Re: unwind-protect within while-no-input

[Stefan Monnier via "Emacs development discussions." <emacs-devel@gnu.org>]

> It seems like it's impossible right now to use unwind-protect robustly
> in face of quits, because a quit triggered in the body forms of the
> unwind-protect will also interrupt the unwind forms.

I believe that is indeed the case.  🙁

> And there is no way to work around this, because binding inhibit-quit
> in the unwind forms can *also* be interrupted.

Yup.  I have seen such problems when I hit `C-g` frantically to get out
of some problematic state, and some of the repetitions end up happening
while we're running the unwind forms.

> Maybe there should be a version of unwind-protect which automatically
> and atomically inhibits quit in the unwind forms?

I tend to agree.  I even considered that this should apply to all unwind
forms, tho I suspect it would be too drastic, making it impossible to
escape problems where the unwind form itself is stuck in an inf-loop.
At the very least things like `C-g` should not be able to interrupt the
unwinding of things like dynamically scoped let-bindings or
`save-excursion`.

Is there a bug-report associated to this thread?

Maybe a "cheaper" answer for your specific problem is to make sure
that once `while-no-input` has started a non-local exit (which it does
with `throw`, IIRC), we mark "this while-no-input" as being done
so it won't cause a nested exit while we process the unwind form of the
original exit.

E.g. with a patch like the one below?  Would it fix your case?


        Stefan


diff --git a/src/keyboard.c b/src/keyboard.c
index 3551a77a9c9..c2c326f434c 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -3801,7 +3816,10 @@ kbd_buffer_store_buffered_event (union buffered_input_event *event,
      as input, set quit-flag to cause an interrupt.  */
   if (!NILP (Vthrow_on_input)
       && !is_ignored_event (event))
-    Vquit_flag = Vthrow_on_input;
+    {
+      Vquit_flag = Vthrow_on_input;
+      Vthrow_on_input = !nil;
+    }
 }
 
 /* Limit help event positions to this range, to avoid overflow problems.  */