GNU bug report logs - #16160
[PATCH] define-derived-mode clobbers syntax tables

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Glenn Morris <rgm <at> gnu.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#16160: closed ([PATCH] define-derived-mode clobbers syntax
 tables)
Date: Tue, 13 Dec 2016 01:05:02 +0000

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Your message dated Mon, 12 Dec 2016 20:04:52 -0500
with message-id <58eg1cbpvv.fsf <at> fencepost.gnu.org>
and subject line Re: bug#16160: [PATCH] define-derived-mode clobbers syntax tables
has caused the debbugs.gnu.org bug report #16160,
regarding [PATCH] define-derived-mode clobbers syntax tables
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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From: Daniel Colascione <dancol <at> dancol.org>
To: bug-emacs <bug-gnu-emacs <at> gnu.org>
Subject: [PATCH] define-derived-mode clobbers syntax tables
Date: Mon, 16 Dec 2013 03:12:43 -0800

Repro:

1) Visit shell script with comments. See comments fontified properly.
2) M-x find-library sh-script RET
3) M-x eval-buffer
4) Return to shell script buffer
5) C-x C-v RET
6) Observe that comments are no longer fontified as comments.

The define-derived-mode macro in sh-script.el emits code that clobbers 
sh-mode-syntax-table because we don't have a :syntax-table argument. I 
think the following patch changes the code to the expected behavior.

~/edev/trunk
$ bzr diff
=== modified file 'lisp/emacs-lisp/derived.el'
--- lisp/emacs-lisp/derived.el	2013-05-27 16:12:52 +0000
+++ lisp/emacs-lisp/derived.el	2013-12-16 11:09:41 +0000
@@ -206,11 +206,11 @@
        ,(if declare-syntax
 	    `(progn
 	       (unless (boundp ',syntax)
-		 (put ',syntax 'definition-name ',child))
-	       (defvar ,syntax (make-syntax-table))
-	       (unless (get ',syntax 'variable-documentation)
-		 (put ',syntax 'variable-documentation
-		      (purecopy ,(format "Syntax table for `%s'." child))))))
+		 (put ',syntax 'definition-name ',child)
+                 (defvar ,syntax (make-syntax-table))
+                 (unless (get ',syntax 'variable-documentation)
+                   (put ',syntax 'variable-documentation
+                        (purecopy ,(format "Syntax table for `%s'." 
child)))))))
        ,(if declare-abbrev
 	    `(progn
 	       (put ',abbrev 'definition-name ',child)


That is, we shouldn't touch the syntax table variable at all unless the 
variable is unbound.

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From: Glenn Morris <rgm <at> gnu.org>
To: 16160-done <at> debbugs.gnu.org
Subject: Re: bug#16160: [PATCH] define-derived-mode clobbers syntax tables
Date: Mon, 12 Dec 2016 20:04:52 -0500

Version: 26.1

>> The repro steps are incorrect. Try these:
>>
>> 1) emacs -Q
>> 2) visit a shell script
>> 3) M-x find-library sh-script RET
>> 4) C-s define-derived-mode RET
>> 5) C-M-x
>> 6) switch to your shell script buffer
>> 7) C-x C-v
>> 8) Observe that the shell script is fontified incorrectly; the syntax
>> table is now clobbered.

Fixed in 8db7b65 along the suggested lines.

So AIUI the issue is that define-derived-mode expands to 

(defvar ,syntax (make-syntax-table))

Normally this has no effect if ,syntax is already bound.
But M-x eval-defun resets defvars to their default settings...
Hence if you selectively re-evaluate only the mode definition from
sh-script.el, you get the reported problem.

So I changed it to explicitly check if ,syntax is bound.

This will however now make things go wrong in the opposite way for
someone who has been playing around with customizing a syntax table that
was actually defined by define-derived-mode and wants to reset it by
re-evaluating the derived mode definition. But AFAICS there's no way to
fix both scenarios, and the second seems less likely (?) to me than the
one reported here.

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