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Re: How to debug modification to a variable value?

From: Tassilo Horn
Subject: Re: How to debug modification to a variable value?
Date: Tue, 26 Jan 2010 09:43:09 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1.91 (gnu/linux)

"Davis Herring" <address@hidden> writes:

Hi Davis,

>> Now, my problem was that under some circumstances after saving, the
>> buffer-local value of `tg-schema-alist' was gone, i.e. set to nil.
> There's a difference between "gone" and "set to nil", even for
> automatically-buffer-local variables.  Does it still have a
> buffer-local value (which is wrong), or is it using the default
> (again)?  C-h v will say.

"Gone" means, it has the buffer-local value nil.  So there seems to be
no call to `kill-local-variable'.

>> I double-checked `greql-set-fontlock-types-regex' that it doesn't
>> modify `tg-schema-alist', and it doesn't.  I also removed all
>> destructive function calls in there, although it operates only on a
>> list created by `mapcar', and that's a copy anyway, right?
> Destructive operations on lists can't set the value of a variable to nil,
> nor can they invoke `kill-local-variable'.  `set[qf]?' is of course
> destructive, as are `kill-local-variable' and `kill-all-local-variables';
> have you looked for all of those?

There's no `kill-[all-]local-variable[s]', and the `setq' of that
variable is isolated in one command (see end of this message), which is
never called internally, only by a keybinding.

>> So what I need is some way to be put in the debugger when the value
>> of `tg-schema-alist' is modified.  Is that feasible?  I tried adding
>> an after advice to `setq' which does exactly that, but that screwed
>> my emacs instance.  I guess it's no good idea to advice such
>> primitives...
> I don't that you can do this in the Emacs debugger, but you should be
> able to use a watchpoint in gdb for this purpose.  Find where the
> buffer-local value of the variable is stored, set the watchpoint, and
> then save.

Sounds good, but how do I do that?  Especially the "find where the
buffer-local value of the variable is stored" part...

> Meanwhile, my psychic powers suggest that you have a `let' binding of
> the variable in question and switch buffers within the `let', or else
> that something is reasserting your major mode so that
> `kill-all-local-variables' is getting called.

Hm, the problem occured while I was using many different frames, which I
didn't use till now, so something in this direction seems possible.  But
between the variable is set buffer-locally to some alist and the
variable is set buffer-locally to nil, there was only one C-x C-s
without switching frames/buffers at all.

> You could test for the latter with the `permanent-local' property.

Oh, I didn't know that property.

,----[ (info "(elisp)Creating Buffer-Local") ]
|    A buffer-local variable is "permanent" if the variable name (a
| symbol) has a `permanent-local' property that is non-`nil'.  Permanent
| locals are appropriate for data pertaining to where the file came from
| or how to save it, rather than with how to edit the contents.

I don't know if this is appropriate for `tg-schema-alist'.  It
determines the completion possibilities and highlighting, and is not
strictly related to the current buffer/file, but to another file a user
can select and switch.

> PS - Sorry if some of these instructions are obvious;

They are not, at least to me.  So thanks a lot!

> I don't know just what you know!

Sure, I just wantet to avoid posting the whole code, if not absolutely
neccessary.  It's free sofware, and you can svn checkout it from [1]
using anonymous/secret as user/password.

But basically, there are two modes: tg-mode has a simple parser for a
schema description language, and greql-mode which is a mode for a graph
query language, which is used for querying graphs conforming to some TG

So tg-mode contains

--8<---------------cut here---------------start------------->8---
(defvar tg-schema-alist nil
  "The schema of the current TG file.")
(make-variable-buffer-local 'tg-schema-alist)
--8<---------------cut here---------------end--------------->8---

and in greql-mode (which requires tg-mode), there is the only function
modifying the value of `tg-schema-alist' by assigning the buffer-local
value of a temporary buffer to the buffer-local value in the query

--8<---------------cut here---------------start------------->8---
(defun greql-set-graph (graph)
  "Set `greql-graph' to GRAPH and parse it with `tg-parse-schema'."
  (interactive "fGraph file: ")
  (setq greql-graph graph)
  (let ((g greql-graph)
        schema-alist unique-name-map)
      (insert-file-contents g)
      (setq schema-alist tg-schema-alist)
      (setq unique-name-map tg-unique-name-hashmap))
    (setq tg-schema-alist schema-alist)
    (setq tg-unique-name-hashmap unique-name-map))
  ;; Setup schema element font locking
--8<---------------cut here---------------end--------------->8---

[1] https://svn.uni-koblenz.de/ist/projects/jgralab/trunk/utils

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