @@ func (tok Token) RenderHome() string {
 	return str
 }
 
-// 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() {
-		return ErrInvalidAddress
-	}
-	if amount < 0 {
-		return ErrInvalidAmount
-	}
-
-	currentAllowance := led.allowance(owner, spender)
-	if currentAllowance < amount {
-		return ErrInsufficientAllowance
-	}
-
-	key := allowanceKey(owner, spender)
-	newAllowance := overflow.Sub64p(currentAllowance, amount)
-
-	if newAllowance == 0 {
-		led.allowances.Remove(key)
-	} else {
-		led.allowances.Set(key, newAllowance)
-	}
-
-	return nil
-}
-
 // Transfer transfers tokens from the specified from address to the specified to address.
 func (led *PrivateLedger) Transfer(from, to address, amount int64) error {
 	if !from.IsValid() {
@@ -162,30 +133,25 @@ 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
 }
 
@@ -276,6 +242,37 @@ func (led *PrivateLedger) Burn(addr address, amount int64) error {
 	return nil
 }
 
+// spendAllowance decreases the allowance of the specified owner and spender.
+func (led *PrivateLedger) spendAllowance(owner, spender address, amount int64) error {
+	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 {
+		return ErrInsufficientAllowance
+	}
+
+	key := allowanceKey(owner, spender)
+	newAllowance := overflow.Sub64p(currentAllowance, amount)
+
+	if newAllowance == 0 {
+		led.allowances.Remove(key)
+	} else {
+		led.allowances.Set(key, newAllowance)
+	}
+
+	return nil
+}
+
 // hasAddr checks if the specified address is a known account in the ledger.
 func (led PrivateLedger) hasAddr(addr address) bool {
 	return led.balances.Has(addr.String())

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

@@ -70,11 +70,11 @@ func TestToken(t *testing.T) {
 	checkBalances(1000, 0, 100)
 	checkAllowances(300, 0, 0, 0, 0, 0)
 
-	urequire.Error(t, adm.SpendAllowance(alice, bob, 2000000))
+	urequire.Error(t, adm.spendAllowance(alice, bob, 2000000))
 	checkBalances(1000, 0, 100)
 	checkAllowances(300, 0, 0, 0, 0, 0)
 
-	urequire.NoError(t, adm.SpendAllowance(alice, bob, 100))
+	urequire.NoError(t, adm.spendAllowance(alice, bob, 100))
 	checkBalances(1000, 0, 100)
 	checkAllowances(200, 0, 0, 0, 0, 0)
 }
@@ -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) {