[java-gnome-hackers] Symol issues with GtkBuilder

[Andrew C. <an...@op...>]

I had a shot at implementing GtkBuilder support in java-gnome. The
public API was easy and was done in a flash. But it doesn't work.

Exception in thread "main" java.text.ParseException: Invalid object type `GtkLabel'
	at org.gnome.gtk.Builder.addFromFile(Builder.java:107)
	at Designer.setupUserInterface(Designer.java:46)
	at Designer.main(Designer.java:80)

Invalid object type? huh?

++

I have copied in the logs of a long conversation I had on #gtk+ today
with Tristan van Berkom, Benjamin Otte, and Johan Dahlin. I'd ask you to
read through it. The end  result appears to be that because java-gnome
reaches GTK via a shared library and not by virtue of being an
executable itself, GtkBuilder won't work as is.

There are two options:
        
        1) implement a vFunc (by C side subclassing GtkBuilder, I think)
        to do ... something
        
        2) manually call dlopen() with RTLD_GLOBAL which does ...
        something

The fact that I was able to hack (2) to work is not interesting. What's
the cost of it? I assume switching it to global would mean an insane
growth in the runtime size of the symbol table. Is that bad? I assume
so. How do we measure it? No idea.

Also, if we did (2) then we'd have to manually list every system library
name in C code. That's fragile, to say the least.

On the other hand, it's not like our symbol table isn't huge already
care of JNIEXPORT [which is the reason to investigate JNI's
RegisterNatives, but that's another story]. But this wouldn't read-only
sharable data space in a .so, right? We want to shrink our memory
footprint, not grow it!

Johan said that the vFunc was just for this sort of occasion. Ok, so
presumably (1) is the better thing to do, but can anyone figure out from
this conversation what, exactly, we're supposed to do there?

My branch is at 'hackers/andrew/builder'. It's a 4.0 branch. With the
patch below it "works", but we can't use that patch as is.

Slightly edited IRC log follows. As you'll read, on quite a number of
occasions I point out that I really don't know enough about linking to
be able to properly assess what is (or isn't) going on. 

Thanks to Tristan, Benjamin, and Johan for their help getting us this
far! Now, my friendly little java-gnome hackers, I need yours...

AfC
Sydney


++

AfC: Hm. I wonder why GtkBuilder would emit "Invalid object type
`GtkLabel'" when parsing a .ui file?

Company: AfC: not yet called gtk_init() or so?
Company: AfC: the gtk_label_get_type() function must have been called
once for the type to be registered

AfC: Company: no, gtk_init() etc called already
AfC: Company: er
AfC: what?
AfC: you're kidding.

Company: no, i'm not
Company: but i'm pretty sure there is some function that does that

AfC: You mean someone has to manually call gtk_label_new() and
gtk_foobar_new() and gtk_everything_else_new() before they can use
GtkBuilder?

Company: AfC: no

tristan: AfC, you you doing this in C ?

Company: AfC: i mean someone has to call gtk_label_get_type()

tristan: AfC, there is a function that creates "gtk_label_get_type" from
"GtkLabel"

AfC: tristan: no, I'm finally getting around to porting our libglade
coverage in java-gnome to GtkBuilder instead. It was easy enough, except
that I've hit this

tristan: it's used along with g_module_symbol() to initialize types

Company: ahhh
Company: more magical than i'd have thought

tristan: AfC, I suspect... not sure... but suspect that the binding has
problems getting the type from name

AfC: bloody hell. Do you have any idea how much we have *not* exposed
the _get_type() functions? They're meaningless in a language binding. 

tristan: I think bindings are supposed to override that part of the
builder
tristan: I mean... do you want builder to create a raw GtkLabel ? or do
you want it to create some java-gnome wrapper object ?
tristan: I suspect that in gtkmm they have some derived code that
returns a wrapper
***tristan checks with builder how thats done for bindings... I dont
recall

Company: i'd expect you let the builder create the label

tristan: AfC, GtkBuilderClass.get_type_from_name()
Company: and then wrap it

AfC: tristan: the proxy object (to use Owen's terminology) is already
fully engineered, that works fine [and worked fine with libglade].
AfC: tristan: we're stuck at add_from_file

tristan: Company, you would expect that to be done where then ?

Company: tristan: in get_object()

AfC: Company: that's what I'd expect, and that's what we're doing

tristan: AfC, I dont know how it worked fine in the past and how it
worked with libglade.

AfC: Company: we're not even at get_object() yet. Given any pointer we
can ... call g_type_name() on ... we can construct our proxy, no problem

Company: AfC: as tristan says, that's not enough

AfC: Company: but I can't understand why gtk_builder_add_from_file()
can't hack it when glade_xml_parse() managed it

AfC: Well.

tristan: different languages seem to have totally different binding sets
tristan: for instance, I'm not sure that when you create a derived
widget with gtkmm, that it even registers a GType for the new class
tristan: when you create an object with python... it makes a GType
tristan: so they are bidirectional
tristan: in a sense

Company: AfC: glade called all gtk get_type() functions afair
Company: AfC: was always tricky when gtk added new widgets and libglade
hadn't been updated yet

tristan: Company, would it be enough to do it in get_object() ?
tristan: if I go and fetch a window or box... and then itterate it's
children

AfC: tristan: no we don't do anything like that. Never needed to. As far
as C is concerned it's just a GtkWhatever. If the developer has created
a Java side subclass that's their business. Things Just Work™. But
that's not the problem here
tristan: do I get widgets as children or wrappers ?

Company: tristan: you get wrappers

AfC: Company: ... called all gtk_get_type_ functions. Oh. Yes, well,
that'd do it

Company: AfC: so why does gtkbuilder not find the existing C function
gtk_label_get_type() when trying to g_module_symbol() it?

tristan: AfC, they *kindof* work then... if you cannot get the derived
class details in GType terms in C... then ... it's impossible for me to
make a useful java-gnome plugin for Glade

Company: AfC: in java-gnome i mean

AfC: Company: hm?

tristan: because I cannot read any properties or signals that the
java-gnome object may have added

AfC: Company: Code is like this:
AfC: Gtk.init()
AfC: builder = new Builder();
AfC: builder.addFromFile("simple.ui");
AfC: crash
AfC: (well, GError → Exception → terminate)

Company: AfC: yes, because java-gnome is doing bad things
Company: AfC: because g_module_symbol(null_module, "gtk_label_get_type")
fails
Company: AfC: and that's java-gnome's fault somehow

AfC: Company: sorry, what is that?

Company: AfC: that is C

AfC: Company: look I know you guys don't give a shit about language
bindings, but maybe we can leave off the "fault" stuff and assume that
the project means well, and has done what was asked of it over the
years?
AfC: So if there's a new initialization pattern that needs to be
supported, fine

Company: AfC: there isn't
Company: AfC: there's just a new way to look up class names
Company: AfC: and it's done by inspecting the running code for an
exported C function

AfC: Company: so you're saying we instantiated the GtkBuilder object
somehow wrong?

tristan: AfC, I do give a shit, and I also wish I could introspect more
from language-binding created classes... I dont see exactly what could
be going wrong

AfC: tristan: no, me neither;

tristan: if your code is running with libgtk+, then "gtk_label_get_type"
MUST be there
tristan: no reason for the C code calling g_module_symbol to fail.

AfC: tristan: what I wrote above is == to this C code, right?

Company: AfC: no, i'm saying that a java-gnome application looks
different to C code than a C application

AfC: gtk_init()
AfC: gtk_builder_new()
AfC: gtk_builder_add_from_file()

tristan: AfC, yes that should really definitely work

AfC: well, that's the sequence of C calls we're making. :(

Company: AfC: no, deep down it's not
Company: AfC: are you linking your java app with gcc?
Company: AfC: is it doing the equivalent of gcc -shared -lgtk-3 ?

AfC: Company: This is GTK 2, but yes
AfC: LINK=/usr/bin/gcc-4.4 -shared + pkg-config --libs

tristan: right, I suppose there could be low-level trickery, that some
obscure calls to ldd/nm might reveal

Company: AfC: so if you run ldd on the generated binary, it will list
libgtk just like when running it on /usr/bin/gtk-demo ?

AfC: Company: there's no binary. There's a shared library loaded at
runtime, but I will double check

Company: AfC: aha!

tristan: i.e. what is the actual symbol name... but if it's linking to a
real libgtk+ lib... I still dont see why there would be no "T
gtk_label_get_type" in there

Company: AfC: shared libraries loaded at runtime may be loaded with
hidden symbols
Company: AfC: so that g_module_symbol() would not find the symbol

AfC: Company: um, ok, that's interesting
AfC: notes that the rest of the GNOME stack works fine, and has for 9
years

Company: yes

tristan: if it obfuscates the symbol name or such, you can reverse the
process by implementing GtkBuilderClass.get_type_from_name()

Company: the rest of the gnome stack doesn't have to resolve C symbols
at runtime

AfC: if [what] obfuscates?
AfC: This is very interesting

tristan: whatever obscure linking method you might use

Company: tristan: it's dlopen()ing libgtk with RTLD_LOCAL

AfC: Um, well, it's doing whatever pkg-config tells it to do

tristan: Company, that means you can find nothing in the global
namespace ?

Company: tristan: actually, it's not dlopen()ing libgtk but
libjava-gnome-plugin.so
Company: tristan: yes

AfC: yes
AfC: (where it is the Java VM process, yes)

tristan: right, so "whoever" is dlopening it.. has access to those
symbols, there is a handle somewhere
tristan: that handle needs to be used in
GtkBuilder->get_type_from_name()
tristan: to get the actual type

AfC: tristan, Company: thanks for your help. I'll be honest and say I
don't really understand what needs to be done;
AfC: tristan, Company: sounds like its not a matter of a different
linker flag but I certainly don't have access to the dlopen call being
made by the VM [nor would any language binding, I imagine]
AfC: tristan, Company: unless it's $something I can do before calling
[say] gtk_init()

Company: AfC: i'd suspect python or other languages have the same
problem
Company: JS, whatever

tristan: AfC, short answer/explanation is that a language binding needs
to know how to get the actual GType from a type name before the type is
actually registered

Company: unless they don't use RTLD_LOCAL
Company: AfC: and as tristan says, you can overwrite the lookup function

AfC: [I don't know that the JVM is, fwiw]

tristan: python works find with GtkBuilder, they create real GObjects
afaik also

AfC: tristan: isn't that circular? g_type_from_name() only works after a
type has been registered, right?
AfC: Hey, if someone can point us at the call that eg Python is making
before they call gtk_init() that magically fixes the problem, then I'll
get it added right away.

tristan: AfC, no it's not circular, I said a binding needs to make that
association *before* the type is registered

AfC: tristan: ah
AfC: tristan: well as it happens we do have such a list

tristan: it's simple: GTK+ provides standard naming convention, and all
the _get_type() stubs are there
tristan: just have to mash up the string a bit and use the _get_type()
to create it

AfC: tristan: so you're saying we have to invoke every _get_type()
possible before attempting to use GtkBuilder.

tristan: GTK+ already does the string mashing part, you can copy that
code pretty easily
tristan: AfC, no... you have to call _get_type() as a result of
GtkBuilder->get_type_from_name()
tristan: in your language binding's wrapper of GtkBuilder... you
override the ->get_type_from_name() vfunc
tristan: and make it do something language binding custom

AfC: this is where I'm lost. There's nothing custom
AfC: we just have a GtkBuilder*

tristan: AfC, that was an ugly issue with libglade... you actually had
to register each type to use them
tristan: so a.) libglade needed updates for new types... and b.)
applications needed to provide modules to load their own custom types
tristan: AfC, you use GtkBuilder directly from java ??
tristan: or you have something in between, right ?
tristan: the in between should use CustomBuilder instead of GtkBuilder
directly
tristan: with ->get_type_from_name() overridden

AfC: We're verging off-topic for #gtk+ and I don't want to further
clutter up the channel when important bug fixing is going on.
AfC: tristan: I was hoping to have GtkBuilder coverage in our 4.1
release to replace the removed libglade, but it sounds like we'll have
to dig a bit harder.
AfC: appreciate your help

tristan: I'll be around, sorry to walk out on you

AfC: tristan: nah, that's cool


(12:19:43) 

jdahlin: AfC: libglade has a huge table which maps widget name to GType,
we wanted to avoid that when rewriting libglade into GtkBuilder

AfC: jdahlin: I see

jdahlin: and as Company pointed out, that caused problems when a new
widget was added in gtk+ but not in that table

AfC: jdahlin: sure, I grok that

jdahlin: so dlsym(NULL, "gtk_widget_get_type") needs to work, not sure
why that isn't working for java-gnome

AfC: jdahlin: yeah. I don't really understand why a shared linking to
GTK is different than an executable linking to GTK [with respect to
however this works in a normal C GTK program] (which is Company's
hypothesis)

jdahlin: AfC: executable linking is done at compile time, while
dlopen/dlsym is runtime
jdahlin: AfC: GtkBuilder creates a bit of problem for C programs as
well, it uses dlsym to find out callbacks specific in the .ui file
jdahlin: so for C programs you need to link with -rdynamic
jdahlin: is java-gnome using jna or jni?

AfC: jdahlin: JNI

tristan|afk: jdahlin, I think he means that... java-gnome being a shared
lib is linking to GTK+... I *think*... that the java-gnome lib is
dlopened by whatever is running this

AfC: tristan|afk: that's correct

jdahlin: AfC: the main executable is the jvm (/usr/bin/java or
whatever), it loads in libjava-gnome.so via dlopen

AfC: So if there's something I have to add to the linker invocation to
build the .so, no problem

jdahlin: for GtkBuilder to work you have to make all *_get_type symbols
accessible in the main namespace for jvm
jdahlin: that's usually accomplished by passing in RTLD_GLOBAL to dlopen

tristan|afk: if I'm guessing Company's hypothesis right... maybe what is
opening java-gnome is doing so without making the symbols global
tristan|afk: so when that code actually ends up running dlsym() with
NULL... it finds nothing

jdahlin: how is libjava-gnome.so linked against libgtk.so?

tristan|afk: jdahlin, or... it can be done by overriding
->get_type_from_name() inside java-gnome... I think, no ?

AfC: If so, that's going to be hard to beat, because I have zero control
over that. The only workaround would be to cause the VM process to load
a *separate* library first, and call dlopen() on libgtk-x11-2.0.so.0

jdahlin: tristan|afk: that would work as well

AfC: jdahlin: well, let's see

jdahlin: AfC: do you know how the VM loads JNI modules?

AfC: jdahlin: no
AfC: jdahlin: more to the point, I have zero influence over it.
AfC: I mean, it's gotta be dlopen() and dlsym(), but with what flags? No
idea
AfC: jdahlin: LINK=/usr/bin/gcc-4.4 -g -shared -Wall -fPIC
AfC: and
AfC: jdahlin: $LINK -o tmp/libgtkjni-4.0.20-dev.so $objects `pkg-config
--libs $modules`
AfC: [sic]

jdahlin: okay, you could fix it up yourself, either by overriding
get_type_from_name as tristan mentioned or just doing something like
dlopen("libgtk.so", RTLD_GLOBAL) in JNI's module initialization function

AfC: jdahlin: I can certainly do that.
AfC: That's what suggested to Company
AfC: though I wasn't sure if that would do anything if invoked from
a .so that is already linked against libgtk-x11-2.0.so.0

jdahlin: it's a bit ugly using RTLD_GLOBAL though, you're polluting the
namespaces
jdahlin: it'll hopefully do the right thing
jdahlin: the same needs to be done for all shared libraries with
*_get_type functions that java-gnome wants to support, it's not just gtk
jdahlin: but gtk+ is arguable the most important one

AfC: As for "a bit ugly", that's what I'm blown away by all this. I've
tried to follow this conversation, but fundamentally I just don't
understand why it works for a C program [without "polluting"] and not
a .so
AfC: anyway, trying my best

jdahlin: AfC: because of a number of reasons, a C program is linked at
compile time and all the functions its using are known
jdahlin: eg, it knows that gtk_label_get_type() is going to be used, but
not gtk_button_get_type() etc
jdahlin: you can check which functions are linked in by running ldd on
the executable
jdahlin: now, dynamic languages uses dlopen() which can specific, make
all functions available or none at all
jdahlin: the JNI authors decided the latter should be the default for
some reason, and it doesn't seem (by 2 minutes googling) that it's
possible to change that

AfC: [ldd + executable = functions? What option is that?]

jdahlin: or if it's objdump/nm, haven't looked at it for a while
jdahlin: objdump -T executable seems to do it here on my system

AfC: ah
AfC: Well
AfC: I added this before the call to gtk_init():

=== modified file 'src/bindings/org/gnome/gtk/GtkMain.c'
--- src/bindings/org/gnome/gtk/GtkMain.c	2010-02-14 20:14:38 +0000
+++ src/bindings/org/gnome/gtk/GtkMain.c	2011-04-08 02:51:05 +0000
@@ -31,6 +31,7 @@
  * wish to do so, delete this exception statement from your version.
  */
 
+#include <dlfcn.h>
 #include <jni.h>
 #include <glib.h>
 #include <gdk/gdk.h>
@@ -56,6 +57,7 @@
 	jobjectArray _args
 )
 {
+	void* handle;
 	int argc;
 	char** argv;
 	gint i;
@@ -90,6 +92,12 @@
 	argv[0] = "";
 	argc++;
 
+	handle = dlopen("libgtk-x11-2.0.so", RTLD_NOW | RTLD_GLOBAL);
+	if (handle == NULL) {
+		bindings_java_throw(env, "dlopen() failed: %s", dlerror());
+		return;
+	}
+
 	// call function
 	gtk_init(&argc, &argv);
 


AfC: and now GtkBuilder works

jdahlin: great

AfC: yeah. But is it great? :)

jdahlin: great that it's working

AfC: Certainly lends strength to Company's hypothesis, that's for sure

jdahlin: is it inconvenient for you to override the GtkBuilder vfunc
get_type_from_name?

AfC: jdahlin: "no", but I'm not sure what I'd be overriding it with?

jdahlin: AfC: implementing it!
jdahlin: you need to subclass it and override it

AfC: "This is mainly used when implementing the GtkBuildable interface
on a type."

jdahlin: sure, but we're outside of "mainly" right now
jdahlin: I added it specifically for these kind of situations
jdahlin: s/added/made it overridable/
jdahlin: gtkmm uses it as well if iirc

AfC: Oh? Ok. I should go look at their code, then

jdahlin: not really
jdahlin: all gtkmm classes are subclasses, and they implement it to map
GtkLabel to their overridden GType
jdahlin: http://git.gnome.org/browse/gtkmm/tree/gtk/src/builder.ccg

AfC: jdahlin: yeah, I'm reading that right now
AfC: All they do is call g_type_from_name() ?

jdahlin: gtkmm is considerably different from a dynamic binding, so you
shouldn't worry too much

AfC: um generally we're mostly the same; Java is a static language too

jdahlin: no, it's not really static

AfC: [except for this business of getting to GTK through a .so rather
than a linked executable]

jdahlin: in the sense that language bindings are loaded via dynamically

AfC: Sure. I means static as in static language API, not a dynamic
language API
AfC: So I assume that GtkBuilder just calls g_type_from_name() itself
internally.

jdahlin: not really no
jdahlin: but it works fine for gtkmm which calls all get_type functions
when the library loads

AfC: Ah
AfC: No, we definitely don't do that. :)
AfC: We could, I guess. Sounds expensive.

jdahlin: maybe it doesn't, but their impl. requires you to call the
_get_type function of a class before using it in gtkbuilder

AfC: which is why g_type_from_name() works
AfC: right
AfC: hm
AfC: Embedding system library names in the source code to pass to
dlopen() is going to be fragile.
AfC: I wondered if -rdynamic to the linker would have the same effect,
but no

jdahlin: kind of fragile yes, but the alternatives are wors
jdahlin: e

AfC: jdahlin: that seems pretty nuts of GTKmm; if I do get_object() on
something I think is be a GtkButton and it turns out to be a
GtkCheckButton (say) it'll crash
AfC: [actually, it won't get past add_from_file() which is my problem
right now]
jdahlin: dunno about the details, you need to check with the gtkmm
authors if you really want to know
AfC: jdahlin: yeah, I'll talk to Murray next time I see him
***jdahlin -> zzz
AfC: jdahlin: do you have an opinion about the "expense" of dlopen(... ,
| RTLD_GLOBAL)?
AfC: jdahlin: ah, g'night. Thanks for your help