Mono: Better handling of missing/outdated API assemblies

Remove the old API assembly invalidation system. It's pretty simple since now the editor has a hard dependency on the API assemblies and SCons takes care of prebuilding them.
If we fail to load a project's API assembly because it was either missing or outdated, we just copy the prebuilt assemblies to the project and try again. We also do this when creating the solution and before building, just in case the user removed them from the disk after they were loaded.
This way the API assemblies will be always loaded successfully. If they are not, it's a bug.

Also fixed:

- EditorDef was behaving like GlobalDef in GodotTools.
- NullReferenceException because we can't serialize System.WeakReference yet. Use Godot.WeakRef in the mean time.

2 files changed, 221 insertions, 298 deletions

diff --git a/modules/mono/mono_gd/gd_mono.cpp b/modules/mono/mono_gd/gd_mono.cpp
index 06fbae019c..096ad0f5e3 100644
--- a/modules/mono/mono_gd/gd_mono.cpp
+++ b/modules/mono/mono_gd/gd_mono.cpp
@@ -59,10 +59,6 @@
 #include "android_mono_config.gen.h"
 #endif
 
-#define OUT_OF_SYNC_ERR_MESSAGE(m_assembly_name) "The assembly '" m_assembly_name "' is out of sync. "                    \
-												 "This error is expected if you just upgraded to a newer Godot version. " \
-												 "Building the project will update the assembly to the correct version."
-
 GDMono *GDMono::singleton = NULL;
 
 namespace {
@@ -304,26 +300,9 @@ void GDMono::initialize() {
 	mono_install_unhandled_exception_hook(&unhandled_exception_hook, NULL);
 
 #ifndef TOOLS_ENABLED
-	if (!DirAccess::exists("res://.mono")) {
-		// 'res://.mono/' is missing so there is nothing to load. We don't need to initialize mono, but
-		// we still do so unless mscorlib is missing (which is the case for projects that don't use C#).
-
-		String mscorlib_fname("mscorlib.dll");
-
-		Vector<String> search_dirs;
-		GDMonoAssembly::fill_search_dirs(search_dirs);
-
-		bool found = false;
-		for (int i = 0; i < search_dirs.size(); i++) {
-			if (FileAccess::exists(search_dirs[i].plus_file(mscorlib_fname))) {
-				found = true;
-				break;
-			}
-		}
-
-		if (!found)
-			return; // mscorlib is missing, do not initialize mono
-	}
+	// Export templates only load the Mono runtime if the project uses it
+	if (!DirAccess::exists("res://.mono"))
+		return;
 #endif
 
 	root_domain = mono_jit_init_version("GodotEngine.RootDomain", "v4.0.30319");
@@ -354,63 +333,48 @@ void GDMono::initialize() {
 
 	_register_internal_calls();
 
-	// The following assemblies are not required at initialization
-#ifdef MONO_GLUE_ENABLED
-	if (_load_api_assemblies()) {
-		// Everything is fine with the api assemblies, load the tools and project assemblies
-
-#if defined(TOOLS_ENABLED)
-		ERR_EXPLAIN("Mono: Failed to load GodotTools assemblies");
-		ERR_FAIL_COND(!_load_tools_assemblies());
-#endif
+	print_verbose("Mono: INITIALIZED");
+}
 
-		_load_project_assembly();
+void GDMono::initialize_load_assemblies() {
 
-	} else {
-		if ((core_api_assembly && (core_api_assembly_out_of_sync || !GDMonoUtils::mono_cache.godot_api_cache_updated))
-#ifdef TOOLS_ENABLED
-				|| (editor_api_assembly && editor_api_assembly_out_of_sync)
+#ifndef MONO_GLUE_ENABLED
+	ERR_EXPLAIN("Mono: This binary was built with `mono_glue=no`; cannot load assemblies");
+	CRASH_NOW();
 #endif
-		) {
-#ifdef TOOLS_ENABLED
-			// The assembly was successfully loaded, but the full api could not be cached.
-			// This is most likely an outdated assembly loaded because of an invalid version in the
-			// metadata, so we invalidate the version in the metadata and unload the script domain.
-
-			if (core_api_assembly_out_of_sync) {
-				ERR_PRINT(OUT_OF_SYNC_ERR_MESSAGE(CORE_API_ASSEMBLY_NAME));
-				metadata_set_api_assembly_invalidated(APIAssembly::API_CORE, true);
-			} else if (!GDMonoUtils::mono_cache.godot_api_cache_updated) {
-				ERR_PRINT("The loaded assembly '" CORE_API_ASSEMBLY_NAME "' is in sync, but the cache update failed");
-				metadata_set_api_assembly_invalidated(APIAssembly::API_CORE, true);
-			}
 
-			if (editor_api_assembly_out_of_sync) {
-				ERR_PRINT(OUT_OF_SYNC_ERR_MESSAGE(EDITOR_API_ASSEMBLY_NAME));
-				metadata_set_api_assembly_invalidated(APIAssembly::API_EDITOR, true);
-			}
+	// Load assemblies. The API and tools assemblies are required,
+	// the application is aborted if these assemblies cannot be loaded.
 
-			print_line("Mono: Proceeding to unload scripts domain because of invalid API assemblies.");
+	_load_api_assemblies();
 
-			Error err = _unload_scripts_domain();
-			if (err != OK) {
-				WARN_PRINT("Mono: Failed to unload scripts domain");
-			}
-#else
-			ERR_PRINT("The loaded API assembly is invalid");
-			CRASH_NOW();
-#endif // TOOLS_ENABLED
-		}
+#if defined(TOOLS_ENABLED)
+	if (!_load_tools_assemblies()) {
+		ERR_EXPLAIN("Mono: Failed to load GodotTools assemblies");
+		CRASH_NOW();
 	}
-#else
-	print_verbose("Mono: Glue disabled, ignoring script assemblies.");
-#endif // MONO_GLUE_ENABLED
+#endif
 
-	print_verbose("Mono: INITIALIZED");
+	// Load the project's main assembly. This doesn't necessarily need to succeed.
+	// The game may not be using .NET at all, or if the project does use .NET and
+	// we're running in the editor, it may just happen to be it wasn't built yet.
+	if (!_load_project_assembly()) {
+		if (OS::get_singleton()->is_stdout_verbose())
+			print_error("Mono: Failed to load project assembly");
+	}
+}
+
+bool GDMono::_are_api_assemblies_out_of_sync() {
+	bool out_of_sync = core_api_assembly && (core_api_assembly_out_of_sync || !GDMonoUtils::mono_cache.godot_api_cache_updated);
+#ifdef TOOLS_ENABLED
+	if (!out_of_sync)
+		out_of_sync = editor_api_assembly && editor_api_assembly_out_of_sync;
+#endif
+	return out_of_sync;
 }
 
-#ifdef MONO_GLUE_ENABLED
 namespace GodotSharpBindings {
+#ifdef MONO_GLUE_ENABLED
 
 uint64_t get_core_api_hash();
 #ifdef TOOLS_ENABLED
@@ -419,13 +383,33 @@ uint64_t get_editor_api_hash();
 uint32_t get_bindings_version();
 
 void register_generated_icalls();
-} // namespace GodotSharpBindings
+
+#else
+
+uint64_t get_core_api_hash() {
+	CRASH_NOW();
+	GD_UNREACHABLE();
+}
+#ifdef TOOLS_ENABLED
+uint64_t get_editor_api_hash() {
+	CRASH_NOW();
+	GD_UNREACHABLE();
+}
 #endif
+uint32_t get_bindings_version() {
+	CRASH_NOW();
+	GD_UNREACHABLE();
+}
+
+void register_generated_icalls() {
+	/* Fine, just do nothing */
+}
+
+#endif // MONO_GLUE_ENABLED
+} // namespace GodotSharpBindings
 
 void GDMono::_register_internal_calls() {
-#ifdef MONO_GLUE_ENABLED
 	GodotSharpBindings::register_generated_icalls();
-#endif
 }
 
 void GDMono::_initialize_and_check_api_hashes() {
@@ -565,12 +549,21 @@ bool GDMono::_load_corlib_assembly() {
 }
 
 #ifdef TOOLS_ENABLED
-static bool copy_api_assembly(const String &p_src_dir, const String &p_dst_dir, const String &p_assembly_name, APIAssembly::Type p_api_type) {
+bool GDMono::copy_prebuilt_api_assembly(APIAssembly::Type p_api_type) {
+
+	bool &api_assembly_out_of_sync = (p_api_type == APIAssembly::API_CORE) ?
+											 GDMono::get_singleton()->core_api_assembly_out_of_sync :
+											 GDMono::get_singleton()->editor_api_assembly_out_of_sync;
+
+	String src_dir = GodotSharpDirs::get_data_editor_prebuilt_api_dir().plus_file("Debug");
+	String dst_dir = GodotSharpDirs::get_res_assemblies_dir();
+
+	String assembly_name = p_api_type == APIAssembly::API_CORE ? CORE_API_ASSEMBLY_NAME : EDITOR_API_ASSEMBLY_NAME;
 
 	// Create destination directory if needed
-	if (!DirAccess::exists(p_dst_dir)) {
-		DirAccess *da = DirAccess::create_for_path(p_dst_dir);
-		Error err = da->make_dir_recursive(p_dst_dir);
+	if (!DirAccess::exists(dst_dir)) {
+		DirAccess *da = DirAccess::create_for_path(dst_dir);
+		Error err = da->make_dir_recursive(dst_dir);
 		memdelete(da);
 
 		if (err != OK) {
@@ -579,21 +572,19 @@ static bool copy_api_assembly(const String &p_src_dir, const String &p_dst_dir,
 		}
 	}
 
-	String assembly_file = p_assembly_name + ".dll";
-	String assembly_src = p_src_dir.plus_file(assembly_file);
-	String assembly_dst = p_dst_dir.plus_file(assembly_file);
+	String assembly_file = assembly_name + ".dll";
+	String assembly_src = src_dir.plus_file(assembly_file);
+	String assembly_dst = dst_dir.plus_file(assembly_file);
 
-	if (!FileAccess::exists(assembly_dst) ||
-			FileAccess::get_modified_time(assembly_src) > FileAccess::get_modified_time(assembly_dst) ||
-			GDMono::get_singleton()->metadata_is_api_assembly_invalidated(p_api_type)) {
+	if (!FileAccess::exists(assembly_dst) || api_assembly_out_of_sync) {
 		DirAccessRef da = DirAccess::create(DirAccess::ACCESS_FILESYSTEM);
 
-		String xml_file = p_assembly_name + ".xml";
-		if (da->copy(p_src_dir.plus_file(xml_file), p_dst_dir.plus_file(xml_file)) != OK)
+		String xml_file = assembly_name + ".xml";
+		if (da->copy(src_dir.plus_file(xml_file), dst_dir.plus_file(xml_file)) != OK)
 			WARN_PRINTS("Failed to copy " + xml_file);
 
-		String pdb_file = p_assembly_name + ".pdb";
-		if (da->copy(p_src_dir.plus_file(pdb_file), p_dst_dir.plus_file(pdb_file)) != OK)
+		String pdb_file = assembly_name + ".pdb";
+		if (da->copy(src_dir.plus_file(pdb_file), dst_dir.plus_file(pdb_file)) != OK)
 			WARN_PRINTS("Failed to copy " + pdb_file);
 
 		Error err = da->copy(assembly_src, assembly_dst);
@@ -603,11 +594,46 @@ static bool copy_api_assembly(const String &p_src_dir, const String &p_dst_dir,
 			return false;
 		}
 
-		GDMono::get_singleton()->metadata_set_api_assembly_invalidated(p_api_type, false);
+		api_assembly_out_of_sync = false;
 	}
 
 	return true;
 }
+
+String GDMono::update_api_assemblies_from_prebuilt() {
+
+#define FAIL_REASON(m_out_of_sync, m_prebuilt_exist)                             \
+	(                                                                            \
+			(m_out_of_sync ?                                                     \
+							String("The assembly is invalidated") :              \
+							String("The assembly was not found")) +              \
+			(m_prebuilt_exist ?                                                  \
+							String(" and the prebuilt assemblies are missing") : \
+							String(" and we failed to copy the prebuilt assemblies")))
+
+	bool api_assembly_out_of_sync = core_api_assembly_out_of_sync || editor_api_assembly_out_of_sync;
+
+	String core_assembly_path = GodotSharpDirs::get_res_assemblies_dir().plus_file(CORE_API_ASSEMBLY_NAME ".dll");
+	String editor_assembly_path = GodotSharpDirs::get_res_assemblies_dir().plus_file(EDITOR_API_ASSEMBLY_NAME ".dll");
+
+	if (!api_assembly_out_of_sync && FileAccess::exists(core_assembly_path) && FileAccess::exists(editor_assembly_path))
+		return String(); // No update needed
+
+	String prebuilt_api_dir = GodotSharpDirs::get_data_editor_prebuilt_api_dir().plus_file("Debug");
+	String prebuilt_core_dll_path = prebuilt_api_dir.plus_file(CORE_API_ASSEMBLY_NAME ".dll");
+	String prebuilt_editor_dll_path = prebuilt_api_dir.plus_file(EDITOR_API_ASSEMBLY_NAME ".dll");
+
+	if (!FileAccess::exists(prebuilt_core_dll_path) || !FileAccess::exists(prebuilt_editor_dll_path))
+		return FAIL_REASON(api_assembly_out_of_sync, /* prebuilt_exist: */ false);
+
+	// Copy the prebuilt Api
+	if (!copy_prebuilt_api_assembly(APIAssembly::API_CORE) || !copy_prebuilt_api_assembly(APIAssembly::API_EDITOR))
+		return FAIL_REASON(api_assembly_out_of_sync, /* prebuilt_exist: */ true);
+
+	return String(); // Updated successfully
+
+#undef FAIL_REASON
+}
 #endif
 
 bool GDMono::_load_core_api_assembly() {
@@ -616,35 +642,15 @@ bool GDMono::_load_core_api_assembly() {
 		return true;
 
 #ifdef TOOLS_ENABLED
-	if (metadata_is_api_assembly_invalidated(APIAssembly::API_CORE)) {
-		String prebuilt_api_dir = GodotSharpDirs::get_data_editor_prebuilt_api_dir().plus_file("Debug");
-		String prebuilt_dll_path = prebuilt_api_dir.plus_file(CORE_API_ASSEMBLY_NAME ".dll");
-		String invalidated_dll_path = get_invalidated_api_assembly_path(APIAssembly::API_CORE);
-
-		if (!FileAccess::exists(prebuilt_dll_path) ||
-				FileAccess::get_modified_time(invalidated_dll_path) == FileAccess::get_modified_time(prebuilt_dll_path)) {
-			print_verbose("Mono: Skipping loading of Core API assembly because it was invalidated");
-			return false;
-		} else {
-			// Copy the prebuilt Api
-			String res_assemblies_dir = GodotSharpDirs::get_res_assemblies_dir();
-			if (!copy_api_assembly(prebuilt_api_dir, res_assemblies_dir, CORE_API_ASSEMBLY_NAME, APIAssembly::API_CORE) ||
-					!copy_api_assembly(prebuilt_api_dir, res_assemblies_dir, EDITOR_API_ASSEMBLY_NAME, APIAssembly::API_EDITOR)) {
-				print_verbose("Mono: Failed to copy prebuilt API. Skipping loading of Core API assembly because it was invalidated");
-				return false;
-			}
-		}
-	}
-#endif
-
+	// For the editor and the editor player we want to load it from a specific path to make sure we can keep it up to date
 	String assembly_path = GodotSharpDirs::get_res_assemblies_dir().plus_file(CORE_API_ASSEMBLY_NAME ".dll");
-
-	bool success = (FileAccess::exists(assembly_path) &&
-						   load_assembly_from(CORE_API_ASSEMBLY_NAME, assembly_path, &core_api_assembly)) ||
-				   load_assembly(CORE_API_ASSEMBLY_NAME, &core_api_assembly);
+	bool success = FileAccess::exists(assembly_path) &&
+				   load_assembly_from(CORE_API_ASSEMBLY_NAME, assembly_path, &core_api_assembly);
+#else
+	bool success = load_assembly(CORE_API_ASSEMBLY_NAME, &core_api_assembly);
+#endif
 
 	if (success) {
-#ifdef MONO_GLUE_ENABLED
 		APIAssembly::Version api_assembly_ver = APIAssembly::Version::get_from_loaded_assembly(core_api_assembly, APIAssembly::API_CORE);
 		core_api_assembly_out_of_sync = GodotSharpBindings::get_core_api_hash() != api_assembly_ver.godot_api_hash ||
 										GodotSharpBindings::get_bindings_version() != api_assembly_ver.bindings_version ||
@@ -654,9 +660,8 @@ bool GDMono::_load_core_api_assembly() {
 
 			_install_trace_listener();
 		}
-#else
-		GDMonoUtils::update_godot_api_cache();
-#endif
+	} else {
+		core_api_assembly_out_of_sync = false;
 	}
 
 	return success;
@@ -668,44 +673,100 @@ bool GDMono::_load_editor_api_assembly() {
 	if (editor_api_assembly)
 		return true;
 
-	if (metadata_is_api_assembly_invalidated(APIAssembly::API_EDITOR)) {
-		String prebuilt_api_dir = GodotSharpDirs::get_data_editor_prebuilt_api_dir().plus_file("Debug");
-		String prebuilt_dll_path = prebuilt_api_dir.plus_file(EDITOR_API_ASSEMBLY_NAME ".dll");
-		String invalidated_dll_path = get_invalidated_api_assembly_path(APIAssembly::API_EDITOR);
-
-		if (!FileAccess::exists(prebuilt_dll_path) ||
-				FileAccess::get_modified_time(invalidated_dll_path) == FileAccess::get_modified_time(prebuilt_dll_path)) {
-			print_verbose("Mono: Skipping loading of Editor API assembly because it was invalidated");
-			return false;
-		} else {
-			// Copy the prebuilt editor Api (no need to copy the core api if we got to this point)
-			String res_assemblies_dir = GodotSharpDirs::get_res_assemblies_dir();
-			if (!copy_api_assembly(prebuilt_api_dir, res_assemblies_dir, EDITOR_API_ASSEMBLY_NAME, APIAssembly::API_EDITOR)) {
-				print_verbose("Mono: Failed to copy prebuilt API. Skipping loading of Editor API assembly because it was invalidated");
-				return false;
-			}
-		}
-	}
-
+	// For the editor and the editor player we want to load it from a specific path to make sure we can keep it up to date
 	String assembly_path = GodotSharpDirs::get_res_assemblies_dir().plus_file(EDITOR_API_ASSEMBLY_NAME ".dll");
-
-	bool success = (FileAccess::exists(assembly_path) &&
-						   load_assembly_from(EDITOR_API_ASSEMBLY_NAME, assembly_path, &editor_api_assembly)) ||
-				   load_assembly(EDITOR_API_ASSEMBLY_NAME, &editor_api_assembly);
+	bool success = FileAccess::exists(assembly_path) &&
+				   load_assembly_from(EDITOR_API_ASSEMBLY_NAME, assembly_path, &editor_api_assembly);
 
 	if (success) {
-#ifdef MONO_GLUE_ENABLED
 		APIAssembly::Version api_assembly_ver = APIAssembly::Version::get_from_loaded_assembly(editor_api_assembly, APIAssembly::API_EDITOR);
 		editor_api_assembly_out_of_sync = GodotSharpBindings::get_editor_api_hash() != api_assembly_ver.godot_api_hash ||
 										  GodotSharpBindings::get_bindings_version() != api_assembly_ver.bindings_version ||
 										  CS_GLUE_VERSION != api_assembly_ver.cs_glue_version;
-#endif
+	} else {
+		editor_api_assembly_out_of_sync = false;
 	}
 
 	return success;
 }
 #endif
 
+bool GDMono::_try_load_api_assemblies() {
+
+	if (!_load_core_api_assembly()) {
+		if (OS::get_singleton()->is_stdout_verbose())
+			print_error("Mono: Failed to load Core API assembly");
+		return false;
+	}
+
+	if (core_api_assembly_out_of_sync || !GDMonoUtils::mono_cache.godot_api_cache_updated)
+		return false;
+
+#ifdef TOOLS_ENABLED
+	if (!_load_editor_api_assembly()) {
+		if (OS::get_singleton()->is_stdout_verbose())
+			print_error("Mono: Failed to load Editor API assembly");
+		return false;
+	}
+
+	if (editor_api_assembly_out_of_sync)
+		return false;
+#endif
+
+	return true;
+}
+
+void GDMono::_load_api_assemblies() {
+
+	if (!_try_load_api_assemblies()) {
+		// The API assemblies are out of sync. Fine, try one more time, but this time
+		// update them from the prebuilt assemblies directory before trying to load them.
+
+		// 1. Unload the scripts domain
+		if (_unload_scripts_domain() != OK) {
+			ERR_EXPLAIN("Mono: Failed to unload scripts domain");
+			CRASH_NOW();
+		}
+
+		// 2. Update the API assemblies
+		String update_error = update_api_assemblies_from_prebuilt();
+		if (!update_error.empty()) {
+			ERR_EXPLAIN(update_error);
+			CRASH_NOW();
+		}
+
+		// 3. Load the scripts domain again
+		if (_load_scripts_domain() != OK) {
+			ERR_EXPLAIN("Mono: Failed to load scripts domain");
+			CRASH_NOW();
+		}
+
+		// 4. Try loading the updated assemblies
+		if (!_try_load_api_assemblies()) {
+			// welp... too bad
+
+			if (_are_api_assemblies_out_of_sync()) {
+				if (core_api_assembly_out_of_sync) {
+					ERR_PRINT("The assembly '" CORE_API_ASSEMBLY_NAME "' is out of sync");
+				} else if (!GDMonoUtils::mono_cache.godot_api_cache_updated) {
+					ERR_PRINT("The loaded assembly '" CORE_API_ASSEMBLY_NAME "' is in sync, but the cache update failed");
+				}
+
+#ifdef TOOLS_ENABLED
+				if (editor_api_assembly_out_of_sync) {
+					ERR_PRINT("The assembly '" EDITOR_API_ASSEMBLY_NAME "' is out of sync");
+				}
+#endif
+
+				CRASH_NOW();
+			} else {
+				ERR_EXPLAIN("Failed to load one of the API assemblies");
+				CRASH_NOW();
+			}
+		}
+	}
+}
+
 #ifdef TOOLS_ENABLED
 bool GDMono::_load_tools_assemblies() {
 
@@ -734,39 +795,11 @@ bool GDMono::_load_project_assembly() {
 
 	if (success) {
 		mono_assembly_set_main(project_assembly->get_assembly());
-	} else {
-		if (OS::get_singleton()->is_stdout_verbose())
-			print_error("Mono: Failed to load project assembly");
 	}
 
 	return success;
 }
 
-bool GDMono::_load_api_assemblies() {
-
-	if (!_load_core_api_assembly()) {
-		if (OS::get_singleton()->is_stdout_verbose())
-			print_error("Mono: Failed to load Core API assembly");
-		return false;
-	}
-
-	if (core_api_assembly_out_of_sync || !GDMonoUtils::mono_cache.godot_api_cache_updated)
-		return false;
-
-#ifdef TOOLS_ENABLED
-	if (!_load_editor_api_assembly()) {
-		if (OS::get_singleton()->is_stdout_verbose())
-			print_error("Mono: Failed to load Editor API assembly");
-		return false;
-	}
-
-	if (editor_api_assembly_out_of_sync)
-		return false;
-#endif
-
-	return true;
-}
-
 void GDMono::_install_trace_listener() {
 
 #ifdef DEBUG_ENABLED
@@ -784,78 +817,6 @@ void GDMono::_install_trace_listener() {
 #endif
 }
 
-#ifdef TOOLS_ENABLED
-String GDMono::_get_api_assembly_metadata_path() {
-
-	return GodotSharpDirs::get_res_metadata_dir().plus_file("api_assemblies.cfg");
-}
-
-void GDMono::metadata_set_api_assembly_invalidated(APIAssembly::Type p_api_type, bool p_invalidated) {
-
-	String section = APIAssembly::to_string(p_api_type);
-	String path = _get_api_assembly_metadata_path();
-
-	Ref<ConfigFile> metadata;
-	metadata.instance();
-	metadata->load(path);
-
-	metadata->set_value(section, "invalidated", p_invalidated);
-
-	String assembly_path = GodotSharpDirs::get_res_assemblies_dir()
-								   .plus_file(p_api_type == APIAssembly::API_CORE ?
-													  CORE_API_ASSEMBLY_NAME ".dll" :
-													  EDITOR_API_ASSEMBLY_NAME ".dll");
-
-	ERR_FAIL_COND(!FileAccess::exists(assembly_path));
-
-	uint64_t modified_time = FileAccess::get_modified_time(assembly_path);
-
-	metadata->set_value(section, "invalidated_asm_modified_time", String::num_uint64(modified_time));
-
-	String dir = path.get_base_dir();
-	if (!DirAccess::exists(dir)) {
-		DirAccessRef da = DirAccess::create(DirAccess::ACCESS_FILESYSTEM);
-		ERR_FAIL_COND(!da);
-		Error err = da->make_dir_recursive(ProjectSettings::get_singleton()->globalize_path(dir));
-		ERR_FAIL_COND(err != OK);
-	}
-
-	Error save_err = metadata->save(path);
-	ERR_FAIL_COND(save_err != OK);
-}
-
-bool GDMono::metadata_is_api_assembly_invalidated(APIAssembly::Type p_api_type) {
-
-	String section = APIAssembly::to_string(p_api_type);
-
-	Ref<ConfigFile> metadata;
-	metadata.instance();
-	metadata->load(_get_api_assembly_metadata_path());
-
-	String assembly_path = GodotSharpDirs::get_res_assemblies_dir()
-								   .plus_file(p_api_type == APIAssembly::API_CORE ?
-													  CORE_API_ASSEMBLY_NAME ".dll" :
-													  EDITOR_API_ASSEMBLY_NAME ".dll");
-
-	if (!FileAccess::exists(assembly_path))
-		return false;
-
-	uint64_t modified_time = FileAccess::get_modified_time(assembly_path);
-
-	uint64_t stored_modified_time = metadata->get_value(section, "invalidated_asm_modified_time", 0);
-
-	return metadata->get_value(section, "invalidated", false) && modified_time <= stored_modified_time;
-}
-
-String GDMono::get_invalidated_api_assembly_path(APIAssembly::Type p_api_type) {
-
-	return GodotSharpDirs::get_res_assemblies_dir()
-			.plus_file(p_api_type == APIAssembly::API_CORE ?
-							   CORE_API_ASSEMBLY_NAME ".dll" :
-							   EDITOR_API_ASSEMBLY_NAME ".dll");
-}
-#endif
-
 Error GDMono::_load_scripts_domain() {
 
 	ERR_FAIL_COND_V(scripts_domain != NULL, ERR_BUG);
@@ -903,11 +864,6 @@ Error GDMono::_unload_scripts_domain() {
 	tools_project_editor_assembly = NULL;
 #endif
 
-	core_api_assembly_out_of_sync = false;
-#ifdef TOOLS_ENABLED
-	editor_api_assembly_out_of_sync = false;
-#endif
-
 	MonoDomain *domain = scripts_domain;
 	scripts_domain = NULL;
 
@@ -944,57 +900,25 @@ Error GDMono::reload_scripts_domain() {
 		return err;
 	}
 
-#ifdef MONO_GLUE_ENABLED
-	if (!_load_api_assemblies()) {
-		if ((core_api_assembly && (core_api_assembly_out_of_sync || !GDMonoUtils::mono_cache.godot_api_cache_updated))
-#ifdef TOOLS_ENABLED
-				|| (editor_api_assembly && editor_api_assembly_out_of_sync)
-#endif
-		) {
-#ifdef TOOLS_ENABLED
-			// The assembly was successfully loaded, but the full api could not be cached.
-			// This is most likely an outdated assembly loaded because of an invalid version in the
-			// metadata, so we invalidate the version in the metadata and unload the script domain.
-
-			if (core_api_assembly_out_of_sync) {
-				ERR_PRINT(OUT_OF_SYNC_ERR_MESSAGE(CORE_API_ASSEMBLY_NAME));
-				metadata_set_api_assembly_invalidated(APIAssembly::API_CORE, true);
-			} else if (!GDMonoUtils::mono_cache.godot_api_cache_updated) {
-				ERR_PRINT("The loaded Core API assembly is in sync, but the cache update failed");
-				metadata_set_api_assembly_invalidated(APIAssembly::API_CORE, true);
-			}
-
-			if (editor_api_assembly_out_of_sync) {
-				ERR_PRINT(OUT_OF_SYNC_ERR_MESSAGE(EDITOR_API_ASSEMBLY_NAME));
-				metadata_set_api_assembly_invalidated(APIAssembly::API_EDITOR, true);
-			}
+	// Load assemblies. The API and tools assemblies are required,
+	// the application is aborted if these assemblies cannot be loaded.
 
-			err = _unload_scripts_domain();
-			if (err != OK) {
-				WARN_PRINT("Mono: Failed to unload scripts domain");
-			}
+	_load_api_assemblies();
 
-			return ERR_CANT_RESOLVE;
-#else
-			ERR_PRINT("The loaded API assembly is invalid");
-			CRASH_NOW();
-#endif
-		} else {
-			return ERR_CANT_OPEN;
-		}
+#if defined(TOOLS_ENABLED)
+	if (!_load_tools_assemblies()) {
+		ERR_EXPLAIN("Mono: Failed to load GodotTools assemblies");
+		CRASH_NOW();
 	}
-
-#ifdef TOOLS_ENABLED
-	ERR_EXPLAIN("Mono: Failed to load GodotTools assemblies");
-	ERR_FAIL_COND_V(!_load_tools_assemblies(), ERR_CANT_OPEN);
 #endif
 
+	// Load the project's main assembly. Here, during hot-reloading, we do
+	// consider failing to load the project's main assembly to be an error.
+	// However, unlike the API and tools assemblies, the application can continue working.
 	if (!_load_project_assembly()) {
+		print_error("Mono: Failed to load project assembly");
 		return ERR_CANT_OPEN;
 	}
-#else
-	print_verbose("Mono: Glue disabled, ignoring script assemblies.");
-#endif // MONO_GLUE_ENABLED
 
 	return OK;
 }
diff --git a/modules/mono/mono_gd/gd_mono.h b/modules/mono/mono_gd/gd_mono.h
index a926bf4126..deebe5fd50 100644
--- a/modules/mono/mono_gd/gd_mono.h
+++ b/modules/mono/mono_gd/gd_mono.h
@@ -104,6 +104,8 @@ class GDMono {
 
 	void _domain_assemblies_cleanup(uint32_t p_domain_id);
 
+	bool _are_api_assemblies_out_of_sync();
+
 	bool _load_corlib_assembly();
 	bool _load_core_api_assembly();
 #ifdef TOOLS_ENABLED
@@ -112,11 +114,8 @@ class GDMono {
 #endif
 	bool _load_project_assembly();
 
-	bool _load_api_assemblies();
-
-#ifdef TOOLS_ENABLED
-	String _get_api_assembly_metadata_path();
-#endif
+	bool _try_load_api_assemblies();
+	void _load_api_assemblies();
 
 	void _install_trace_listener();
 
@@ -157,9 +156,8 @@ public:
 #endif
 
 #ifdef TOOLS_ENABLED
-	void metadata_set_api_assembly_invalidated(APIAssembly::Type p_api_type, bool p_invalidated);
-	bool metadata_is_api_assembly_invalidated(APIAssembly::Type p_api_type);
-	String get_invalidated_api_assembly_path(APIAssembly::Type p_api_type);
+	bool copy_prebuilt_api_assembly(APIAssembly::Type p_api_type);
+	String update_api_assemblies_from_prebuilt();
 #endif
 
 	static GDMono *get_singleton() { return singleton; }
@@ -203,6 +201,7 @@ public:
 	Error finalize_and_unload_domain(MonoDomain *p_domain);
 
 	void initialize();
+	void initialize_load_assemblies();
 
 	GDMono();
 	~GDMono();