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Re: [Emacs-diffs] feature/byte-unwind-protect 916094a 2/2: Add new bytec

From: Stefan Monnier
Subject: Re: [Emacs-diffs] feature/byte-unwind-protect 916094a 2/2: Add new bytecodes for unwind-protect
Date: Tue, 23 Jan 2018 11:15:06 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

>>> +   CASE (Bpushunwindprotect): /* New in 27.1.  */
>>> +     {
>>> +       struct handler *c = push_handler (Qt, CATCHER_ALL);
> Stefan> Hmm... using this "catch+rethrow" method interferes with the
> Stefan> debug-on-error stacktrace:
> Hmm, yeah.
> Any ideas for fixing this?

Probably change the maybe_call_debugger code so it ignores
CATCHER_ALL somehow.

> One idea would be to capture the stack trace when CATCHER_ALL is
> targeted, then have a way to pass this to signal or throw.

In general, it'd be nice, yes, except that:

> But often the stack trace isn't needed.


> Stefan> I also wonder if there can be places where we call something like
> Stefan> `unbind_to` which should run those new unwind-forms but won't find it
> Stefan> because it won't look at the handlers [ offhand, I can't think of any
> Stefan> place where that would happen, and I think it's probably OK, but
> Stefan> I can't quite convince myself that it definitely is.  ]
> If this happens then surely it's already a problem for catch and
> condition-case?

But having looked a bit more carefully at the code, I think we're OK, indeed.

> Stefan> All in all, I'd really prefer if we kept the unwind forms in
> Stefan> the specpdl.  Was there a particular reason why you used the
> Stefan> handlers instead?
> This is how catch and condition-case are implemented.

I know.  But until now all the unwind forms are pushed to the specpdl,
whereas the catcher and condition-case handlers go to the
handlers stack.

Your patch changes that by moving the unwind forms to the handlers stack
(tho it keeps the "built-in unwinds" on the specpdl).

> I do agree there's no reason to have a separate handler list.  In fact
> this puzzled me for a while.  However I think it's a separate cleanup.

Yes, merging the two is a different issue (last I looked at that code
I was also tempted to go that way, but the two are used quite
differently, so it's not clear there'd be much benefit).


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