dbusServices: Refactor service utilities from fileUtils into dbusUtils

To enable porting services to ECMAScript modules independently of
the shell, split DBus service utility functions into a new file,
dbusUtils.js

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2365>

js/misc/dbusUtils.js

@@ -0,0 +1,68 @@
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+/* exported loadInterfaceXML, loadSubInterfaceXML */
+
+const Config = imports.misc.config;
+const { Gio, GLib } = imports.gi;
+
+let _ifaceResource = null;
+
+/**
+ * @private
+ */
+function _ensureIfaceResource() {
+    if (_ifaceResource)
+        return;
+
+    // don't use global.datadir so the method is usable from tests/tools
+    let dir = GLib.getenv('GNOME_SHELL_DATADIR') || Config.PKGDATADIR;
+    let path = `${dir}/gnome-shell-dbus-interfaces.gresource`;
+    _ifaceResource = Gio.Resource.load(path);
+    _ifaceResource._register();
+}
+
+/**
+ * @param {string} iface the interface name
+ * @returns {string | null} the XML string or null if it is not found
+ */
+function loadInterfaceXML(iface) {
+    _ensureIfaceResource();
+
+    let uri = `resource:///org/gnome/shell/dbus-interfaces/${iface}.xml`;
+    let f = Gio.File.new_for_uri(uri);
+
+    try {
+        let [ok_, bytes] = f.load_contents(null);
+        return new TextDecoder().decode(bytes);
+    } catch (e) {
+        log(`Failed to load D-Bus interface ${iface}`);
+    }
+
+    return null;
+}
+
+/**
+ * @param {string} iface the interface name
+ * @param {string} ifaceFile the interface filename
+ * @returns {string | null} the XML string or null if it is not found
+ */
+function loadSubInterfaceXML(iface, ifaceFile) {
+    let xml = loadInterfaceXML(ifaceFile);
+    if (!xml)
+        return null;
+
+    let ifaceStartTag = `<interface name="${iface}">`;
+    let ifaceStopTag = '</interface>';
+    let ifaceStartIndex = xml.indexOf(ifaceStartTag);
+    let ifaceEndIndex = xml.indexOf(ifaceStopTag, ifaceStartIndex + 1) + ifaceStopTag.length;
+
+    let xmlHeader = '<!DOCTYPE node PUBLIC\n' +
+        '\'-//freedesktop//DTD D-BUS Object Introspection 1.0//EN\'\n' +
+        '\'http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd\'>\n' +
+        '<node>\n';
+    let xmlFooter = '</node>';
+
+    return (
+        xmlHeader +
+        xml.substr(ifaceStartIndex, ifaceEndIndex - ifaceStartIndex) +
+        xmlFooter);
+}