bug#34769: Bug in emacs 26.1 gdb-send match-string applied after non-matching string-match

bug#34769: Bug in emacs 26.1 gdb-send match-string applied after non-matching string-match

[Tobias Zawada]

Dear Emacs maintainers,

At the end of gdb-send one finds the following lines:

  (let* ((control-command-p (string-match gdb-control-commands-regexp string))
         (command-arg (match-string 3 string))

It is wrong to call (match-string 3 string) if (string-match gdb-control-commands-regexp string) returned nil.
The doc of match-string says:
"Return string of text matched by last search."
That means match-string only returns sensible results if the last match was successful.

Possible correction:

  (let* ((control-command-p (string-match gdb-control-commands-regexp string))
         (command-arg (and control-command-p (match-string 3 string)))


System info:
GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.30) of 2018-05-29

Best regards,
Tobias

bug#34769: Bug in emacs 26.1 gdb-send match-string applied after non-matching string-match

[Eli Zaretskii]

> Date: Wed, 6 Mar 2019 12:27:03 +0100 (CET)
> From: Tobias Zawada <i_inbox@tn-home.de>
> 
> At the end of gdb-send one finds the following lines:
> 
>   (let* ((control-command-p (string-match gdb-control-commands-regexp string))
>          (command-arg (match-string 3 string))
> 
> It is wrong to call (match-string 3 string) if (string-match gdb-control-commands-regexp string) returned nil.
> The doc of match-string says:
> "Return string of text matched by last search."
> That means match-string only returns sensible results if the last match was successful.

That is true, and a cleanup is a good idea.  But please note that the
undefined results are not used if control-command-p is nil, so the
issue is not that serious.

Thanks.

bug#34769: Bug in emacs 26.1 gdb-send match-string applied after non-matching string-match

[Tobias Zawada]

Dear Eli,
thanks for looking into this.

> > It is wrong to call (match-string 3 string) if (string-match gdb-control-commands-regexp string) returned nil.
> ...
> That is true, and a cleanup is a good idea. But please note that the
> undefined results are not used if control-command-p is nil, so the
> issue is not that serious.

The match data is that one of the previous successful matching operation.
One gets an args-out-of-range signal if the indexes in the match data are larger than the width of the string argument to match-string. That is how I discovered the error.

A reconstruction with gdb-send is too involved.
Instead I demonstrate the effect with the following orgmode source code block inclusive results:

#+BEGIN_SRC emacs-lisp
(string-match "\\(1\\) *\\(2\\)" "1          2")
(string-match "\\(3 \\(4\\)\\)" "1 2")
(append
 (match-data)
 (condition-case err
     (match-string 2 "1 2")
     (error (list err))))
#+END_SRC

#+RESULTS:
| 0 | 12 | 0 | 1 | 11 | 12 | (args-out-of-range 1 2 11 12) |

Best regards,
Tobias

bug#34769: Bug in emacs 26.1 gdb-send match-string applied after non-matching string-match

[Eli Zaretskii]

> Date: Wed, 6 Mar 2019 21:10:54 +0100 (CET)
> From: Tobias Zawada <i_inbox@tn-home.de>
> Cc: 34769@debbugs.gnu.org
> 
> Dear Eli,
> thanks for looking into this.
> 
> > > It is wrong to call (match-string 3 string) if (string-match gdb-control-commands-regexp string) returned nil.
> > ...
> > That is true, and a cleanup is a good idea. But please note that the
> > undefined results are not used if control-command-p is nil, so the
> > issue is not that serious.
> 
> The match data is that one of the previous successful matching operation.
> One gets an args-out-of-range signal if the indexes in the match data are larger than the width of the string argument to match-string. That is how I discovered the error.

You are right; I've now fixed this on the emacs-26 branch.