jsonchema: add cycle detection (#77)

jsonschema/infer.go

@@ -37,17 +37,19 @@ import (
 //   - unsafe pointers
 //
 // The types must not have cycles.
+// It will return an error if there is a cycle in the types.
 func For[T any]() (*Schema, error) {
 	// TODO: consider skipping incompatible fields, instead of failing.
-	s, err := forType(reflect.TypeFor[T]())
+	seen := make(map[reflect.Type]bool)
+	s, err := forType(reflect.TypeFor[T](), seen)
 	if err != nil {
 		var z T
 		return nil, fmt.Errorf("For[%T](): %w", z, err)
 	}
 	return s, nil
 }
 
-func forType(t reflect.Type) (*Schema, error) {
+func forType(t reflect.Type, seen map[reflect.Type]bool) (*Schema, error) {
 	// Follow pointers: the schema for *T is almost the same as for T, except that
 	// an explicit JSON "null" is allowed for the pointer.
 	allowNull := false
@@ -56,6 +58,16 @@ func forType(t reflect.Type) (*Schema, error) {
 		t = t.Elem()
 	}
 
+	// Check for cycles
+	// User defined types have a name, so we can skip those that are natively defined
+	if t.Name() != "" {
+		if seen[t] {
+			return nil, fmt.Errorf("cycle detected for type %v", t)
+		}
+		seen[t] = true
+		defer delete(seen, t)
+	}
+
 	var (
 		s   = new(Schema)
 		err error
@@ -81,14 +93,14 @@ func forType(t reflect.Type) (*Schema, error) {
 			return nil, fmt.Errorf("unsupported map key type %v", t.Key().Kind())
 		}
 		s.Type = "object"
-		s.AdditionalProperties, err = forType(t.Elem())
+		s.AdditionalProperties, err = forType(t.Elem(), seen)
 		if err != nil {
 			return nil, fmt.Errorf("computing map value schema: %v", err)
 		}
 
 	case reflect.Slice, reflect.Array:
 		s.Type = "array"
-		s.Items, err = forType(t.Elem())
+		s.Items, err = forType(t.Elem(), seen)
 		if err != nil {
 			return nil, fmt.Errorf("computing element schema: %v", err)
 		}
@@ -114,7 +126,7 @@ func forType(t reflect.Type) (*Schema, error) {
 			if s.Properties == nil {
 				s.Properties = make(map[string]*Schema)
 			}
-			s.Properties[info.Name], err = forType(field.Type)
+			s.Properties[info.Name], err = forType(field.Type, seen)
 			if err != nil {
 				return nil, err
 			}

jsonschema/infer_test.go

@@ -91,3 +91,96 @@ func TestForType(t *testing.T) {
 		})
 	}
 }
+
+func TestForWithMutation(t *testing.T) {
+	// This test ensures that the cached schema is not mutated when the caller
+	// mutates the returned schema.
+	type S struct {
+		A int
+	}
+	type T struct {
+		A int `json:"A"`
+		B map[string]int
+		C []S
+		D [3]S
+		E *bool
+	}
+	s, err := jsonschema.For[T]()
+	if err != nil {
+		t.Fatalf("For: %v", err)
+	}
+	s.Required[0] = "mutated"
+	s.Properties["A"].Type = "mutated"
+	s.Properties["C"].Items.Type = "mutated"
+	s.Properties["D"].MaxItems = jsonschema.Ptr(10)
+	s.Properties["D"].MinItems = jsonschema.Ptr(10)
+	s.Properties["E"].Types[0] = "mutated"
+
+	s2, err := jsonschema.For[T]()
+	if err != nil {
+		t.Fatalf("For: %v", err)
+	}
+	if s2.Properties["A"].Type == "mutated" {
+		t.Fatalf("ForWithMutation: expected A.Type to not be mutated")
+	}
+	if s2.Properties["B"].AdditionalProperties.Type == "mutated" {
+		t.Fatalf("ForWithMutation: expected B.AdditionalProperties.Type to not be mutated")
+	}
+	if s2.Properties["C"].Items.Type == "mutated" {
+		t.Fatalf("ForWithMutation: expected C.Items.Type to not be mutated")
+	}
+	if *s2.Properties["D"].MaxItems == 10 {
+		t.Fatalf("ForWithMutation: expected D.MaxItems to not be mutated")
+	}
+	if *s2.Properties["D"].MinItems == 10 {
+		t.Fatalf("ForWithMutation: expected D.MinItems to not be mutated")
+	}
+	if s2.Properties["E"].Types[0] == "mutated" {
+		t.Fatalf("ForWithMutation: expected E.Types[0] to not be mutated")
+	}
+	if s2.Required[0] == "mutated" {
+		t.Fatalf("ForWithMutation: expected Required[0] to not be mutated")
+	}
+}
+
+type x struct {
+	Y y
+}
+type y struct {
+	X []x
+}
+
+func TestForWithCycle(t *testing.T) {
+	type a []*a
+	type b1 struct{ b *b1 } // unexported field should be skipped
+	type b2 struct{ B *b2 }
+	type c1 struct{ c map[string]*c1 } // unexported field should be skipped
+	type c2 struct{ C map[string]*c2 }
+
+	tests := []struct {
+		name      string
+		shouldErr bool
+		fn        func() error
+	}{
+		{"slice alias (a)", true, func() error { _, err := jsonschema.For[a](); return err }},
+		{"unexported self cycle (b1)", false, func() error { _, err := jsonschema.For[b1](); return err }},
+		{"exported self cycle (b2)", true, func() error { _, err := jsonschema.For[b2](); return err }},
+		{"unexported map self cycle (c1)", false, func() error { _, err := jsonschema.For[c1](); return err }},
+		{"exported map self cycle (c2)", true, func() error { _, err := jsonschema.For[c2](); return err }},
+		{"cross-cycle x -> y -> x", true, func() error { _, err := jsonschema.For[x](); return err }},
+		{"cross-cycle y -> x -> y", true, func() error { _, err := jsonschema.For[y](); return err }},
+	}
+
+	for _, test := range tests {
+		test := test // prevent loop shadowing
+		t.Run(test.name, func(t *testing.T) {
+			err := test.fn()
+			if test.shouldErr && err == nil {
+				t.Errorf("expected cycle error, got nil")
+			}
+			if !test.shouldErr && err != nil {
+				t.Errorf("unexpected error: %v", err)
+			}
+		})
+	}
+}