Allow shutting down Emacs even if there are errors in kill-emacs-hook

* lisp/subr.el (run-hook-query-error-with-timeout): New function
(bug#28542).

* src/emacs.c (Fkill_emacs): Use it to allow exiting Emacs even if
there are errors in kill-emacs-hook.
(syms_of_emacs): Define the symbol.

etc/NEWS

@@ -1365,6 +1365,13 @@ This new command (bound to 'C-c C-l') regenerates the current hunk.
 
 ** Miscellaneous
 
+---
+*** Errors in 'kill-emacs-hook' no longer prevents Emacs from shutting down.
+If a function in that hook signals an error in an interactive Emacs,
+the user will be prompted whether to continue or not.  If the user
+doesn't answer within five seconds, Emacs will continue shutting down
+anyway.
+
 ---
 *** iso-transl is now preloaded.
 This means that keystrokes like 'Alt-[' are defined by default,

lisp/subr.el

@@ -5909,4 +5909,22 @@ returned list are in the same order as in TREE.
 (defconst regexp-unmatchable "\\`a\\`"
   "Standard regexp guaranteed not to match any string at all.")
 
+(defun run-hook-query-error-with-timeout (hook)
+  "Run HOOK, catching errors, and querying the user about whether to continue.
+If a function in HOOK signals an error, the user will be prompted
+whether to continue or not.  If the user doesn't respond,
+evaluation will continue if the user doesn't respond within five
+seconds."
+  (run-hook-wrapped
+   hook
+   (lambda (fun)
+     (condition-case err
+         (funcall fun)
+       (error
+        (unless (y-or-n-p-with-timeout (format "Error %s; continue?" err)
+                                       5 t)
+          (error err))))
+     ;; Continue running.
+     nil)))
+
 ;;; subr.el ends here

src/emacs.c

@@ -2368,7 +2368,7 @@ all of which are called before Emacs is actually killed.  */
   if (noninteractive)
     safe_run_hooks (Qkill_emacs_hook);
   else
-    run_hook (Qkill_emacs_hook);
+    call1 (Qrun_hook_query_error_with_timeout, Qkill_emacs_hook);
 
 #ifdef HAVE_X_WINDOWS
   /* Transfer any clipboards we own to the clipboard manager.  */
@@ -2890,6 +2890,8 @@ syms_of_emacs (void)
   DEFSYM (Qrisky_local_variable, "risky-local-variable");
   DEFSYM (Qkill_emacs, "kill-emacs");
   DEFSYM (Qkill_emacs_hook, "kill-emacs-hook");
+  DEFSYM (Qrun_hook_query_error_with_timeout,
+	  "run-hook-query-error-with-timeout");
 
 #ifdef HAVE_UNEXEC
   defsubr (&Sdump_emacs);