[#252] patcher: Fix applying patch from the same offset

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

object/patcher/patcher.go

@@ -190,6 +190,8 @@ func (p *patcher) copyRange(ctx context.Context, rng *objectSDK.Range) error {
 }
 
 func (p *patcher) applyPatch(ctx context.Context, payloadPatch *objectSDK.PayloadPatch, offset uint64) (newOffset uint64, err error) {
+	newOffset = offset
+
 	// write the original payload chunk before the start of the patch
 	if payloadPatch.Range.GetOffset() > offset {
 		rng := new(objectSDK.Range)

object/patcher/patcher_test.go

@@ -508,6 +508,31 @@ func TestPatch(t *testing.T) {
 			originalObjectPayload: []byte("0123456789ABCDEF"),
 			patchedPayload:        []byte("0123456789aaaaaDEFbbbbb"),
 		},
+		{
+			name: "starting from the same offset",
+			patches: []objectSDK.Patch{
+				{
+					PayloadPatch: &objectSDK.PayloadPatch{
+						Range: rangeWithOffestWithLength(8, 3),
+						Chunk: []byte("1"),
+					},
+				},
+				{
+					PayloadPatch: &objectSDK.PayloadPatch{
+						Range: rangeWithOffestWithLength(11, 0),
+						Chunk: []byte("2"),
+					},
+				},
+				{
+					PayloadPatch: &objectSDK.PayloadPatch{
+						Range: rangeWithOffestWithLength(11, 0),
+						Chunk: []byte("3"),
+					},
+				},
+			},
+			originalObjectPayload: []byte("abcdefghijklmnop"),
+			patchedPayload:        []byte("abcdefgh123lmnop"),
+		},
 		{
 			name: "a few patches: various modifiactions",
 			patches: []objectSDK.Patch{