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: Daniel Colascione <dancol <at> dancol.org>
Subject: bug#16160: closed (Re: bug#16160: [PATCH] define-derived-mode
 clobbers syntax tables)
Date: Tue, 13 Dec 2016 01:05:02 +0000

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Your bug report

#16160: [PATCH] define-derived-mode clobbers syntax tables

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 16160 <at> debbugs.gnu.org.

-- 
16160: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16160
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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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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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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