A couple of enhancements to user-visible threading semantics

doc/classes/EditorImportPlugin.xml

@@ -120,6 +120,13 @@
 		<link title="Import plugins">$DOCS_URL/tutorials/plugins/editor/import_plugins.html</link>
 	</tutorials>
 	<methods>
+		<method name="_can_import_threaded" qualifiers="virtual const">
+			<return type="bool" />
+			<description>
+				Tells whether this importer can be run in parallel on threads, or, on the contrary, it's only safe for the editor to call it from the main thread, for one file at a time.
+				If this method is not overridden, it will return [code]true[/code] by default (i.e., safe for parallel importing).
+			</description>
+		</method>
 		<method name="_get_import_options" qualifiers="virtual const">
 			<return type="Dictionary[]" />
 			<param index="0" name="path" type="String" />

editor/import/editor_import_plugin.cpp

@@ -186,6 +186,15 @@ Error EditorImportPlugin::import(const String &p_source_file, const String &p_sa
 	ERR_FAIL_V_MSG(ERR_METHOD_NOT_FOUND, "Unimplemented _import in add-on.");
 }
 
+bool EditorImportPlugin::can_import_threaded() const {
+	bool ret = false;
+	if (GDVIRTUAL_CALL(_can_import_threaded, ret)) {
+		return ret;
+	} else {
+		return ResourceImporter::can_import_threaded();
+	}
+}
+
 Error EditorImportPlugin::_append_import_external_resource(const String &p_file, const Dictionary &p_custom_options, const String &p_custom_importer, Variant p_generator_parameters) {
 	HashMap<StringName, Variant> options;
 	List<Variant> keys;
@@ -213,5 +222,6 @@ void EditorImportPlugin::_bind_methods() {
 	GDVIRTUAL_BIND(_get_import_order)
 	GDVIRTUAL_BIND(_get_option_visibility, "path", "option_name", "options")
 	GDVIRTUAL_BIND(_import, "source_file", "save_path", "options", "platform_variants", "gen_files");
+	GDVIRTUAL_BIND(_can_import_threaded);
 	ClassDB::bind_method(D_METHOD("append_import_external_resource", "path", "custom_options", "custom_importer", "generator_parameters"), &EditorImportPlugin::_append_import_external_resource, DEFVAL(Dictionary()), DEFVAL(String()), DEFVAL(Variant()));
 }

editor/import/editor_import_plugin.h

@@ -52,6 +52,7 @@ class EditorImportPlugin : public ResourceImporter {
 	GDVIRTUAL0RC(int, _get_import_order)
 	GDVIRTUAL3RC(bool, _get_option_visibility, String, StringName, Dictionary)
 	GDVIRTUAL5RC(Error, _import, String, String, Dictionary, TypedArray<String>, TypedArray<String>)
+	GDVIRTUAL0RC(bool, _can_import_threaded)
 
 	Error _append_import_external_resource(const String &p_file, const Dictionary &p_custom_options = Dictionary(), const String &p_custom_importer = String(), Variant p_generator_parameters = Variant());
 
@@ -69,6 +70,7 @@ class EditorImportPlugin : public ResourceImporter {
 	virtual void get_import_options(const String &p_path, List<ImportOption> *r_options, int p_preset) const override;
 	virtual bool get_option_visibility(const String &p_path, const String &p_option, const HashMap<StringName, Variant> &p_options) const override;
 	virtual Error import(const String &p_source_file, const String &p_save_path, const HashMap<StringName, Variant> &p_options, List<String> *r_platform_variants, List<String> *r_gen_files, Variant *r_metadata = nullptr) override;
+	virtual bool can_import_threaded() const override;
 	Error append_import_external_resource(const String &p_file, const HashMap<StringName, Variant> &p_custom_options = HashMap<StringName, Variant>(), const String &p_custom_importer = String(), Variant p_generator_parameters = Variant());
 };
 

scene/main/node.h

@@ -558,7 +558,9 @@ class Node : public Object {
 	_FORCE_INLINE_ bool is_readable_from_caller_thread() const {
 		if (current_process_thread_group == nullptr) {
 			// No thread processing.
-			return is_current_thread_safe_for_nodes();
+			// Only accessible if node is outside the scene tree
+			// or access will happen from a node-safe thread.
+			return !data.inside_tree || is_current_thread_safe_for_nodes();
 		} else {
 			// Thread processing.
 			return true;