fix crash on x % 0`

1 % 0 now returns an error (as does Postgres 9.3; MySQL goes for NULL)

found via go-fuzz

sql/parser/eval.go

@@ -19,6 +19,7 @@ package parser
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"math"
 	"reflect"
@@ -231,7 +232,11 @@ var binOps = map[binArgs]func(Datum, Datum) (Datum, error){
 	},
 
 	binArgs{Mod, intType, intType}: func(left Datum, right Datum) (Datum, error) {
-		return left.(DInt) % right.(DInt), nil
+		r := right.(DInt)
+		if r == 0 {
+			return nil, errors.New("zero modulus")
+		}
+		return left.(DInt) % r, nil
 	},
 	binArgs{Mod, floatType, floatType}: func(left Datum, right Datum) (Datum, error) {
 		return DFloat(math.Mod(float64(left.(DFloat)), float64(right.(DFloat)))), nil

sql/parser/eval_test.go

@@ -41,6 +41,8 @@ func TestEvalExpr(t *testing.T) {
 		{`5 % 3`, `2`, nil},
 		// Division is always done on floats.
 		{`4 / 5`, `0.8`, nil},
+		{`1 / 0`, `+Inf`, nil},
+		{`-1.0 * (1.0 / 0.0)`, `-Inf`, nil},
 		// Grouping
 		{`1 + 2 + (3 * 4)`, `15`, nil},
 		// Unary operators.
@@ -204,9 +206,11 @@ func TestEvalExprError(t *testing.T) {
 		expr     string
 		expected string
 	}{
+		{`1 % 0`, `zero modulus`},
 		{`'1' + '2'`, `unsupported binary operator:`},
 		{`'a' + 0`, `unsupported binary operator:`},
 		{`1.1 # 3.1`, `unsupported binary operator:`},
+		{`1/0.0`, `unsupported binary operator:`},
 		{`~0.1`, `unsupported unary operator:`},
 		{`'10' > 2`, `unsupported comparison operator:`},
 		{`1 IN ('a', 'b')`, `unsupported comparison operator:`},