Re: MI: "^running" issues

[Jim Ingham <jingham@apple.com>]

On Sep 6, 2007, at 11:07 AM, Vladimir Prus wrote:

> Jim Ingham wrote:
>
>> We got the asynchronous mode working for the Mac OS X target in the
>> Apple gdb sources.  It definitely took a bunch of work beyond what is
>> in the current FSF sources to get all the details working (things  
>> like
>> breakpoint commands that restart the target and command files and  
>> some
>> other bits like that needed attending to...)  It wasn't deadly hard  
>> to
>> do, though.
>>
>> We use it mostly so that Xcode can query the target's state at any
>> point - basically a backstop when it looks like maybe the UI and gdb
>> have gotten out of sync.  That way you can always unambiguously get
>> the tri-state answer - the target's stopped; the target's running;
>> gdb's gone south.  This is actually pretty useful to have around,
>> though in a perfect world would not be necessary...
>
> I see. So, the problem is that in real world those "^running" and
> "*stopped" are not necessary always output when needed, so you can
> get out of sync?

Those are always bugs, and we fix them when we find them, but yes that  
did happen - hasn't happened in a while though.  What happens more  
often now for us is that we rely a lot on calling functions to inspect  
opaque data types when the target stops.  This often goes bad - people  
right data inspectors that can deadlock for instance...  So it's  
useful to have some "resync with gdb" action  that check's gdb's state  
and does the right thing based on what it is.

>
>
>> We also use -exec-interrupt.  It's certainly true that you can  
>> achieve
>> the same thing by sending ^C to the target, but it's much more  
>> regular
>> to do everything you're doing with the target through the same  
>> control
>> channel.  And of course, this moves to gdb the knowledge of any
>> funniness with interrupting a running program - which is where it
>> belongs.
>
> Hmm, if you send ^C to gdb, then gdb can handle interrupting the  
> program
> just fine.

Sure, but you're still using two ways to talk to gdb.

>
>
>> Another stronger reason why async was originally done was the
>> experience of merging gdb with Tcl/Tk for the Insight debugger.  One
>> of the big reasons for the instability of that project is that when
>> the target is running, gdb is blocked, so if you want to service  
>> other
>> events - like window system events in the case of Insight - you've  
>> got
>> no good way to do that.  We ended up having to run a timer and try to
>> service events in the timer interrupt - a dubious practice at best.
>>
>> It's possible to run the interpreter in a separate thread, and let it
>> and gdb communicate through some side channel.  But most interpreters
>> have some mechanism for handling events, and it is much cleaner to
>> have gdb be just another event source.
>
> I presume this is only relevant when gdb is combined with some other
> code in the same process? Last time I've asked about this, I was
> told that it not supported, because gdb does fancy things with  
> signals,
> or something like that.

Yes, this would be to support an in-process interpreter.  But I don't  
know about problems with signals.  That part didn't cause Tcl any  
difficulty, but that was a long time ago too.

Jim