vim-patch:9.2.0443: GUI: cancelling save dialog overwrites or discards unnamed buffer (#39617)

Problem:  When closing gvim with an unsaved unnamed buffer, choosing
          "Yes" in the "Save changes?" dialog and then "Cancel" in the
          file selection dialog either silently writes the buffer to a
          file named "Untitled" (overwriting any existing file with
          that name) or discards the buffer altogether
          (vibs29, after v9.1.0265).
Solution: In dialog_changed(), if browse_save_fname() leaves the buffer
          without a file name, treat it as a cancel and return without
          saving.  Also stop clearing the modified flag in the restore
          path on write failure, so the unsaved changes are kept and
          the caller (e.g. gui_shell_closed()) can also cancel the
          close.  Pre-fill the file dialog with "Untitled" to match
          the preceding "Save changes to ..." prompt.  Add a test for
          the write-failure path (Hirohito Higashi).

fixes:  vim/vim#20132
closes: vim/vim#20143

https://github.com/vim/vim/commit/cf947e7ef09fc8a63f447b54b107585b8f3b7abe

Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>

src/nvim/ex_cmds2.c

@@ -223,12 +223,11 @@ void dialog_changed(buf_T *buf, bool checkall)
       }
     }
 
-    // restore to empty when write failed
+    // restore to empty when write failed or was cancelled
     if (empty_bufname) {
       buf->b_fname = NULL;
       XFREE_CLEAR(buf->b_ffname);
       XFREE_CLEAR(buf->b_sfname);
-      unchanged(buf, true, false);
     }
   } else if (ret == VIM_NO) {
     unchanged(buf, true, false);

test/old/testdir/test_buffer.vim

@@ -344,6 +344,28 @@ func Test_goto_buf_with_confirm()
   close!
 endfunc
 
+" Test for issue #20132: when saving an unnamed buffer fails the modified
+" flag must be kept, otherwise the buffer is silently discarded.
+func Test_dialog_changed_keep_modified_on_write_fail()
+  CheckUnix
+  CheckNotGui
+  CheckFeature dialog_con
+  CheckNotFeature dialog_con_gui
+  CheckNotRoot
+
+  call writefile(['existing'], 'Untitled', 'D')
+  call setfperm('Untitled', 'r--r--r--')
+
+  new
+  call setline(1, 'test')
+  call feedkeys('y', 'L')
+  silent! confirm bdel
+  call assert_true(&modified)
+  call assert_equal(['existing'], readfile('Untitled'))
+
+  bw!
+endfunc
+
 " Test for splitting buffer with 'switchbuf'
 func Test_buffer_switchbuf()
   new Xswitchbuf