Add rsync-compatible ignore-existing option

tests/integration/test_features.py

@@ -214,6 +214,29 @@ class TestIncremental:
             assert f.read() == b"hello world\n"
 
 
+class TestIgnoreExisting:
+    def test_ignore_existing_preserves_existing_and_transfers_new(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, port=shared_server.port)
+        assert result.returncode == 0
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        existing_file = os.path.join(received, "small.txt")
+        with open(existing_file, "wb") as f:
+            f.write(b"destination content\n")
+        new_source = os.path.join(SOURCE_DIR, "new.txt")
+        with open(new_source, "wb") as f:
+            f.write(b"new file\n")
+
+        result, _ = run_client(SOURCE_DIR, DEST_DIR,
+                               flags=["--ignore-existing"], port=shared_server.port)
+        assert result.returncode == 0, f"Sync failed: {(result.stderr or result.stdout)[:200]}"
+        with open(existing_file, "rb") as f:
+            assert f.read() == b"destination content\n"
+        with open(os.path.join(received, "new.txt"), "rb") as f:
+            assert f.read() == b"new file\n"
+
+
 class TestDelete:
     def test_delete_removes_extra_files(self, shared_server):
         clean_dir(DEST_DIR)

tests/test_client_cli.c

@@ -181,6 +181,19 @@ static void test_parse_args_valid_port() {
   config_delete(cfg);
 }
 
+static void test_parse_args_ignore_existing() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--ignore-existing", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 4, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, 0);
+  EXPECT_TRUE(cfg->ignore_existing);
+
+  config_delete(cfg);
+}
+
 /* Test parse_args rejects port > 65535 */
 static void test_parse_args_invalid_port() {
   Config* cfg = config_create();
@@ -352,6 +365,7 @@ void test_client_cli() {
   test_parse_args_help();
   test_parse_args_version();
   test_parse_args_valid_port();
+  test_parse_args_ignore_existing();
   test_parse_args_invalid_port();
   test_parse_args_non_numeric_port();
   test_parse_args_invalid_server_port();

tests/test_config.c

@@ -126,6 +126,7 @@ static void test_config_send_receive() {
   send_cfg->use_metadata = true;
   send_cfg->compression_level = 5;
   send_cfg->chunk_size = 1024;
+  send_cfg->ignore_existing = true;
 
   /* Use socketpair for bidirectional communication */
   int p[2];
@@ -160,6 +161,8 @@ static void test_config_send_receive() {
         ok = false;
       if (recv_cfg->chunk_size != 1024)
         ok = false;
+      if (!recv_cfg->ignore_existing)
+        ok = false;
     }
     config_delete(recv_cfg);
     close(p[0]);

tests/test_file.c

@@ -88,6 +88,38 @@ static void test_file_save_to_disk() {
   rmdir("test_save_tmp");
 }
 
+static void test_file_save_to_disk_ignore_existing() {
+  const char* path = "test_ignore_existing_tmp/existing.txt";
+  EXPECT_TRUE(file_write_to_disk(path, "old", 3, false, false));
+
+  File* file = file_create("existing.txt");
+  EXPECT_NOT_NULL(file);
+  file->data->data = malloc(3);
+  EXPECT_NOT_NULL(file->data->data);
+  memcpy(file->data->data, "new", 3);
+  file->data->size = 3;
+
+  Config* config = config_create();
+  EXPECT_NOT_NULL(config);
+  config->ignore_existing = true;
+  EXPECT_TRUE(file_save_to_disk("test_ignore_existing_tmp", file, config));
+
+  FILE* stream = fopen(path, "rb");
+  char content[4] = {0};
+  EXPECT_NOT_NULL(stream);
+  // cppcheck-suppress knownConditionTrueFalse
+  if (stream) {
+    EXPECT_EQ_INT((int)fread(content, 1, 3, stream), 3);
+    fclose(stream);
+  }
+  EXPECT_EQ_STR(content, "old");
+
+  file_destroy(file);
+  config_delete(config);
+  unlink(path);
+  rmdir("test_ignore_existing_tmp");
+}
+
 static void test_file_write_to_disk_basic() {
   const char* content = "Basic file_write_to_disk test";
   EXPECT_TRUE(file_write_to_disk("test_file_write_to_disk_basic.txt", content, strlen(content),
@@ -458,6 +490,7 @@ void test_file() {
   test_file_load_data();
   test_file_load_data_missing_file();
   test_file_save_to_disk();
+  test_file_save_to_disk_ignore_existing();
   test_file_write_to_disk_basic();
   test_file_write_to_disk_creates_dirs();
   test_file_write_to_disk_does_not_follow_symlink();