From: Olivier Andrieu <oandrieu at nerim.net>
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Message-ID: <16234.5256.398860.836871 at karryall.dnsalias.org>
Date: Thu, 18 Sep 2003 22:24:40 +0200
To: Richard Jones <rich at annexia.org>
Cc: lablgtk at kaba.or.jp
Subject: Re: Exception handling bug in GMain.Main (was: Re: Trapping exceptions at the top level? (repost))
In-Reply-To: <20030918182103.GA26041 at redhat.com>
References: <20030912101328J.garrigue at kurims.kyoto-u.ac.jp>
	<20030918182103.GA26041@redhat.com>

 Richard Jones [Thursday 18 September 2003] :
 >
 > On Fri, Sep 12, 2003 at 10:13:28AM +0900, Jacques Garrigue wrote:
 > > [This mail was sent by Richard Jones <rich@annexia.org>]
 > > let rec run_main () =
 > >   try
 > >     GMain.Main.main ()
 > >   with
 > >     Sys_error (...) ->
 > >       (* Some code to post a message_box describing this error. *);
 > >       run_main ()
 > 
 > I actually tried to implement this, but I've hit a Gtk/lablgtk bug.
 > Attached is the minimal example I made to reproduce the bug.

Mind that even without the lablgtk bug, this wouldn't work : you're
calling GMain.main () many times, but you're only calling GMain.quit
twice.

 > Crash:
 > 
 > GLib-GObject-ERROR **: file gsignal.c: line 646 (emission_pop):
 > should not be reached aborting...
 > Aborted

Ouch. I do not get the message, only a segfault. Anyway ...

 > It looks like GMain.Main.main can't safely have exceptions passing
 > through it.

Yeah, the stacktrace under gdb is a mess : all the Gtkbutton signal
are stuck in the middle of their invocation but the caml program has
jumped to a completely different part of the program (with a new main
loop etc.)

I'm not sure it's safe to let exceptions escape the callbacks ever.  A
simple solution is to modify GtkSignal to trap all exceptions raised
by callbacks and run a user-exception handler (like yours that display
a message box). That way the main loop is never exited because of a jump
due to an exception.

I tried it, it works quite well. Here's how it looks :

,----
| let display_error title msg =
|   let icon = GMisc.image () in
|   icon#set_stock `DIALOG_ERROR;
|   icon#set_icon_size `DIALOG;
|   GToolbox.message_box ~title ~icon msg
| 
| let my_exn_handler = function
|   | Unix_error (err, syscall, filename) ->
|       display_error
| 	"Filesystem error"
| 	(syscall ^ ": " ^ filename ^ ": " ^ error_message err)
|   | Sys_error err ->
|       display_error
| 	"System error"
| 	err
|     | Failure err ->
| 	display_error
| 	  "Internal error"
| 	  ("You probably found a bug in this software:\n\n" ^ err)
|     | exn -> ()
| 
| let _ = GtkSignal.user_exn_handler := my_exn_handler
| 
| let () = 
|   (* rest of the program ... *)
`----

-- 
   Olivier