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Re: Different behaviour while debugging?


From: Thorsten Jolitz
Subject: Re: Different behaviour while debugging?
Date: Wed, 20 Feb 2013 16:45:35 +0100
User-agent: Gnus/5.130002 (Ma Gnus v0.2) Emacs/24.2 (gnu/linux)

"Drew Adams" <address@hidden> writes:

>> I have the very strange case that a program - in some special 
>> situation
>> - does not work as expected, but when I instrument the relevant
>> function(s) for edebug and single-step through them, everythings works
>> as expected and the output is fine. 
>> 
>> There are no errors involved, so 'toggle-debug-on-error does not help.
>> Its just that the program should put a buffer temporarily into a given
>> major-mode and uncomment some comment-lines with 'uncomment-region'
>> before turning-on Org-mode. 
>
> Yes, it can happen that using the debugger gives you different behavior,
> especially when there are changes of selected window or current buffer or 
> frame
> focus, or there is interaction with a timer or some events.  It's a case of 
> the
> observer affecting the observation, in some sense.
>
> The debugger is pretty useful most of the time, but there are cases where you
> need to find another solution.  Typically you can insert calls to `message' in
> the code to be debugged, perhaps with accompanying `sleep-for' or `sit-for'
> calls.

I inserted 'message calls in every condition-case of 'cond, and the
*Messages* buffer showed that the correct condition-cases are simply not
entered when run in normal mode, but are entered when in debugging mode. 

> Before you do that, however, try the normal Emacs debugger, `debug', instead 
> of
> `edebug'.  You can use `M-x debug-on-entry' to do that.  Or you can insert 
> calls
> `(debug)' in the code itself.  See the manual or `C-h f debug' for more info
> about using `debug'.

I tried `M-x debug-on-entry', and just like with edebug, everything
works fine as expected. 


Here the relevant code part with the message calls:
,--------------------------------------------------------------------------------
| [...]
|  (while (not (eobp))
|    (message "Entering while-loop ...")
|    (cond
|     ;; empty line (do nothing)
|     ((looking-at "^[[:space:]]*$"))
|     ;; comment line after comment line or at
|     ;; beginning of buffer
|     ((and
|       (message "Before sleep [comline after comline or bobp] ...")
|       (progn (sleep-for 1) 
|       (message "After sleep [comline after comline or bobp] ..."))
|       (message "Before checking condition [comline after comline or bobp] 
...")
|       (save-excursion
|         (eq (comment-on-line-p) (point-at-bol)))
|       (or (bobp) last-line-comment-p))
|      (message "After checking condition [comline after comline or bobp] ...")
|       (message "Entering comline after comline or bobp ...")
|      (uncomment-region (point-at-bol) (point-at-eol))
|      (setq last-line-comment-p t))
| [...]
`--------------------------------------------------------------------------------

Here the different - but only partly correct - output when using sleep
for 1 second:

,--------------------------------------------------------------
| Entering outorg-convert-to-org....
| html-mode
| Entering while-loop ...
| Before sleep [comline after comline or bobp] ...
| After sleep [comline after comline or bobp] ...
| Before checking condition [comline after comline or bobp] ...
| Entering codeline after codeline ....
| Entering while-loop ... [2 times]
| Before sleep [comline after comline or bobp] ...
| After sleep [comline after comline or bobp] ...
| Before checking condition [comline after comline or bobp] ...
| Entering comline after codeline ....
| Entering while-loop ... [2 times]
| Before sleep [comline after comline or bobp] ...
| After sleep [comline after comline or bobp] ...
| Before checking condition [comline after comline or bobp] ...
| Entering codeline after comline ....
`--------------------------------------------------------------

here again the entirely incorrect output after outcommenting  

       ,-------------------------------------------------------------
       | (progn (sleep-for 1) 
       | (message "After sleep [comline after comline or bobp] ..."))
       `-------------------------------------------------------------

in the code example above:

,--------------------------------------------------------------
| Entering outorg-convert-to-org....
| html-mode
| Entering while-loop ...
| Before sleep [comline after comline or bobp] ...
| Before checking condition [comline after comline or bobp] ...
| Entering codeline after codeline ....
| Entering while-loop ... [2 times]
| Before sleep [comline after comline or bobp] ...
| Before checking condition [comline after comline or bobp] ...
| Entering codeline after codeline ....
| Entering while-loop ... [2 times]
| Before sleep [comline after comline or bobp] ...
| Before checking condition [comline after comline or bobp] ...
| Entering codeline after codeline ....
`--------------------------------------------------------------


from this evidence, I would conclude that slowing down the program by 1
second makes it partly behaving correct, but *really* slowing it down by
single-stepping in the debugger (no matter if debug or edebug) makes it
behave correctly. 

Not really what I would expect...

-- 
cheers,
Thorsten




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