Fix handling of looping structs

* tests: add case for looping structure (stack overflow)

* Add tracking for already seen structs

Fixes #8

Co-authored-by: Lucas Bremgartner <lucas@bremis.ch>

errchkjson.go

@@ -120,7 +120,7 @@ func (e *errchkjson) handleJSONMarshal(pass *analysis.Pass, ce *ast.CallExpr, fn
 		t = t.(*types.Pointer).Elem()
 	}
 
-	err := e.jsonSafe(t, 0)
+	err := e.jsonSafe(t, 0, map[types.Type]struct{}{})
 	if err != nil {
 		if _, ok := err.(unsupported); ok {
 			pass.Reportf(ce.Pos(), "`%s` for %v", fnName, err)
@@ -149,7 +149,11 @@ const (
 	unsupportedBasicTypes   = types.IsComplex
 )
 
-func (e *errchkjson) jsonSafe(t types.Type, level int) error {
+func (e *errchkjson) jsonSafe(t types.Type, level int, seenTypes map[types.Type]struct{}) error {
+	if _, ok := seenTypes[t]; ok {
+		return nil
+	}
+
 	if types.Implements(t, textMarshalerInterface()) {
 		return fmt.Errorf("unsafe type `%s` found", t.String())
 	}
@@ -176,20 +180,21 @@ func (e *errchkjson) jsonSafe(t types.Type, level int) error {
 		}
 
 	case *types.Array:
-		err := e.jsonSafe(ut.Elem(), level+1)
+		err := e.jsonSafe(ut.Elem(), level+1, seenTypes)
 		if err != nil {
 			return err
 		}
 		return nil
 
 	case *types.Slice:
-		err := e.jsonSafe(ut.Elem(), level+1)
+		err := e.jsonSafe(ut.Elem(), level+1, seenTypes)
 		if err != nil {
 			return err
 		}
 		return nil
 
 	case *types.Struct:
+		seenTypes[t] = struct{}{}
 		exported := 0
 		for i := 0; i < ut.NumFields(); i++ {
 			if !ut.Field(i).Exported() {
@@ -202,7 +207,7 @@ func (e *errchkjson) jsonSafe(t types.Type, level int) error {
 					continue
 				}
 			}
-			err := e.jsonSafe(ut.Field(i).Type(), level+1)
+			err := e.jsonSafe(ut.Field(i).Type(), level+1, seenTypes)
 			if err != nil {
 				return err
 			}
@@ -214,7 +219,7 @@ func (e *errchkjson) jsonSafe(t types.Type, level int) error {
 		return nil
 
 	case *types.Pointer:
-		err := e.jsonSafe(ut.Elem(), level+1)
+		err := e.jsonSafe(ut.Elem(), level+1, seenTypes)
 		if err != nil {
 			return err
 		}
@@ -225,7 +230,7 @@ func (e *errchkjson) jsonSafe(t types.Type, level int) error {
 		if err != nil {
 			return err
 		}
-		err = e.jsonSafe(ut.Elem(), level+1)
+		err = e.jsonSafe(ut.Elem(), level+1, seenTypes)
 		if err != nil {
 			return err
 		}

errchkjson_test.go

@@ -39,3 +39,10 @@ func TestNoExportedField(t *testing.T) {
 	testdata := analysistest.TestData()
 	analysistest.Run(t, testdata, errchkjson, "noexport")
 }
+
+func TestLoop(t *testing.T) {
+	errchkjson := errchkjson.NewAnalyzer()
+
+	testdata := analysistest.TestData()
+	analysistest.Run(t, testdata, errchkjson, "loop")
+}

testdata/src/loop/a.go

@@ -0,0 +1,18 @@
+package example
+
+import (
+	"encoding/json"
+)
+
+type entry struct {
+	Name   string `json:"name"`
+	Fields schema `json:"fields"`
+}
+
+type schema []entry
+
+// JSONMarshalStructWithLoop contains a struct with a loop.
+func JSONMarshalStructWithLoop() {
+	var structWithLoop schema
+	_, _ = json.Marshal(structWithLoop)
+}