Improve fuzz tests

Signed-off-by: John Howard <[email protected]>

v2/fuzz_test.go

@@ -1,8 +1,11 @@
 package jsonpatch_test
 
 import (
+	"encoding/json"
 	"testing"
 
+	jp "github.com/evanphx/json-patch"
+	"github.com/stretchr/testify/assert"
 	"gomodules.xyz/jsonpatch/v2"
 )
 
@@ -17,6 +20,35 @@ func FuzzCreatePatch(f *testing.F) {
 	add(empty, simpleA)
 	add(point, lineString)
 	f.Fuzz(func(t *testing.T, a, b []byte) {
-		_, _ = jsonpatch.CreatePatch(a, b)
+		checkFuzz(t, a, b)
 	})
 }
+
+func checkFuzz(t *testing.T, src, dst []byte) {
+	patch, err := jsonpatch.CreatePatch(src, dst)
+	if err != nil {
+		// Ok to error, src or dst may be invalid
+		t.Skip()
+	}
+
+	// Applying library only works with arrays and structs, no primitives
+	// We still do CreatePatch to make sure it doesn't panic
+	if isPrimitive(src) || isPrimitive(dst) {
+		return
+	}
+
+	data, err := json.Marshal(patch)
+	assert.Nil(t, err)
+
+	p2, err := jp.DecodePatch(data)
+	assert.Nil(t, err)
+
+	d2, err := p2.Apply(src)
+	assert.Nil(t, err)
+
+	assert.JSONEq(t, string(dst), string(d2))
+}
+
+func isPrimitive(data []byte) bool {
+	return data[0] != '{' && data[0] != '['
+}