Changes necessary for hot reload to work

SConstruct

@@ -173,6 +173,14 @@ opts.Add(
     )
 )
 
+opts.Add(
+    BoolVariable(
+        key="use_hot_reload",
+        help="Enable the extra accounting required to support hot reload.",
+        default=(env.get("target", "template_debug") != "template_release"),
+    )
+)
+
 # Add platform options
 tools = {}
 for pl in platforms:
@@ -238,6 +246,9 @@ if env["arch"] == "":
             print("Unsupported CPU architecture: " + host_machine)
             Exit()
 
+if env["use_hot_reload"]:
+    env.Append(CPPDEFINES=["HOT_RELOAD_ENABLED"])
+
 tool = Tool(env["platform"], toolpath=["tools"])
 
 if tool is None or not tool.exists(env):

gdextension/gdextension_interface.h

@@ -264,6 +264,7 @@ typedef void (*GDExtensionClassReference)(GDExtensionClassInstancePtr p_instance
 typedef void (*GDExtensionClassUnreference)(GDExtensionClassInstancePtr p_instance);
 typedef void (*GDExtensionClassCallVirtual)(GDExtensionClassInstancePtr p_instance, const GDExtensionConstTypePtr *p_args, GDExtensionTypePtr r_ret);
 typedef GDExtensionObjectPtr (*GDExtensionClassCreateInstance)(void *p_userdata);
+typedef GDExtensionClassInstancePtr (*GDExtensionClassRecreateInstance)(void *p_userdata, GDExtensionObjectPtr p_object);
 typedef void (*GDExtensionClassFreeInstance)(void *p_userdata, GDExtensionClassInstancePtr p_instance);
 typedef GDExtensionClassCallVirtual (*GDExtensionClassGetVirtual)(void *p_userdata, GDExtensionConstStringNamePtr p_name);
 
@@ -285,6 +286,7 @@ typedef struct {
 	GDExtensionClassGetVirtual get_virtual_func; // Queries a virtual function by name and returns a callback to invoke the requested virtual function.
 	GDExtensionClassGetRID get_rid_func;
 	void *class_userdata; // Per-class user data, later accessible in instance bindings.
+	GDExtensionClassRecreateInstance recreate_instance_func;
 } GDExtensionClassCreationInfo;
 
 typedef void *GDExtensionClassLibraryPtr;

include/godot_cpp/classes/wrapped.hpp

@@ -51,6 +51,15 @@ class Wrapped {
 	friend void postinitialize_handler(Wrapped *);
 
 protected:
+#ifdef HOT_RELOAD_ENABLED
+	struct RecreateInstance {
+		GDExtensionClassInstancePtr wrapper;
+		GDExtensionObjectPtr owner;
+		RecreateInstance *next;
+	};
+	inline static RecreateInstance *recreate_instance = nullptr;
+#endif
+
 	virtual const StringName *_get_extension_class_name() const; // This is needed to retrieve the class name before the godot object has its _extension and _extension_instance members assigned.
 	virtual const GDExtensionInstanceBindingCallbacks *_get_bindings_callbacks() const = 0;
 
@@ -104,6 +113,17 @@ void free_c_property_list(GDExtensionPropertyInfo *plist);
 
 } // namespace godot
 
+#ifdef HOT_RELOAD_ENABLED
+#define _GDCLASS_RECREATE(m_class, m_inherits)                                                    \
+	m_class *new_instance = (m_class *)memalloc(sizeof(m_class));                                \
+	Wrapped::RecreateInstance recreate_data = { new_instance, obj, Wrapped::recreate_instance }; \
+	Wrapped::recreate_instance = &recreate_data;                                                 \
+	memnew_placement(new_instance, m_class);                                                     \
+	return new_instance;
+#else
+#define _GDCLASS_RECREATE(m_class, m_inherits) return nullptr;
+#endif
+
 // Use this on top of your own classes.
 // Note: the trail of `***` is to keep sane diffs in PRs, because clang-format otherwise moves every `\` which makes
 // every line of the macro different
@@ -187,6 +207,10 @@ public:
 		return new_object->_owner;                                                                                                                                                     \
 	}                                                                                                                                                                                  \
                                                                                                                                                                                        \
+	static GDExtensionClassInstancePtr recreate(void *data, GDExtensionObjectPtr obj) {                                                                                                \
+		_GDCLASS_RECREATE(m_class, m_inherits);                                                                                                                                        \
+	}                                                                                                                                                                                  \
+                                                                                                                                                                                       \
 	static void notification_bind(GDExtensionClassInstancePtr p_instance, int32_t p_what) {                                                                                            \
 		if (p_instance && m_class::_get_notification()) {                                                                                                                              \
 			if (m_class::_get_notification() != m_inherits::_get_notification()) {                                                                                                     \

include/godot_cpp/core/class_db.hpp

@@ -192,6 +192,7 @@ void ClassDB::_register_class(bool p_virtual) {
 		&ClassDB::get_virtual_func, // GDExtensionClassGetVirtual get_virtual_func;
 		nullptr, // GDExtensionClassGetRID get_rid;
 		(void *)&T::get_class_static(), // void *class_userdata;
+		T::recreate,
 	};
 
 	internal::gdextension_interface_classdb_register_extension_class(internal::library, cl.name._native_ptr(), cl.parent_name._native_ptr(), &class_info);

src/classes/wrapped.cpp

@@ -49,6 +49,23 @@ void Wrapped::_postinitialize() {
 }
 
 Wrapped::Wrapped(const StringName p_godot_class) {
+#ifdef HOT_RELOAD_ENABLED
+	if (unlikely(Wrapped::recreate_instance)) {
+		RecreateInstance *recreate_data = Wrapped::recreate_instance;
+		RecreateInstance *previous = nullptr;
+		while (recreate_data) {
+			if (recreate_data->wrapper == this) {
+				_owner = recreate_data->owner;
+				if (previous) {
+					previous->next = recreate_data->next;
+				}
+				return;
+			}
+			previous = recreate_data;
+			recreate_data = recreate_data->next;
+		}
+	}
+#endif
 	_owner = godot::internal::gdextension_interface_classdb_construct_object(reinterpret_cast<GDExtensionConstStringNamePtr>(p_godot_class._native_ptr()));
 }
 

src/core/class_db.cpp

@@ -354,6 +354,9 @@ void ClassDB::deinitialize(GDExtensionInitializationLevel p_level) {
 		for (auto method : cl.method_map) {
 			memdelete(method.second);
 		}
+
+		classes.erase(*i);
+		class_register_order.erase((i + 1).base());
 	}
 }
 

tools/linux.py

@@ -14,6 +14,9 @@ def generate(env):
     if env["use_llvm"]:
         clang.generate(env)
         clangxx.generate(env)
+    elif env["use_hot_reload"]:
+        # Required for extensions to truly unload.
+        env.Append(CXXFLAGS=["-fno-gnu-unique"])
 
     env.Append(CCFLAGS=["-fPIC", "-Wwrite-strings"])
     env.Append(LINKFLAGS=["-Wl,-R,'$$ORIGIN'"])