fix(parity): scope -R --delete to the transferred prefix subtree

A general -R transfer places its files below the reconstructed prefix, so
marking the whole receive root as the delete scope deleted unrelated
sibling directories (data loss; rsync keeps them).  Use the prefix itself
as the root marker when it is non-empty, in both the single-threaded and
multithreaded pipelines.

src/client/client_send.c

@@ -335,6 +335,24 @@ static bool append_implied_dir_times(const Config* config, ArrayList* dir_entrie
   return ok;
 }
 
+/* The delete-walk root scope for a full (non---files-from) transfer: rsync
+ * confines --delete to the directories it actually transferred.  A plain
+ * recursive run mirrors the source under the receive root, so "." (the whole
+ * tree) is correct; an -R run transfers only the reconstructed prefix subtree,
+ * so the walk is scoped to that prefix instead.  Returns a malloc'd wire path
+ * (or "."), or NULL on allocation failure. */
+static char* delete_scope_root_marker(const Config* config) {
+  if (config->relative && config->files_from_set == NULL && config->send_directory) {
+    char* prefix = scanner_relative_prefix(config->send_directory);
+    if (!prefix)
+      return NULL;
+    if (prefix[0] != '\0')
+      return prefix;
+    free(prefix);
+  }
+  return str_dup(".");
+}
+
 /* True when some --files-from entry is an ancestor-or-equal directory of
  * `rel` (an empty entry -- the whole tree "." -- counts as the root). */
 static bool file_list_ancestor_listed(const FileListSet* set, const char* rel) {
@@ -2561,7 +2579,7 @@ int send_files(Config* config) {
        receive root, so mark the root itself (the "." sentinel) and let the
        scanner record nothing extra. */
     if (config->files_from_set == NULL) {
-      char* root_marker = str_dup(".");
+      char* root_marker = delete_scope_root_marker(config);
       if (!root_marker || !array_list_add(synced_dirs, root_marker)) {
         free(root_marker);
         goto send_fail;
@@ -2918,7 +2936,7 @@ int send_files_multithreaded(Config** config_ptr) {
       return 1;
     }
     if (config->files_from_set == NULL) {
-      char* root_marker = str_dup(".");
+      char* root_marker = delete_scope_root_marker(config);
       if (!root_marker || !array_list_add(context->synced_dirs, root_marker)) {
         free(root_marker);
         pipeline_context_sender_destroy(context);

tests/integration/test_parity_selection.py

@@ -141,6 +141,34 @@ class TestDirsOneLevel:
         assert result.returncode == 0, result.stderr[:300]
         assert _tree(rdst) == _tree(dest)
 
+    @requires_rsync
+    @pytest.mark.ci
+    def test_relative_delete_scope_matches_rsync(self):
+        """-R --delete must be confined to the transferred prefix subtree so a
+        sibling destination directory survives (rsync parity)."""
+        source = _make_tree(os.path.join(TEST_DATA_DIR, "sel_delscope_src"))
+        dest = os.path.join(TEST_DATA_DIR, "sel_delscope_dst")
+        rdst = os.path.join(TEST_DATA_DIR, "sel_delscope_rdst")
+        spec = source + "/./foo"
+        for root in (dest, rdst):
+            clean_dir(root)
+            os.makedirs(os.path.join(root, "foo"))
+            with open(os.path.join(root, "foo", "extra.txt"), "wb") as fh:
+                fh.write(b"extra\n")
+            os.makedirs(os.path.join(root, "unrelated"))
+            with open(os.path.join(root, "unrelated", "keep.txt"), "wb") as fh:
+                fh.write(b"keep\n")
+        with ServerManager() as server:
+            server.start(extra_args=["--allow-delete"])
+            r = _rsync(["-aR", "--delete", spec, rdst + "/"])
+            assert r.returncode == 0, r.stderr
+            result, _ = run_client(spec, dest, flags=["-a", "-R", "--delete"],
+                                   port=server.port)
+            assert result.returncode == 0, result.stderr[:300]
+        assert (os.path.isfile(os.path.join(dest, "unrelated", "keep.txt"))
+                == os.path.isfile(os.path.join(rdst, "unrelated", "keep.txt")))
+        assert _tree(dest) == _tree(rdst)
+
 
 class TestClientAliases:
     """#5: safe rsync option aliases accepted client-side."""