add test for issue 107 (#108)

* add test for issue 107

* fix issue 107

json/codec.go

@@ -16,13 +16,32 @@ import (
 	"github.com/segmentio/asm/keyset"
 )
 
+const (
+	// 1000 is the value used by the standard encoding/json package.
+	//
+	// https://cs.opensource.google/go/go/+/refs/tags/go1.17.3:src/encoding/json/encode.go;drc=refs%2Ftags%2Fgo1.17.3;l=300
+	startDetectingCyclesAfter = 1000
+)
+
 type codec struct {
 	encode encodeFunc
 	decode decodeFunc
 }
 
-type encoder struct{ flags AppendFlags }
-type decoder struct{ flags ParseFlags }
+type encoder struct {
+	flags AppendFlags
+	// ptrDepth tracks the depth of pointer cycles, when it reaches the value
+	// of startDetectingCyclesAfter, the ptrSeen map is allocated and the
+	// encoder starts tracking pointers it has seen as an attempt to detect
+	// whether it has entered a pointer cycle and needs to error before the
+	// goroutine runs out of stack space.
+	ptrDepth uint32
+	ptrSeen  map[unsafe.Pointer]struct{}
+}
+
+type decoder struct {
+	flags ParseFlags
+}
 
 type encodeFunc func(encoder, []byte, unsafe.Pointer) ([]byte, error)
 type decodeFunc func(decoder, []byte, unsafe.Pointer) ([]byte, error)

json/encode.go

@@ -2,6 +2,7 @@ package json
 
 import (
 	"encoding"
+	"fmt"
 	"math"
 	"reflect"
 	"sort"
@@ -815,6 +816,18 @@ func (e encoder) encodeEmbeddedStructPointer(b []byte, p unsafe.Pointer, t refle
 
 func (e encoder) encodePointer(b []byte, p unsafe.Pointer, t reflect.Type, encode encodeFunc) ([]byte, error) {
 	if p = *(*unsafe.Pointer)(p); p != nil {
+		if e.ptrDepth++; e.ptrDepth >= startDetectingCyclesAfter {
+			if _, seen := e.ptrSeen[p]; seen {
+				// TODO: reconstruct the reflect.Value from p + t so we can set
+				// the erorr's Value field?
+				return b, &UnsupportedValueError{Str: fmt.Sprintf("encountered a cycle via %s", t)}
+			}
+			if e.ptrSeen == nil {
+				e.ptrSeen = make(map[unsafe.Pointer]struct{})
+			}
+			e.ptrSeen[p] = struct{}{}
+			defer delete(e.ptrSeen, p)
+		}
 		return encode(e, b, p)
 	}
 	return e.encodeNull(b, nil)

json/json.go

@@ -55,7 +55,7 @@ type UnsupportedValueError = json.UnsupportedValueError
 
 // AppendFlags is a type used to represent configuration options that can be
 // applied when formatting json output.
-type AppendFlags uint
+type AppendFlags uint32
 
 const (
 	// EscapeHTML is a formatting flag used to to escape HTML in json strings.
@@ -74,7 +74,7 @@ const (
 
 // ParseFlags is a type used to represent configuration options that can be
 // applied when parsing json input.
-type ParseFlags uint
+type ParseFlags uint32
 
 func (flags ParseFlags) has(f ParseFlags) bool {
 	return (flags & f) != 0

json/json_test.go

@@ -1591,6 +1591,28 @@ func TestGithubIssue44(t *testing.T) {
 	}
 }
 
+type issue107Foo struct {
+	Bar *issue107Bar
+}
+
+type issue107Bar struct {
+	Foo *issue107Foo
+}
+
+func TestGithubIssue107(t *testing.T) {
+	f := &issue107Foo{}
+	b := &issue107Bar{}
+	f.Bar = b
+	b.Foo = f
+
+	_, err := Marshal(f) // must not crash
+	switch err.(type) {
+	case *UnsupportedValueError:
+	default:
+		t.Errorf("marshaling a cycling data structure was expected to return an unsupported value error but got %T", err)
+	}
+}
+
 type rawJsonString string
 
 func (r *rawJsonString) UnmarshalJSON(b []byte) error {