Re: [Gtk2hs-devel] threaded RTS and finalizers

[Axel S. <Axe...@en...>]

Dear Simon, Duncan,

-threaded now works with Gtk2Hs. See below.


On Tue, 2009-11-10 at 14:14 +0000, Simon Marlow wrote:
> On 10/11/2009 13:52, Axel Simon wrote:

> >> Does the traceback in my original message give you enough information?
> >> (reproduced at the end of this message)
> >
> > Well, you omitted 'various' stuff. I agree that something is calling
> > back into C when it shouldn't. Are there any function names between
> > frame 6 and 44 that would give me a clue where we've been negligent?
> 
> Ah yes, sorry.  Here's the full trace:
> 
> (gdb) where
> #0  0x0000003c0aa0af19 in pthread_cond_wait@@GLIBC_2.3.2 () from 
> /lib64/libpthread.so.0
> #1  0x00000000005ac6e9 in waitCondition ()
> #2  0x00000000005c67ab in waitForReturnCapability ()
> #3  0x00000000005a3f17 in rts_lock ()
> #4  0x00000000004ce397 in SystemziGlibziGObject_d1wV ()
> #5  0x0000003c10cd8eed in ?? () from /usr/lib64/libgtk-x11-2.0.so.0
> #6  0x0000003c0ba0d6a8 in g_object_unref () from /lib64/libgobject-2.0.so.0
> #7  0x0000003c10ccebc7 in gtk_tree_view_remove_column () from 

I understand now where this error is. There is a call in your program to
cellLayoutSetAttributes. This function installs certain callbacks that
Gtk+ tries to free in the stack trace above. The call stack above is
triggered by

> #41 0x0000003c0ba0d632 in g_object_unref ()
from /lib64/libgobject-2.0.so.0
> #42 0x00000000005abdfd in runAllCFinalizers ()

... and I suppose runAllCFinalizers is the GC and an 'unsafe' call from
Haskell. Thus, it is actually only possible  to use the C function
freeHaskellPtr. I do that now.


>    (3) use Haskell finalizers with idleAdd as a temporary workaround.
> 
> (3) is by far the easiest solution, but it may have unacceptable 
> performance, I'd interested to know if that's the case.  The runtime 
> does batch multiple finalizers so they don't each get a separate Haskell 
> thread, so it might not be too bad.
> 
> I'm not completely averse to doing (1) if it's absolutely necessary, but 
> I'd like to do something that avoids having a dependency on a future 
> version of GHC.  And all things being equal, we should avoid adding new 
> features to GHC because they're hard to remove again.

So now there are two kinds of finalizers: 

(1) those that directly free the C GObject which are used in Cairo and
(in the future) Pango, Pixbuf, and all other libraries whose objects do
not call back into Haskell. 

(2) all finalizers of Gtk+ and libraries that build upon Gtk+
(sourceview, etc). These call a special finalizer that enqueues the
object to be finalized into an array and install a an g_idle_add handler
that will run the finalizers from the main iteration loop of Gtk+ and,
thus, from the right thread and from a safe call.

All objects in (2) can therefore call back to Haskell while they are
being destroyed. I have therefore moved the possibility to add a WeakRef
(a function that is called when a GObject is finalized) to from GObject
to Graphics.UI.Gtk.Abstract.Object which means they can only be
installed on objects that fall into category (2).

So the darcs version of Gtk2Hs with -threaded should now work as
expected. You're still responsible to call Gtk2Hs function from the same
Haskell thread. But you can create Cairo drawings and Pagno stuff in
different threads and pass them to the main thread.

Cheers,
Axel.