implement Coin.Multiply overflow check

resolve #326

errors/errors.go

@@ -63,6 +63,10 @@ var (
 	// ErrExpired stands for expired entities, normally has to do with block height expirations
 	ErrExpired = Register(15, "expired")
 
+	// ErrOverflow s returned when a computation cannot be completed
+	// because the result value exceeds the type.
+	ErrOverflow = Register(16, "an operation cannot be completed due to value overflow")
+
 	// ErrPanic is only set when we recover from a panic, so we know to redact potentially sensitive system info
 	ErrPanic = Register(111222, "panic")
 )

x/coin.go

@@ -75,22 +75,54 @@ func (c Coin) Divide(pieces int64) (Coin, Coin, error) {
 	return one, rest, nil
 }
 
-// Multiply returns the result of a coin value multiplication.
-func (c Coin) Multiply(times int64) Coin {
-	whole := c.Whole * times
-	frac := c.Fractional * times
+// Multiply returns the result of a coin value multiplication. This method can
+// fail if the result would overflow maximum coin value.
+func (c Coin) Multiply(times int64) (Coin, error) {
+	if times == 0 || (c.Whole == 0 && c.Fractional == 0) {
+		return Coin{Ticker: c.Ticker}, nil
+	}
+
+	whole, ok := mul64(c.Whole, times)
+	if !ok {
+		return Coin{}, errors.ErrOverflow
+
+	}
+	frac, ok := mul64(c.Fractional, times)
+	if !ok {
+		return Coin{}, errors.ErrOverflow
+	}
 
 	// Normalize if fractional value overflows.
 	if frac > FracUnit {
-		whole += frac / FracUnit
+		if n := whole + frac/FracUnit; n < whole {
+			return Coin{}, errors.ErrOverflow
+		} else {
+			whole = n
+		}
 		frac = frac % FracUnit
 	}
 
-	return Coin{
+	res := Coin{
 		Ticker:     c.Ticker,
 		Whole:      whole,
 		Fractional: frac,
 	}
+	return res, nil
+}
+
+// Borrowed from
+// https://github.com/JohnCGriffin/overflow/blob/master/overflow_impl.go#L336
+func mul64(a, b int64) (int64, bool) {
+	if a == 0 || b == 0 {
+		return 0, true
+	}
+	c := a * b
+	if (c < 0) == ((a < 0) != (b < 0)) {
+		if c/b == a {
+			return c, true
+		}
+	}
+	return c, false
 }
 
 // Add combines two coins.

x/coin_test.go

@@ -2,6 +2,7 @@ package x
 
 import (
 	"fmt"
+	"math"
 	"testing"
 
 	"github.com/iov-one/weave/errors"
@@ -340,16 +341,17 @@ func TestCoinDivide(t *testing.T) {
 
 func TestCoinMultiply(t *testing.T) {
 	cases := map[string]struct {
-		coin  Coin
-		times int64
-		want  Coin
+		coin    Coin
+		times   int64
+		want    Coin
+		wantErr error
 	}{
 		"zero value coin": {
 			coin:  NewCoin(0, 0, "DOGE"),
 			times: 666,
 			want:  NewCoin(0, 0, "DOGE"),
 		},
-		"multiply": {
+		"simple multiply": {
 			coin:  NewCoin(1, 0, "DOGE"),
 			times: 3,
 			want:  NewCoin(3, 0, "DOGE"),
@@ -369,12 +371,48 @@ func TestCoinMultiply(t *testing.T) {
 			times: -2,
 			want:  NewCoin(-2, -2, "DOGE"),
 		},
+		"overflow of a negative and a positive value": {
+			coin:    NewCoin(math.MaxInt64, 0, "DOGE"),
+			times:   -math.MaxInt64,
+			wantErr: errors.ErrOverflow,
+		},
+		"overflow of two negative values": {
+			coin:    NewCoin(-math.MaxInt64, 0, "DOGE"),
+			times:   -math.MaxInt64,
+			wantErr: errors.ErrOverflow,
+		},
+		"overflow of two positive values": {
+			coin:    NewCoin(math.MaxInt64, 0, "DOGE"),
+			times:   math.MaxInt64,
+			wantErr: errors.ErrOverflow,
+		},
+		"overflow with a big multiply": {
+			coin:    NewCoin(1000, 0, "DOGE"),
+			times:   math.MaxInt64 / 10,
+			wantErr: errors.ErrOverflow,
+		},
+		"overflow with a small multiply": {
+			coin:    NewCoin(math.MaxInt64/10, 0, "DOGE"),
+			times:   1000,
+			wantErr: errors.ErrOverflow,
+		},
+		"overflow when normalizing": {
+			coin:    NewCoin(math.MaxInt64-1, math.MaxInt64-1, "DOGE"),
+			times:   1,
+			wantErr: errors.ErrOverflow,
+		},
 	}
 	for testName, tc := range cases {
 		t.Run(testName, func(t *testing.T) {
-			got := tc.coin.Multiply(tc.times)
-			if !got.Equals(tc.want) {
-				t.Fatalf("got %v", got)
+			got, err := tc.coin.Multiply(tc.times)
+			if !errors.Is(tc.wantErr, err) {
+				t.Logf("got coin: %+v", got)
+				t.Fatalf("got error %v", err)
+			}
+			if tc.wantErr == nil {
+				if !got.Equals(tc.want) {
+					t.Fatalf("got %v", got)
+				}
 			}
 		})
 	}

x/distribution/handler.go

@@ -277,7 +277,10 @@ func distribute(db weave.KVStore, ctrl CashController, source weave.Address, rec
 		}
 
 		for _, r := range recipients {
-			amount := one.Multiply(int64(r.Weight / div))
+			amount, err := one.Multiply(int64(r.Weight / div))
+			if err != nil {
+				return errors.Wrap(err, "cannot multiply chunk")
+			}
 			// Chunk is too small to be distributed.
 			if amount.IsZero() {
 				continue