Re: help!

2014/1/22 Jan Wielemaker <[email protected]>

> Hi Carlo,
>
>
> On 01/21/2014 10:28 AM, Carlo Capelli wrote:
>
>> Hi Jan
>>
>> Sorry to bother you, but I was trying to understand why I can't catch
>> exceptions when calling on background thread, and stumbled upon a weird
>> behaviour of the standard - not Qt - console.
>>
>> Briefly, ?- help. hangs.
>>
>> carlo@ubuntu-carlo-64:~$ swipl
>> % .plrc compiled 0.07 sec, 1,574 clauses
>> Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 7.1.5-DIRTY)
>> Copyright (c) 1990-2013 University of Amsterdam, VU Amsterdam
>> SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
>> and you are welcome to redistribute it under certain conditions.
>> Please visit http://www.swi-prolog.org for details.
>>
>> For help, use ?- help(Topic). or ?- apropos(Word).
>>
>> ?- help.
>> ^C^C
>> [forced] Action (h for help) ? goals
>>   [23] pce_principal:send(@2182115/pui_toc, root(toc_folder('Manual',
>> manual, resource(manual), resource(book))))
>>   [20] pce_principal:new(_G6283, pui_toc)
>>   [19] Send-method on @pui_help_window/pui_manual:
>> pui_manual->table_of_contents
>>   [17] pce_principal:send(@pui_help_window/pui_manual, table_of_contents)
>>   [14] pce_principal:send(new(@pui_help_window/pui_manual, pui_manual),
>> open)
>>
>> [forced] Action (h for help) ?
>>
>> Note the double ^C...
>>
>
> I can't reproduce that.  Looks like a deadlock.  Note that xpce is not
> multi-threaded.  It has a big lock around all its operations to make it
> thread-safe.  There is in_pce_thread/1 to run asynchronously in the
> pce thread.
>
> If the default behaviour is as above, either your compilation is corrupted
> or you have something in your ~/.plrc that breaks this.
>
>
>  In Qt console, win_menu:help, called on background, hangs, and the main
>> thread got confused, hangs itself... It's a bit unfortunate, as this is
>> probably one of the first menu a casual user will try...
>>
>
> I submitted a patch for xpce to use in_pce_thread/1 for help/0 and quite
> a few others.  This might fix this.
>
>
Thanks Jan, going to try immediately...


>  Since I hope the problems are (at least loosely) related, I wonder if some
>> setting from C side could inhibit exceptions from working. Seems related
>> to
>> threading, since try {} catch {} works as expected in a simple setting
>> (let's say, from main).
>>
>> BTW, the reason ?- help. doesn't work get explained after two abort...
>>
>> [forced] Action (h for help) ? abort
>> ERROR: pce(object) `@pui_help_window' does not exist
>>     Exception: (6) online_help:help ? abort
>> % Execution Aborted
>> ?- help.
>>
>> SWI-Prolog [thread 1]: received fatal signal 11 (segv)
>> Stack trace labeled "crash":
>>    [0] crashHandler+0x36
>>    [1] dispatch_signal+0x44d
>>    [2] __restore_rt+(nil)
>>    [3] pthread_cond_signal+0xa
>> Annullato
>>
>
> Using ^C sends a Prolog signal.  These are handled synchronously at
> a safe point in the execution.  Some foreign code simply blocks and
> does not handle (Prolog) signals.  Now, if you hit ^C the second
> time before the first one was honored, it will say [forced] and
> call the interrupt handler directly from the C signal function
> rather than through the Prolog signal handler.  If you abort, you
> basically do a longjmp() out of the current environment, leaving
> the system in an unknown state.  Anything may happen, in particular
> if you break forcefully out of thread synchronization primitives
> because these are not signal-safe.
>
>         Cheers --- Jan
>
>
>
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