fix: operation marshaling with typed nil value

operation.go

@@ -3,6 +3,7 @@ package jsondiff
 import (
 	"encoding/json"
 	"strings"
+	"unsafe"
 
 	"github.com/tidwall/gjson"
 )
@@ -56,10 +57,14 @@ func (jn jsonNull) MarshalJSON() ([]byte, error) {
 // MarshalJSON implements the json.Marshaler interface.
 func (o Operation) MarshalJSON() ([]byte, error) {
 	type op Operation
+
 	if !o.marshalWithValue() {
 		o.Value = nil
-	} else if o.Value == nil {
-		o.Value = jsonNull{}
+	} else {
+		// Generic check that passes for nil and type nil interface values.
+		if (*[2]uintptr)(unsafe.Pointer(&o.Value))[1] == 0 {
+			o.Value = jsonNull{}
+		}
 	}
 	if !o.hasFrom() {
 		o.From = emptyPtr

operation_test.go

@@ -7,7 +7,7 @@ func TestOperationMarshalJSON(t *testing.T) {
 		Op  Operation
 		Out string // marshaled output
 	}{
-		// Replace operations should ALWAYS be marshaled
+		// Replace and add operations should ALWAYS be marshaled
 		// with a value, even if it is null (override omitempty).
 		{
 			Operation{
@@ -17,6 +17,14 @@ func TestOperationMarshalJSON(t *testing.T) {
 			},
 			`{"op":"replace","path":"/foo/bar","value":null}`,
 		},
+		{
+			Operation{
+				Type:  OperationReplace,
+				Path:  "/foo/bar",
+				Value: typeNilIface(),
+			},
+			`{"op":"replace","path":"/foo/bar","value":null}`,
+		},
 		{
 			Operation{
 				Type:  OperationReplace,
@@ -25,6 +33,31 @@ func TestOperationMarshalJSON(t *testing.T) {
 			},
 			`{"op":"replace","path":"/foo/bar","value":"foo"}`,
 		},
+		{
+			// assigned interface
+			Operation{
+				Type:  OperationAdd,
+				Path:  "",
+				Value: nil,
+			},
+			`{"op":"add","path":"","value":null}`,
+		},
+		{
+			// unassigned interface Value
+			Operation{
+				Type: OperationAdd,
+				Path: "",
+			},
+			`{"op":"add","path":"","value":null}`,
+		},
+		{
+			Operation{
+				Type:  OperationAdd,
+				Path:  "",
+				Value: typeNilIface(),
+			},
+			`{"op":"add","path":"","value":null}`,
+		},
 		{
 			// Remove operation should NEVER be marshaled with
 			// a value.
@@ -101,6 +134,15 @@ func TestNilPatchString(t *testing.T) {
 	s := p.String()
 
 	if s != "" {
-		t.Errorf("stringified operation mismatch, got %q, wantempty string", s)
+		t.Errorf("stringified operation mismatch, got %q, want empty string", s)
 	}
 }
+
+func typeNilIface() interface{} {
+	var i *int
+	var p interface{}
+
+	p = i
+
+	return p
+}

testdata/tests/root.json

@@ -130,21 +130,21 @@
     "before": "foobar",
     "after": null,
     "patch": [
-        { "op": "add", "path": "" }
+        { "op": "add", "path": "", "value": null }
     ]
 }, {
     "name": "remove document (number)",
     "before": 1337,
     "after": null,
     "patch": [
-        { "op": "add", "path": "" }
+        { "op": "add", "path": "", "value": null }
     ]
 }, {
     "name": "remove document (boolean)",
     "before": true,
     "after": null,
     "patch": [
-        { "op": "add", "path": "" }
+        { "op": "add", "path": "", "value": null }
     ]
 }, {
     "name": "remove document (object)",
@@ -154,7 +154,7 @@
     },
     "after": null,
     "patch": [
-        { "op": "add", "path": "" }
+        { "op": "add", "path": "", "value": null }
     ]
 }, {
     "name": "remove document (array)",
@@ -163,7 +163,7 @@
     ],
     "after": null,
     "patch": [
-        { "op": "add", "path": "" }
+        { "op": "add", "path": "", "value": null }
     ]
 }, {
     "name": "replace array with object",