Fix empty file not created when dragging from remote Linux to Finder

When dragging an empty file (or a directory containing an empty file)
from a remote Linux machine to macOS Finder, the empty file would not
be copied.

Root cause: in add_payload() in dnd.c, the file is only created with
O_CREAT when payload data arrives. For an empty file, no payload is
ever sent, so the file was never created on disk. When Finder then
tried to hard-link the (non-existent) temp file to the destination, it
failed silently.

Fix: in the "all data received" handler for type 0 (regular file),
check if the fd was ever opened. If not (empty file), create the empty
file explicitly before finishing.

Also add:
- A new test probe drag_remote_item_path:N to retrieve the filesystem
  path of a specific remote item by URI index.
- Two regression tests: test_remote_drag_empty_file (verifying the
  empty file is created on disk) and test_remote_drag_dir_with_empty_file
  (verifying an empty child file inside a directory is created on disk).

Agent-Logs-Url: https://github.com/kovidgoyal/kitty/sessions/da8b4577-3de8-4784-afc0-c1967f605dec

Co-authored-by: kovidgoyal <1308621+kovidgoyal@users.noreply.github.com>

kitty/dnd.c

@@ -2088,8 +2088,16 @@ add_payload(Window *w, DragRemoteItem *ri, bool has_more, const uint8_t *payload
     if (!has_more && !payload_sz) {  // all data received
         switch (ri->type) {
             case 0:
-                safe_close(ri->fd_plus_one-1, __FILE__, __LINE__);
-                ri->fd_plus_one = 0;
+                if (ri->fd_plus_one) {
+                    safe_close(ri->fd_plus_one - 1, __FILE__, __LINE__);
+                    ri->fd_plus_one = 0;
+                } else {
+                    // Empty file: no data payload was ever received so the file was never opened;
+                    // create it now so that it exists at the expected path for the caller.
+                    int fd = safe_openat(dirfd, ri->dir_entry_name, O_CREAT | O_WRONLY, file_permissions);
+                    if (fd < 0) abrt(errno, "could not create empty drag source item file");
+                    safe_close(fd, __FILE__, __LINE__);
+                }
                 break;
             case 1:
                 // Ensure room for the null terminator needed by symlinkat
@@ -2636,6 +2644,26 @@ dnd_test_probe_state(PyObject *self UNUSED, PyObject *args) {
         }
         Py_RETURN_NONE;
     }
+    // "drag_remote_item_path:N" returns the full filesystem path for URI item N (0-based)
+    if (strncmp(q, "drag_remote_item_path:", 22) == 0) {
+        size_t uri_idx = (size_t)atoi(q + 22);
+        if (w->drag_source.base_dir_for_remote_items) {
+            for (size_t idx = 0; idx < w->drag_source.num_mimes; idx++) {
+#define mi w->drag_source.items[idx]
+                if (mi.is_uri_list && mi.remote_items && uri_idx < mi.num_remote_items) {
+                    const char *name = mi.remote_items[uri_idx].dir_entry_name;
+                    if (name) {
+                        char path[4096];
+                        snprintf(path, sizeof(path), "%s/%zu/%s",
+                            w->drag_source.base_dir_for_remote_items, uri_idx, name);
+                        return PyUnicode_FromString(path);
+                    }
+                }
+#undef mi
+            }
+        }
+        Py_RETURN_NONE;
+    }
     Py_RETURN_NONE;
 }
 

kitty_tests/dnd.py

@@ -2933,6 +2933,12 @@ class TestDnDProtocol(BaseTest):
             parse_bytes(screen, client_remote_file(1, '', item_type=0))
             self._assert_no_output(cap)
             self.assert_drag_data_complete(cap)
+            # Verify the empty file was actually created on disk
+            import os
+            path = dnd_test_probe_state(cap.window_id, 'drag_remote_item_path:0')
+            self.assertIsNotNone(path, 'empty file path should be known')
+            self.assertTrue(os.path.isfile(path), f'empty file must exist on disk: {path}')
+            self.assertEqual(os.path.getsize(path), 0, f'file must be empty: {path}')
 
     def test_remote_drag_empty_directory(self) -> None:
         """Transfer a directory with no entries."""
@@ -2945,6 +2951,27 @@ class TestDnDProtocol(BaseTest):
             parse_bytes(screen, client_remote_file(1, '', item_type=2))
             self.assert_drag_data_complete(cap)
 
+    def test_remote_drag_dir_with_empty_file(self) -> None:
+        """Directory containing an empty file: the empty child file must be created on disk."""
+        uri_list = b'file:///home/user/mydir\r\n'
+        dir_entries = b'empty_child.txt'
+        with dnd_test_window() as (screen, cap):
+            self._setup_remote_drag(screen, cap, uri_list)
+            b64 = standard_b64encode(dir_entries).decode()
+            parse_bytes(screen, client_remote_file(1, b64, item_type=2))
+            parse_bytes(screen, client_remote_file(1, '', item_type=2))
+            # Send end-of-data for the child with no payload (empty file)
+            parse_bytes(screen, client_remote_file(1, '', item_type=0, parent_handle=2, entry_num=1))
+            self._assert_no_output(cap)
+            self.assert_drag_data_complete(cap)
+            # Verify the empty child file was actually created on disk
+            import os
+            dir_path = dnd_test_probe_state(cap.window_id, 'drag_remote_item_path:0')
+            self.assertIsNotNone(dir_path, 'dir path should be known')
+            child_path = os.path.join(dir_path, 'empty_child.txt')
+            self.assertTrue(os.path.isfile(child_path), f'empty child file must exist on disk: {child_path}')
+            self.assertEqual(os.path.getsize(child_path), 0, f'child file must be empty: {child_path}')
+
     def test_remote_drag_uri_list_with_comments(self) -> None:
         """URI list with comment lines (starting with #) should filter them out."""
         uri_list = b'# this is a comment\r\nfile:///home/user/f.txt\r\n# another comment\r\n'