bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Dan Čermák]

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This is a forward of https://bugzilla.redhat.com/show_bug.cgi?id=1950158

Emacs 27.2 freezes when editing a file that ends with .mod (i.e. it
opens the file in Modula 2 mode) after one enters a ':'. The UI simply
hangs until one interrupts it with C-g.

This happens with emacs -Q as well as with the Emacs flatpak (thus I
assume that it is not Fedora specific).


Cheers,

Dan

In GNU Emacs 27.2 (build 1, x86_64-redhat-linux-gnu, GTK+ Version 3.24.27, cairo version 1.16.0)
 of 2021-03-27 built on buildvm-x86-31.iad2.fedoraproject.org
Windowing system distributor 'Fedora Project', version 11.0.12011000
System Description: Fedora 33 (Server Edition)

Recent messages:

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bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Eli Zaretskii]

> From: Dan Čermák <dan.cermak@cgc-instruments.com>
> Date: Sun, 25 Apr 2021 09:53:17 +0200
> 
> This is a forward of https://bugzilla.redhat.com/show_bug.cgi?id=1950158
> 
> Emacs 27.2 freezes when editing a file that ends with .mod (i.e. it
> opens the file in Modula 2 mode) after one enters a ':'. The UI simply
> hangs until one interrupts it with C-g.
> 
> This happens with emacs -Q as well as with the Emacs flatpak (thus I
> assume that it is not Fedora specific).

I cannot reproduce this using the recipe in the Bugzilla report,
neither in Emacs 27.2 nor in the current master branch.  I see no
hang.

If C-g interrupts the hang, please set debug-on-signal non-nil, repeat
the recipe, press C-g, and post the Lisp backtrace here.

Thanks.

bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Gregory Heytings]

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>
> I cannot reproduce this using the recipe in the Bugzilla report, neither 
> in Emacs 27.2 nor in the current master branch.  I see no hang.
>

The recipe isn't precise enough:

emacs -Q foo.mod
set SPC PROD SPC : SPC

Emacs hangs after the second SPC.  This bug is an old one, Emacs 24 has 
the same problem.  In fact an ever shorter recipe also works:

emacs -Q foo.mod
: SPC

>
> If C-g interrupts the hang, please set debug-on-signal non-nil, repeat 
> the recipe, press C-g, and post the Lisp backtrace here.
>

You probably meant "debug-on-quit".  Here is the backtrace:

smie-default-backward-token()
m2-smie-refine-colon()
m2-smie-backward-token()
smie-blink-matching-open()
self-insert-command(1 32)
funcall-interactively(self-insert-command 1 32)
call-interactively(self-insert-command nil nil)
command-execute(self-insert-command)

And here is the patch.

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From 462ca0654d2aa2a2b6506f2e9a1a9ceb8e683cad Mon Sep 17 00:00:00 2001
From: Gregory Heytings <gregory@heytings.org>
Date: Sun, 25 Apr 2021 10:33:48 +0000
Subject: [PATCH] Fix infloop in Modula-2 mode

* lisp/progmodes/modula2.el (m2-smie-refine-colon): Stop looping when
point does not move with forward-sexp (Bug#48011)
---
 lisp/progmodes/modula2.el | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lisp/progmodes/modula2.el b/lisp/progmodes/modula2.el
index 2a0374aa81..5756d91831 100644
--- a/lisp/progmodes/modula2.el
+++ b/lisp/progmodes/modula2.el
@@ -201,7 +201,10 @@ m2-smie-refine-colon
          ((zerop (length tok))
           (let ((forward-sexp-function nil))
             (condition-case nil
-                (forward-sexp -1)
+                (let ((p (point)))
+                  (forward-sexp -1)
+                  (if (= p (point))
+                      (setq res ":")))
               (scan-error (setq res ":")))))
          ((member tok '("|" "OF" "..")) (setq res ":-case"))
          ((member tok '(":" "END" ";" "BEGIN" "VAR" "RECORD" "PROCEDURE"))
-- 
2.30.2

bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Dan Čermák]

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Gregory Heytings <gregory@heytings.org> writes:

> And here is the patch.

I can confirm that the patch solves the problem for me.

Can it be pushed to the Emacs repository?


Cheers,

Dan

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bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Eli Zaretskii]

> From: Dan Čermák <dan.cermak@cgc-instruments.com>
> Cc: 48011@debbugs.gnu.org
> Date: Mon, 26 Apr 2021 12:40:59 +0200
> 
> Gregory Heytings <gregory@heytings.org> writes:
> 
> > And here is the patch.
> 
> I can confirm that the patch solves the problem for me.
> 
> Can it be pushed to the Emacs repository?

We are waiting for Gregory's legal paperwork to come though, and will
install then.

Thanks for testing the patch.

bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Lars Ingebrigtsen]

Gregory Heytings <gregory@heytings.org> writes:

> And here is the patch.

Thanks; applied to Emacs 28.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

> This is a forward of https://bugzilla.redhat.com/show_bug.cgi?id=1950158
> Emacs 27.2 freezes when editing a file that ends with .mod (i.e. it
> opens the file in Modula 2 mode) after one enters a ':'. The UI simply
> hangs until one interrupts it with C-g.

First, I was super happy to see a bug report about modula2.el's
indentation code (which I kind of presumed was never used).  But I'm
starting to think that the real problem here is that you actually didn't
open a Modula-2 file.

What kind of file was that `.mod` file?


        Stefan

bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Gregory Heytings]

>> This is a forward of 
>> https://bugzilla.redhat.com/show_bug.cgi?id=1950158 Emacs 27.2 freezes 
>> when editing a file that ends with .mod (i.e. it opens the file in 
>> Modula 2 mode) after one enters a ':'. The UI simply hangs until one 
>> interrupts it with C-g.
>
> First, I was super happy to see a bug report about modula2.el's 
> indentation code (which I kind of presumed was never used).
>

Alas, the original bug report says: "I'm not actually programming in 
Modula-2, but the product I'm using typically names files with the .mod 
suffix."

>
> But I'm starting to think that the real problem here is that you 
> actually didn't open a Modula-2 file.
>
> What kind of file was that `.mod` file?
>

Why is this important?  Even if the file is not actually a Modula-2 file, 
Emacs should not freeze when the user types ":": with emacs -Q foo.mod, 
type "a:", and Emacs freezes (until the user presses C-g).

bug#48011: 27.2; Emacs freezes in modula 2 mode when entering a :

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

>> What kind of file was that `.mod` file?
> Why is this important?

Because maybe we should change the auto-mode-alist rule not to use
`modula2-mode` for `.mod` files.

> Even if the file is not actually a Modula-2 file,
> Emacs should not freeze when the user types ":":

No doubt.  And AFAIK this has now been fixed.

But even if `modula2-mode` doesn't freeze, using it for non-Modula-2
files is not necessarily a good idea ;-)


        Stefan