halting problem :: The Mirror

:: ~24 min read

This is a rough strawman I put together after various discussions I had during GUADEC 2023 on the future of the GObject type system, introspection, language bindings, and the GNOME application development platform. It's honestly a lot, and it probably looks more like word vomit than an actual proposal; but I think it can be used as a springboard for discussions towards a new direction for the lowest levels of the stack, considering the current resources and challenges. Again: it's a strawman, and as such it'll require further discussion with multiple stakeholders, so don't plan on anything written down here in its present form.

The GObject type system has been serving the GNOME community for more than 20 years. We have based an entire application development platform on top of the features it provides, and the rules that it enforces; we have integrated multiple programming languages on top of that, and in doing so, we expanded the scope of the GNOME platform in a myriad of directions. Unlike GTK, the GObject API hasn’t seen any major change since its introduction: aside from deprecations and little new functionality, the API is exactly the same today as it was when GLib 2.0 was released in March 2002. If you transported a GNOME developer from 2003 to 2023, they would have no problem understanding a newly written GObject class; though, they would likely appreciate the levels of boilerplate reduction, and the performance improvements that have been introduced over the years.

While having a stable API last this long is definitely a positive, it also imposes a burden on maintainers and users, because any change has to be weighted against the possibility of introducing unintended regressions in code that uses undefined, or undocumented, behaviour. There’s a lot of leeway when it comes to playing games with C, and GObject has dark corners everywhere.

The other burden is that any major change to a foundational library like GObject cascades across the entire platform. Releasing GLib 3.0 today would necessitate breaking API in the entirety of the GNOME stack and further beyond; it would require either a hard to execute “flag day”, or an impossibly long transition, reverberating across downstreams for years to come. Both solutions imply amounts of work that are simply not compatible with a volunteer-based project and ecosystem, especially the current one where volunteers of core components are now stretched thin across too many projects.

And yet, we are now at a cross-roads: our foundational code base has reached the point where recruiting new resources capable of affecting change on the project has become increasingly difficult; where any attempt at performance improvement is heavily counterbalanced by the high possibility of introducing world-breaking regressions; and where fixing the safety and ergonomics of idiomatic code requires unspooling twenty years of limitations inherent to the current design.

Something must be done if we want to improve the coding practices, the performance, and the safety of the platform without a complete rewrite.

The Mirror

‘Many things I can command the Mirror to reveal,’ she answered, ‘and to some I can show what they desire to see. But the Mirror also show things unbidden, and those are often stranger and more profitable than things we wish to behold. What you will see, if you leave the Mirror free to work, I cannot tell. For it shows things that were, and things that are, and things that yet may be. But which it is that he sees, even the wisest cannot always tell. Do you wish to look?’ — Lady Galadriel, “The Lords of the Rings”, Volume 1: The Fellowship of the Ring, Book 2: The Ring Goes South

In order to properly understand what we want to achieve, we need to understand the problem space that the type system is meant to solve, and the constraints upon which the type system was implemented. We do that by holding GObject up to Galadriel’s Mirror, and gazing into its surface.

Things that were

History became legend. Legend became myth. — Lady Galadriel, “The Lord of the Rings: The Fellowship of the Ring”

Before GObject there was GtkObject. It was a simpler time, it was a simpler stack. You added types only for the widgets and objects that related to the UI toolkit, and everything else was C89, with a touch of undefined behaviour, like calling function pointers with any number of arguments. Properties were “arguments”, likes were florps, and the timeline went sideways.

We had a class initialisation and an instance initialisation functions; properties were stored in a global hash table, but the property multiplexer pair of functions was stored on the type data instead of using the class structure. Types did not have private data: you only had keyed fields. No interfaces, only single inheritance. GtkObject was reference counted, and had an initially “floating” reference, to allow transparent ownership transfer from child to parent container when writing C code, and make the life of every language binding maintainer miserable in the process. There were weak references attached to an instance that worked by invoking a callback when the instance’s reference count reached zero. Signals operated exactly as they do today: large hash table of signal information, indexed by an integer.

None of this was thread safe. After all, GTK was not thread safe either, because X11 was not thread safe; and we’re talking about 1997: who even had hardware capable of multi-threading at the time? NPTL wasn’t even a thing, yet.

The introduction of GObject in 2001 changed some of the rules—mainly, around the idea of having dynamic types that could be loaded and unloaded in order to implement plugins. The basic design of the type system, after all, came from Beast, a plugin-heavy audio application, and it was extended to subsume the (mostly) static use cases of GTK. In order to support unloading, the class aspect of the type system was allowed to be cleaned up, but the type data had to be registered and never unloaded; in other words, once a type was registered, it was there forever.

Arguments” were renamed to properties, and were extended to include more than basic types, provide validations, and notify of changes; the overall design was still using a global hash table to store all the properties across all types. Properties were tied to the GObject type, but the property definition existed as a separate type hierarchy that was designed to validate values, but not manage fields inside a class. Signals were ported wholesale, with minimal changes mainly around the marshalling of values and abstracting closures.

The entire plan was to have GObject as one of the base classes at the root of a specific hierarchy, with all the required functionality for GTK to inherit from for its own GtkObject, while leaving open the possibility of creating other hierarchies, or even other roots with different functionality, for more lightweight objects.

These constraints were entirely intentional; the idea was to be able to port GTK to the new type system, and to an out of tree GLib, during the 1.3 development phase, and minimise the amount of changes necessary to make the transition work not just inside GTK, but inside of GNOME too.

Little by little, the entire GObject layer was ported towards thread safety in the only way that worked without breaking the type system: add global locks around everything; use read-write locks for the type data; lock the access and traversal of the property hash table and of the signals table. The only real world code bases that actively exercised multi-threading support were GStreamer and the GNOME VFS API that was mainly used by Nautilus.

With the 3.0 API, GTK dropped the GtkObject base type: the whole floating reference mechanism was moved to GObject, and a new type was introduced to provide the “initial” floating reference to derived types. Around the same time, a thread-safe version of weak references for GObject appeared as a separate API, which confused the matter even more.

Things that are

Darkness crept back into the forests of the world. Rumour grew of a Shadow in the East, whispers of a Nameless Fear. — Lady Galadriel, “The Lord of the Rings: The Fellowship of the Ring”

Let’s address the elephant in the room: it’s completely immaterial how many lines of code you have to deal with when creating a new type. It’s a one-off cost, and for most cases, it’s a matter of using the existing macros. The declaration and definition macros have the advantages of enforcing a series of best practices, and keep the code consistent across multiple projects. If you don’t want to deal with boilerplate when using C, you chose the wrong language to begin with. The existence of excessive API is mainly a requirement to allow other languages to integrate their type system with GObject’s own.

The dynamic part of the type system has gotten progressively less relevant. Yes: you can still create plugins, and those can register types; but classes are never unloaded, just like their type data. There is some attempt at enforcing an order of operations: you cannot just add an interface after a type has been instantiated any more; and while you can add properties and signals after class initialisation, it’s mainly a functionality reserved for specific language bindings to maintain backward compatibility.

Yes, defining properties is boring, and could probably be simplified, but the real cost is not in defining and installing a GParamSpec: it’s in writing the set and get property multiplexers, validating the values, boxing and unboxing the data, and dealing with the different property flags; none of those things can be wrapped in some fancy C preprocessor macros—unless you go into the weeds with X macros. The other, big cost of properties is their storage inside a separate, global, lock-happy hash table. The main use case of this functionality—adding entirely separate classes of properties with the same semantics as GObject properties, like style properties and child properties in GTK—has completely fallen out of favour, and for good reasons: it cannot be managed by generic code; it cannot be handled by documentation generators without prior knowledge; and, for the same reason, it cannot be introspected. Even calling these “properties” is kind of a misnomer: they are value validation objects that operate only when using the generic (and less performant) GObject accessor API, something that is constrained to things like UI definition files in GTK, or language bindings. If you use the C accessors for your own GObject type, you’ll have to implement validation yourself; and since idiomatic code will have the generic GObject accessors call the public C API of your type, you get twice the amount of validation for no reason whatsoever.

Signals have mostly been left alone, outside of performance improvements that were hard to achieve within the constraints of the existing implementation; the generic FFI-based closure turned out to be a net performance loss, and we’re trying to walk it back even for D-Bus, which was the main driver for it to land in the first place. Marshallers are now generated with a variadic arguments variant, to reduce the amount of boxing and unboxing of GValue containers. Still, there’s not much left to squeeze out of the old GSignal API.

The atomic nature of the reference counting can be a costly feature, especially for code bases that are by necessity single-threaded; the fact that the reference count field is part of the (somewhat) public API prevents fundamental refactorings, like switching to biased reference counting for faster operations on the same thread that created an instance. The lack of room on GObject also prevents storing the thread ID that owns the instance, which in turn prevents calling the GObjectClass.dispose() and GObjectClass.finalize() virtual functions on the right thread, and requires scheduling the destruction of an object on a separate main context, or locking the contents of an object at a further cost.

Things that yet may be

The quest stands upon the edge of a knife: stray but a little, and it will fail to the ruin of all. Yet hope remains, while the company is true. — Lady Galadriel, “The Lord of the Rings: The Fellowship of the Ring”

Over the years, we have been strictly focusing on GObject: speeding up its internals, figuring out ways to improve property registration and performance, adding new API and features to ensure it behaved more reliably. The type system has also been improved, mainly to streamline its use in idiomatic GObject code bases. Not everything worked: properties are still a problem; weak references and pointers are a mess, with two different API that interact badly with GObject; signals still exists on a completely separate plane; GObject is still wildly inefficient when it comes to locking.

The thesis of this strawman is that we reached the limits of backwards compatibility of GObject, and any attempt at improving it will inevitably lead to a more brittle code, rife with potential regressions. The typical answer, in this case, would be to bump the API/ABI of GObject, remove the mistakes of the past, and provide a new idiomatic approach. Sadly, doing so not only would require a level of resources we, as the GLib project stewards, cannot provide, but it would also completely break the entire ecosystem in a way that is not recoverable. Either nobody would port to the new GObject-3.0 API; or the various projects that depend on GObject would inevitably fracture, following whichever API version they can commit to; in the meantime, downstream distributors would suffer the worst effects of the shared platform we call “Linux”.

Between inaction and slow death, and action with catastrophic consequences, there’s the possibility of a third option: what if we stopped trying to emulate Java, and have a single “god” type?

Our type system is flexible enough to support partitioning various responsibilities, and we can defer complexity where it belongs: into faster moving dependencies, that have the benefit of being able to iterate and change at a much higher rate than the foundational library of the platform. What’s the point of shoving every possible feature into the base class, in order to cover ever increasingly complex use cases across multiple languages, when we can let consumers decide to opt into their own well-defined behaviours? What GObject ought to provide is a set of reliable types that can be combined in expressive ways, and that can be inspected by generic API.

A new, old base type

We already have a derivable type, called GTypeInstance. Typed instances don’t have any memory management: once instantiated, they can only be moved, or freed. All our objects already are typed instances, since GObject inherits from it. Contrary to the current common practices we should move towards using GTypeInstance for our types.

There’s a distinct lack of convenience API for defining typed instances, mostly derived from the fact that GTypeInstance is seen as a form of “escape hatch” for projects to use in order to avoid GObject. In practice, there’s nothing that prevents us from improving the convenience of creating new instantiatable/derivable types, especially if we start using them more often. The verbose API must still exist, to allow language bindings and introspection to handle this kind of types, but just like we made convenience macros for declaring and defining GObject types, we can provide macros for new typed instances, and for setting up a GValue table.

Optional functionality

Typed instances require a wrapper API to free their contents before calling g_type_free_instance(). Nothing prevents us from adding a GFinalizable interface that can be implemented by a GTypeInstance, though: interfaces exist at the type system level, and do not require GObject to work.

typedef struct {
  void (* finalize) (GFinalizable *self);
} GFinalizableInterface;

If a typed instance provides an implementation of GFinalizable, then g_type_free_instance() can free the contents of the instance by calling g_finalizable_finalize().

This interface is optional, in case your typed instance just contains simple values, like:

typedef struct {
  GTypeInstance parent;

  bool is_valid;
  double x1, y1;
  double x2, y2;
} Box;

and does not require deallocations outside of the instance block.

A similar interface can be introduced for cloning instances, allowing a copy operation alongside a move:

typedef struct {
  GClonable * (* clone) (GClonable *self);
} GClonable;

We could then introduce g_type_instance_clone() as a generic entry point that either used GClonable, or simply allocated a new instance and called memcpy() on it, using the size of the instance (and eventual private data) known to the type system.

The prior art for this kind of functionality exists in GIO, in the form of the GInitable and GAsyncInitable interfaces; unfortunately, those interfaces require GObject, and they depend on GCancellable and GAsyncResult objects, which prevent us from moving them into the lower level API.

Typed containers and life time management

The main functionality provided by GObject is garbage collection through reference counting: you acquire a (strong) reference when you need to access an instance, and release it when you don’t need the instance any more. If the reference you released was the last one, the instance gets finalized.

Of course, once you introduce strong references you open the door to a veritable bestiary of other type of references:

  • weak references, used to keep a “pointer” to the instance, and get a notification when the last reference drops
  • floating references, used as a C convenience to allow ownership transfer of newly constructed “child” objects to their “parent”
  • toggle references, used by language bindings that acquire a strong reference on an instance they wrap with a native object; when the toggle reference gets triggered it means that the last reference being held is the one on the native wrapper, and the wrapper can be dropped causing the instance to be finalized

All of these types of reference exist inside GObject, but since they were introduced over the years, they are bolted on top of the base class using the keyed data storage, which comes with its own costly locking and ordering; they are also managed through the finalisation code, which means there are re-entrancy issues or undefined ordering behaviours that routinely crop up over the years, especially when trying to optimise construction and destruction phases.

None of this complexity is, strictly speaking, necessary; we don’t care about an instance being reference counted: a “parent” object can move the memory of a “child” typed instance directly into its own code. What we care about is that, whenever other code interacts with ours, we can hand out a reference to that memory, so that ownership is maintained.

Other languages and standard libraries have the same concept:

These constructs are not part of a base class: they are wrappers around instances. This means you’re not handing out a reference to an instance: you are handing out a reference to a container, which holds the instance for you. The behaviour of the value is made explicit by the type system, not implicit to the type.

A simple implementation of a typed “reference counted” container would provide us with both strong and weak references:

typedef struct _GRc GRc;
typedef struct _GWeak GWeak;

GRc *g_rc_new (GType data_type, gpointer data);

GRc *g_rc_acquire (GRc *rc);
void g_rc_release (GRc *rc);

gpointer g_rc_get_data (GRc *rc);

GWeak *g_rc_downgrade (GRc *rc);
GRc *g_weak_upgrade (GWeak *weak);

bool g_weak_is_empty (GWeak *weak);
gpointer g_weak_get_data (GWeak *weak);

Alongside this type of containers, we could also have a specialisation for atomic reference counted containers; or pinned containers, which guarantee that an object is kept in the same memory location; or re-implement referenced containers inside each language binding, to ensure that the behaviour is tailored to the memory management of those languages.

Specialised types

Container types introduce the requirement of having the type system understand that an object can be the product of two types: the type of the container, and the type of the data. In order to allow properties, signals, and values to effectively provide introspection of this kind of container types we are going to need to introduce “specialised” types:

  • GRc exists as a “generic”, abstract type in the type system
  • any instance of GRc that contains a instance of type A gets a new type in the type system

A basic implementation would look like:

GRc *
g_rc_new (GType data_type, gpointer data)
  // Returns an existing GType if something else already
  // has registered the same GRc<T>
  GType rc_type =
    g_generic_type_register_static (G_TYPE_RC, data_type);

  // Instantiates GRc, but gives it the type of
  // GRc<T>; there is only the base GRc class
  // and instance initialization functions, as
  // GRc<T> is not a pure derived type
  GRc *res = (GRc *) g_type_create_instance (rc_type);
  res->data = data;

  return res;

Any instance of type GRc<A> satisfies the “is-a” relationship with GRc, but it is not a purely derived type:

GType rc_type =
  ((GTypeInstance *) rc)->g_class.g_type;
g_assert_true (g_type_is_a (rc_type, G_TYPE_RC));

The GRc<A> type does not have a different instance or class size, or its own class and instance initialisation functions; it’s still an instance of the GRc type, with a different GType. The GRc<A> type only exists at run time, as it is the result of the type instantiation; you cannot instantiate a plain GRc, or derive your type from GRc in order to create your own reference counted type, either:

GRc *rc = g_type_create_instance (G_TYPE_RC);

typedef GRc GtkWidget;

You can only use a GRc inside your own instance:

typedef struct {
  // GRc<GtkWidget>
  GRc *parent;
  // GRc<GtkWidget>
  GRc *first_child;
  // GRc<GtkWidget>
  GRc *next_sibling;

  // ...
} GtkWidgetPrivate;

Tuple types

Tuples are generic containers of N values, but right now we don’t have any way of formally declaring them into the type system. A hack is to use arrays of similarly typed values, but with the deprecation of GValueArray—which is a bad type that does not allow reference counting, and does not give you guarantees anyway—we only have C arrays and pointer types.

Registering a new tuple type would work like a generic type: a base GTuple abstract type as the “parent”, and a number of types:

typedef struct _GTuple GTuple;

GTuple *
g_tuple_new_int (size_t n_elements,
                 int elements[])
  GType tuple_type =
    g_tuple_type_register_static (G_TYPE_TUPLE, n_elements, G_TYPE_INT);

  GTuple *res = g_type_create_instance (tuple_type);
  for (size_t i = 0; i < n_elements; i++)
    g_tuple_add (res, elements[i]);

  return res;

We can also create specialised tuple types, like pairs:

typedef struct _GPair GPair;

GPair *
g_pair_new (GType this_type,
            GType that_type,

This would give use the ability to standardise our API around fundamental types, and reduce the amount of ad hoc container types that libraries have to define and bindings have to wrap with native constructs.

Sum types

Of course, once we start with specialised types, we end up with sum types:

typedef enum {
} ShapeKind;

typedef struct {
  GTypeInstance parent;

  ShapeKind kind;

  union {
    struct { Point origin; float side; };
    struct { Point origin; Size size; };
    struct { Point center; float radius; };
  } shape;
} Shape;

As of right now, discriminated unions don’t have any special handling in the type system: they are generally boxed types, or typed instances, but they require type-specific API to deal with the discriminator field and type. Since we have types for enumerations and instances, we can register them at the same time, and provide offsets for direct access:

g_sum_type_register_static (const char *name,
                            size_t class_size,
                            size_t instance_size,
                            GType tag_enum_type,
                            offset_t tag_field);

This way it’s possible to ask the type system for:

  • the offset of the tag in an instance, for direct access
  • all the possible values of the tag, by inspecting its GEnum type

From then on, we can easily build types like Option and Result:

typedef enum {
} GResultKind;

typedef struct {
  GTypeInstance parent;

  GResultKind type;
  union {
    GValue value;
    GError *error;
  } result;
} GResult;

// ...
g_sum_type_register_static ("GResult",
                            sizeof (GResultClass),
                            sizeof (GResult),
                            offsetof (GResult, type));

// ...
GResult *
g_result_new_boolean (gboolean value)
  GType res_type =
    g_generic_type_register_static (G_TYPE_RESULT,
  GResult *res =
    g_type_create_instance (res_type);
  g_value_set_boolean (&res->result.value, value);

  return res;

// ...
g_autoptr (GResult) result = obj_finish (task);
switch (g_result_get_kind (result)) {
  case G_RESULT_OK:
    g_print ("Result: %s\n",
      g_result_get_boolean (result)
        ? "true"
        : "false");

    g_printerr ("Error: %s\n",
      g_result_get_error_message (result));

// ...
g_autoptr (GResult) result =
  g_input_stream_read_bytes (stream);
if (g_result_is_error (result)) {
  // ...
} else {
  g_autoptr (GBytes) data = g_result_get_boxed (result);
  // ...

Consolidating GLib and GType

Having the type system in a separate shared library did make sense back when GLib was spun off from GTK; after all, GLib was mainly a set of convenient data types for a language that lacked a decent standard library. Additionally, not many C projects were interested in the type system, as it was perceived as a big chunk of functionality in an era where space was at a premium. These days, the smallest environment capable of running GLib code is plenty capable of running the GObject type system as well. The separation between GLib data types and the GObject type system has created data types that are not type safe, and work by copying data, by having run time defined destructor functions, or by storing pointers and assuming everything will be fine. This leads to code duplication between shared libraries, and prevents the use of GLib data types in the public API, lest the introspection information gets lost.

Moving the type system inside GLib would allow us to have properly typed generic container types, like a GVector replacing GArray, GPtrArray, GByteArray, as well as the deprecated GValueArray; or a GMap and a GSet, replacing GHashTable, GSequence, and GtkRBTree. Even the various list models could be assembled on top of these new types, and moved out of GTK.

Current consumers of GLib-only API would still have their basic C types, but if they don’t want to link against a slightly bigger shared library that includes GTypeInstance, GTypeInterface, and the newly added generic, tuple, and sum types, then they would probably be better served by projects like c-util instead.


Instead of bolting properties on top of GParamSpec, we can move their definition into the type system; after all, properties are a fundamental part of a type, so it does not make sense to bind them to the class instantiation. This would also remove the long-standing issue of properties being available for registration long after a class has been initialised; it would give us the chance to ship a utility for inspecting the type system to get all the meta-information on the hierarchy and generating introspection XML without having to compile a small binary.

If we move property registration to the type registration we can also finally move away from multiplexed accessors, and use direct instance field access where applicable:

GPropertyBuilder builder;

g_property_builder_init (&builder,
  G_TYPE_STRING, "name");
// Stop using flags, and use proper setters; since
// there's no use case for unsetting the readability
// flag, we don't even need a boolean argument
g_property_builder_set_readwrite (&builder);
// The offset is used for read and write access...
g_property_builder_set_private_offset (&builder,
  offsetof (GtkWidgetPrivate, name));
// ... unless an accessor function is provided; in
// this case we want setting a property to go through
// a function
g_property_builder_set_setter_func (&builder,

// Register the property into the type; we return the
// offset of the property into the type node, so we can
// access the property definition with a fast look up
properties[NAME] =
  g_type_add_instance_property (type,
    g_property_builder_end (&builder));

Accessing the property information would then be a case of looking into the type system under a single reader lock, instead of traversing all properties in a glorified globally locked hash table.

Once we have a property registered in the type system, accessing it is a matter of calling API on the GProperty object:

gtk_widget_set_name (GtkWidget *widget,
                     const char *name)
  GProperty *prop =
    g_type_get_instance_property (GTK_TYPE_WIDGET,

  g_property_set (prop, name);


Moving signal registration into the type system would allow us to subsume the global locking into the type locks; it would also give us the chance to simplify some of the complexity for re-emission and hooks:

GSignalBuilder builder;

g_signal_builder_init (&builder, "insert-text");
g_signal_builder_set_args (&builder, 3,
  (GSignalArg[]) {
    { .name = "text", .gtype = G_TYPE_STRING },
    { .name = "length", .gtype = G_TYPE_SIZE },
    { .name = "position", .gtype = G_TYPE_OFFSET },
g_signal_builder_set_retval (&builder,
g_signal_builder_set_class_offset (&builder,
  offsetof (EditableClass, insert_text));

signals[INSERT_TEXT] =
  g_type_add_class_signal (type,
    g_signal_builder_end (&builder));

By taking the chance of moving signals out of the their own namespace we can also move to a model where each class is responsible for providing the API necessary to connect and emit signals, as well as providing callback types for each signal. This would allow us to increase type safety, and reduce the reliance on generic API:

typedef offset_t (* EditableInsertText) (Editable *self,
                                         const char *text,
                                         size_t length,
                                         offset_t position);

unsigned long
editable_connect_insert_text (Editable *self,
                              EditableInsertText callback,
                              gpointer user_data,
                              GSignalFlags flags);

editable_emit_insert_text (Editable *self,
                           const char *text,
                           size_t length,
                           offset_t position);

Extending the type system

Some of the metadata necessary to provide properly typed properties and signals is missing from the type system. For instance, by design, there is no type representing a uint16_t; we are supposed to create a GParamSpec to validate the value of a G_TYPE_INT in order to fit in the 16bit range. Of course, this leads to excessive run time validation, and relies on C’s own promotion rules for variadic arguments; it also does not work for signals, as those do not use GParamSpec. More importantly, though, the missing connection between C types and GTypes prevents gathering proper introspection information for properties and signal arguments: if we only have the GType we cannot generate the full metadata that can be used by documentation and language bindings, unless we’re willing to lose specificity.

Not only the type system should be sufficient to contain all the standard C types that are now available, we also need the type system to provide us with enough information to be able to serialise those types into the introspection data, if we want to be able to generate code like signal API, type safe bindings, or accurate documentation for properties and signal handlers.


Introspection exists outside of GObject mainly because of dependencies; the parser, abstract syntax tree, and transformers are written in Python and interface with a low level C tokeniser. Adding a CPython dependency to GObject is too much of a stretch, especially when it comes to bootstrapping a system. While we could keep the dependency optional, and allow building GObject without support for introspection, keeping the code separate is a simpler solution.

Nevertheless, GObject should not ignore introspection. The current reflection API inside GObject should generate data that is compatible with the libgirepository API and with its GIR parser. Currently, gobject-introspection is tasked with generating a small C executable, compiling it, running it to extract metadata from the type system, as well as the properties and signals of a GObject type, and generate XML that can be parsed and included into the larger GIR metadata for the rest of the ABI being introspected. GObject should ship a pre-built binary, instead; it should dlopen the given library or executable, extract all the type information, and emit the introspection data. This would not make gobject-introspection more cross-compilable, but it would simplify its internals and its distributability. We would not need to know how to compile and run C code from a Python script, for one; a simple executable wrapper around a native copy of the GObject-provided binary would be enough.

Ideally, we could move the girepository API into GObject itself, and allow it to load the binary data compiled out of the XML; language bindings loading the data at run time would then need to depend on GObject instead of an additional library, and we could ship the GIR → typelib compiler directly with GLib, leaving gobject-introspection to deal only with the parsing of C headers, docblocks, and annotations, to generate the XML representation of the C/GObject ABI.

There and back again

And the ship went out into the High Sea and passed on into the West, until at last on a night of rain Frodo smelled a sweet fragrance on the air and heard the sound of singing that came over the water. And then it seemed to him that as in his dream in the house of Bombadil, the grey rain-curtain turned all to silver glass and was rolled back, and he beheld white shores and beyond them a far green country under a swift sunrise. — “The Lord of the Rings”, Volume 3: The Return of the King, Book 6: The End of the Third Age

The hard part of changing a project in a backward compatible way is resisting the temptation of fixing the existing design. Some times it’s necessary to backtrack the chain of decisions, and consider the extant code base a dead branch; not because the code is wrong, or bug free, but because any attempt at doubling down on the same design will inevitably lead to breakage. In this sense, it’s easy to just declare “maintenance bankruptcy”, and start from a new major API version: breaks allow us to fix the implementation, at the cost of adapting to new API. For instance, widgets are still the core of GTK, even after 4 major revisions; we did not rename them to “elements” or “actors”, and we did not change how the windows are structured. You are still supposed to build a tree of widgets, connect callbacks to signals, and let the main event loop run. Porting has been painful because of underlying changes in the graphics stack, or because of portability concerns, but even with the direction change of favouring composition over inheritance, the knowledge on how to use GTK has been transferred from GTK 1 to 4.

We cannot do the same for GObject. Changing how it is implemented implies changing everything that depends on it; it means introducing behavioural changes in subtle, and hard to predict ways. Luckily for us, the underlying type system is still flexible and nimble enough that it can give us the ability to change direction, and implement an entirely different approach to object orientation—one that is more in line with languages like modern C++ and Rust. By following new approaches we can slowly migrate our platform to other languages over time, with a smaller impedance mismatch caused by the current design of our object model. Additionally, by keeping the root of the type system, we maintain the ability to provide a stable C ABI that can be consumed by multiple languages, which is the strong selling point of the GNOME ecosystem.

Why do all of this work, though? Compared to a full API break, this proposal has the advantage of being tractable and realistic; I cannot overemphasise enough how little appetite there is for a “GObject 3.0” in the ecosystem. The recent API bump from libsoup2 to libsoup3 has clearly identified that changes deep into the stack end up being too costly an effort: some projects have found it easier to switch to another HTTP library altogether, rather than support two versions of libsoup for a while; other projects have decided to drop compatibility with libsoup2, forcing the hand of every reverse dependency both upstream and downstream. Breaking GObject would end up breaking the ecosystem, with the hope of a “perfect” implementation way down the line and with very few users on one side, and a dead branch used by everybody else on the other.

Of course, the complexity of the change is not going to be trivial, and it will impact things like the introspection metadata and the various language bindings that exist today; some bindings may even require a complete redesign. Nevertheless, by implementing this new object model and leaving GObject alone, we buy ourselves enough time and space to port our software development platform towards a different future.

Maybe this way we will get to save the Shire; and even if we give up some things, or even lose them, we still get to keep what matters.

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