Add rsync-compatible existing option

tests/integration/test_features.py

@@ -214,6 +214,34 @@ class TestIncremental:
             assert f.read() == b"hello world\n"
 
 
+class TestExisting:
+    def test_existing_updates_existing_and_skips_new(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0, f"Initial sync failed: {(result.stderr or result.stdout)[:200]}"
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        source_file = os.path.join(SOURCE_DIR, "small.txt")
+        new_source_file = os.path.join(SOURCE_DIR, "new-existing-test.txt")
+        with open(source_file, "wb") as f:
+            f.write(b"updated existing content\n")
+        with open(new_source_file, "wb") as f:
+            f.write(b"this file must not be created\n")
+
+        try:
+            result, _ = run_client(SOURCE_DIR, DEST_DIR,
+                                   flags=["-M", "--existing"], port=shared_server.port)
+            assert result.returncode == 0, f"--existing sync failed: {(result.stderr or result.stdout)[:200]}"
+
+            with open(os.path.join(received, "small.txt"), "rb") as f:
+                assert f.read() == b"updated existing content\n"
+            assert not os.path.exists(os.path.join(received, "new-existing-test.txt"))
+        finally:
+            os.unlink(new_source_file)
+            with open(source_file, "wb") as f:
+                f.write(b"hello world\n")
+
+
 class TestDelete:
     def test_delete_removes_extra_files(self, shared_server):
         clean_dir(DEST_DIR)