Return errors instead of panicking on unhashable map keys

vm/vm.go

@@ -254,6 +254,25 @@ func equal(lhsV, rhsV reflect.Value) bool {
 	return reflect.DeepEqual(lhsV.Interface(), rhsV.Interface())
 }
 
+// isHashable returns true if the value can be used as a map key without
+// panicking. A nil interface is a valid map key even though
+// reflect.Value.Comparable reports false for it.
+func isHashable(v reflect.Value) bool {
+	if v.Kind() == reflect.Interface && v.IsNil() {
+		return true
+	}
+	return v.Comparable()
+}
+
+// hashableTypeString returns the type string of the value for error messages,
+// unwrapping interfaces to show the dynamic type.
+func hashableTypeString(v reflect.Value) string {
+	if v.Kind() == reflect.Interface && !v.IsNil() {
+		v = v.Elem()
+	}
+	return v.Type().String()
+}
+
 func getMapIndex(key reflect.Value, aMap reflect.Value) reflect.Value {
 	if aMap.IsNil() {
 		return nilValue
@@ -264,6 +283,10 @@ func getMapIndex(key reflect.Value, aMap reflect.Value) reflect.Value {
 	if err != nil {
 		return nilValue
 	}
+	if !isHashable(key) {
+		// an unhashable key can never be in a map
+		return nilValue
+	}
 
 	// From reflect MapIndex:
 	// It returns the zero Value if key is not found in the map or if v represents a nil map.

vm/vmContainers_test.go

@@ -874,6 +874,14 @@ func TestMaps(t *testing.T) {
 		{Script: `b[1] = 1`, RunError: fmt.Errorf("undefined symbol 'b'")},
 		{Script: `z.y.x = 1`, RunError: fmt.Errorf("undefined symbol 'z'")},
 
+		// unhashable map keys should error instead of panic
+		{Script: `{[1,2]: 3}`, RunError: fmt.Errorf("type []interface {} cannot be used as map key")},
+		{Script: `a = {}; a[[1,2]] = 3`, RunError: fmt.Errorf("type []interface {} cannot be used as map key"), Output: map[string]interface{}{"a": map[interface{}]interface{}{}}},
+		{Script: `a = {}; a[{"x":1}] = 3`, RunError: fmt.Errorf("type map[interface {}]interface {} cannot be used as map key"), Output: map[string]interface{}{"a": map[interface{}]interface{}{}}},
+		{Script: `a = {}; a[[1,2]]`, RunOutput: nil, Output: map[string]interface{}{"a": map[interface{}]interface{}{}}},
+		{Script: `a = {"b": 1}; delete(a, [1,2])`, RunError: fmt.Errorf("type []interface {} cannot be used as map key in delete"), Output: map[string]interface{}{"a": map[interface{}]interface{}{"b": int64(1)}}},
+		{Script: `a = {}; a[nil] = 1; a[nil]`, RunOutput: int64(1)},
+
 		{Script: `{}`, RunOutput: map[interface{}]interface{}{}},
 		{Script: `{"b": nil}`, RunOutput: map[interface{}]interface{}{"b": nil}},
 		{Script: `{"b": true}`, RunOutput: map[interface{}]interface{}{"b": true}},

vm/vmExpr.go

@@ -86,6 +86,11 @@ func (runInfo *runInfoStruct) invokeExpr() {
 					return
 				}
 				key = runInfo.rv
+				if !isHashable(key) {
+					runInfo.err = newStringError(expr, "type "+hashableTypeString(key)+" cannot be used as map key")
+					runInfo.rv = nilValue
+					return
+				}
 
 				runInfo.expr = expr.Values[i]
 				runInfo.invokeExpr()
@@ -132,6 +137,11 @@ func (runInfo *runInfoStruct) invokeExpr() {
 				runInfo.rv = nilValue
 				return
 			}
+			if !isHashable(key) {
+				runInfo.err = newStringError(expr, "type "+hashableTypeString(key)+" cannot be used as map key")
+				runInfo.rv = nilValue
+				return
+			}
 
 			runInfo.expr = expr.Values[i]
 			runInfo.invokeExpr()

vm/vmLetExpr.go

@@ -179,6 +179,11 @@ func (runInfo *runInfoStruct) invokeLetExpr() {
 				runInfo.rv = nilValue
 				return
 			}
+			if !isHashable(runInfo.rv) {
+				runInfo.err = newStringError(expr, "type "+hashableTypeString(runInfo.rv)+" cannot be used as map key")
+				runInfo.rv = nilValue
+				return
+			}
 
 			value, runInfo.err = convertReflectValueToType(value, item.Type().Elem())
 			if runInfo.err != nil {

vm/vmStmt.go

@@ -714,6 +714,11 @@ func (runInfo *runInfoStruct) runSingleStmt() {
 				runInfo.rv = nilValue
 				return
 			}
+			if !isHashable(runInfo.rv) {
+				runInfo.err = newStringError(stmt, "type "+hashableTypeString(runInfo.rv)+" cannot be used as map key in delete")
+				runInfo.rv = nilValue
+				return
+			}
 			item.SetMapIndex(runInfo.rv, reflect.Value{})
 			runInfo.rv = nilValue
 		default: