File unlock errors now issue warnings (Bug#46397)

The primary idea is to allow `kill-buffer' and `kill-emacs' to
complete even if Emacs has trouble unlocking the buffer's file.

* lisp/userlock.el (userlock--handle-unlock-error): New function, call
`display-error'.
* src/filelock.c (unlock_file_body): New function, do what
'unlock_file' used to.
(unlock_file_handle_error): New function, call
`userlock--handle-unlock-error' with the captured error.
(unlock_file): Handle `file-error' conditions by calling the handler
defined above.
* test/src/filelock-tests.el (filelock-tests-kill-buffer-spoiled):
(filelock-tests-unlock-spoiled): Modify to test new behavior.

1 files changed, 23 insertions, 3 deletions

diff --git a/src/filelock.c b/src/filelock.c
index 373fc00a42c..446a262a1ce 100644
--- a/src/filelock.c
+++ b/src/filelock.c
@@ -719,8 +719,8 @@ lock_file (Lisp_Object fn)
     }
 }
 
-void
-unlock_file (Lisp_Object fn)
+static Lisp_Object
+unlock_file_body (Lisp_Object fn)
 {
   char *lfname;
   USE_SAFE_ALLOCA;
@@ -737,6 +737,23 @@ unlock_file (Lisp_Object fn)
     report_file_errno ("Unlocking file", filename, err);
 
   SAFE_FREE ();
+  return Qnil;
+}
+
+static Lisp_Object
+unlock_file_handle_error (Lisp_Object err)
+{
+  call1 (intern ("userlock--handle-unlock-error"), err);
+  return Qnil;
+}
+
+void
+unlock_file (Lisp_Object fn)
+{
+  internal_condition_case_1 (unlock_file_body,
+			     fn,
+			     list1(Qfile_error),
+			     unlock_file_handle_error);
 }
 
 #else  /* MSDOS */
@@ -790,7 +807,10 @@ DEFUN ("unlock-buffer", Funlock_buffer, Sunlock_buffer,
        0, 0, 0,
        doc: /* Unlock the file visited in the current buffer.
 If the buffer is not modified, this does nothing because the file
-should not be locked in that case.  */)
+should not be locked in that case.  It also does nothing if the
+current buffer is not visiting a file, or is not locked.  Handles file
+system errors by calling `display-warning' and continuing as if the
+error did not occur.  */)
   (void)
 {
   if (SAVE_MODIFF < MODIFF