Add checks for nil before string casts in VM (#654)

Author: antonmedv

expr_test.go

@@ -2678,3 +2678,37 @@ func TestExpr_crash(t *testing.T) {
 	_, err = expr.Compile(string(content))
 	require.Error(t, err)
 }
+
+func TestExpr_nil_op_str(t *testing.T) {
+	// Let's test operators, which do `.(string)` in VM, also check for nil.
+
+	var str *string = nil
+	env := map[string]any{
+		"nilString": str,
+	}
+
+	tests := []struct{ code string }{
+		{`nilString == "str"`},
+		{`nilString contains "str"`},
+		{`nilString matches "str"`},
+		{`nilString startsWith "str"`},
+		{`nilString endsWith "str"`},
+
+		{`"str" == nilString`},
+		{`"str" contains nilString`},
+		{`"str" matches nilString`},
+		{`"str" startsWith nilString`},
+		{`"str" endsWith nilString`},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.code, func(t *testing.T) {
+			program, err := expr.Compile(tt.code)
+			require.NoError(t, err)
+
+			output, err := expr.Run(program, env)
+			require.NoError(t, err)
+			require.Equal(t, false, output)
+		})
+	}
+}

vm/vm.go

@@ -274,31 +274,50 @@ func (vm *VM) Run(program *Program, env any) (_ any, err error) {
 		case OpMatches:
 			b := vm.pop()
 			a := vm.pop()
+			if runtime.IsNil(a) || runtime.IsNil(b) {
+				vm.push(false)
+				break
+			}
 			match, err := regexp.MatchString(b.(string), a.(string))
 			if err != nil {
 				panic(err)
 			}
-
 			vm.push(match)
 
 		case OpMatchesConst:
 			a := vm.pop()
+			if runtime.IsNil(a) {
+				vm.push(false)
+				break
+			}
 			r := program.Constants[arg].(*regexp.Regexp)
 			vm.push(r.MatchString(a.(string)))
 
 		case OpContains:
 			b := vm.pop()
 			a := vm.pop()
+			if runtime.IsNil(a) || runtime.IsNil(b) {
+				vm.push(false)
+				break
+			}
 			vm.push(strings.Contains(a.(string), b.(string)))
 
 		case OpStartsWith:
 			b := vm.pop()
 			a := vm.pop()
+			if runtime.IsNil(a) || runtime.IsNil(b) {
+				vm.push(false)
+				break
+			}
 			vm.push(strings.HasPrefix(a.(string), b.(string)))
 
 		case OpEndsWith:
 			b := vm.pop()
 			a := vm.pop()
+			if runtime.IsNil(a) || runtime.IsNil(b) {
+				vm.push(false)
+				break
+			}
 			vm.push(strings.HasSuffix(a.(string), b.(string)))
 
 		case OpSlice: