Do not allow to override builtin function

Author: antonmedv

builtin/builtin.go

@@ -12,13 +12,19 @@ import (
 )
 
 type Function struct {
-	Name     string
-	Func     func(args ...interface{}) (interface{}, error)
+	Name  string
+	Func  func(args ...interface{}) (interface{}, error)
+	Types []reflect.Type
+
+	builtin  bool // true if function is builtin
 	Builtin1 func(arg interface{}) interface{}
-	Types    []reflect.Type
 	Validate func(args []reflect.Type) (reflect.Type, error)
 }
 
+func (f Function) Builtin() bool {
+	return f.builtin
+}
+
 var (
 	Index map[string]int
 	Names []string
@@ -28,6 +34,7 @@ func init() {
 	Index = make(map[string]int)
 	Names = make([]string, len(Functions))
 	for i, fn := range Functions {
+		fn.builtin = true
 		Index[fn.Name] = i
 		Names[i] = fn.Name
 	}

builtin/builtin_test.go

@@ -187,3 +187,34 @@ func TestBuiltin_memory_limits(t *testing.T) {
 		})
 	}
 }
+
+func TestBuiltin_disallow_builtins_override(t *testing.T) {
+	t.Run("via env", func(t *testing.T) {
+		env := map[string]interface{}{
+			"len": func() int { return 42 },
+			"repeat": func(a string) string {
+				return a
+			},
+		}
+		assert.Panics(t, func() {
+			_, _ = expr.Compile(`string(len("foo")) + repeat("0", 2)`, expr.Env(env))
+		})
+	})
+	t.Run("via expr.Function", func(t *testing.T) {
+		length := expr.Function("len",
+			func(params ...interface{}) (interface{}, error) {
+				return 42, nil
+			},
+			new(func() int),
+		)
+		repeat := expr.Function("repeat",
+			func(params ...interface{}) (interface{}, error) {
+				return params[0], nil
+			},
+			new(func(string) string),
+		)
+		assert.Panics(t, func() {
+			_, _ = expr.Compile(`string(len("foo")) + repeat("0", 2)`, length, repeat)
+		})
+	})
+}

conf/config.go

@@ -95,4 +95,24 @@ func (c *Config) Check() {
 			}
 		}
 	}
+	for name, t := range c.Types {
+		if kind(t.Type) != reflect.Func {
+			continue
+		}
+		for _, b := range builtin.Names {
+			if b == name {
+				panic(fmt.Errorf(`cannot override builtin %s(); it is already defined in expr`, name))
+			}
+		}
+	}
+	for _, f := range c.Functions {
+		if f.Builtin() {
+			continue
+		}
+		for _, b := range builtin.Names {
+			if b == f.Name {
+				panic(fmt.Errorf(`cannot override builtin %s(); it is already defined in expr`, f.Name))
+			}
+		}
+	}
 }

conf/types_table.go

@@ -121,6 +121,13 @@ func dereference(t reflect.Type) reflect.Type {
 	return t
 }
 
+func kind(t reflect.Type) reflect.Kind {
+	if t == nil {
+		return reflect.Invalid
+	}
+	return t.Kind()
+}
+
 func FieldName(field reflect.StructField) string {
 	if taggedName := field.Tag.Get("expr"); taggedName != "" {
 		return taggedName