[libcalamares] Add macro CONFIG_PREVENT_EDITING to handle uneditable fields

Boilerplate code for avoiding accidental setting of an internal
field when the UI is editable and the underlying data isn't.

src/libcalamares/modulesystem/Config.h

@@ -54,6 +54,25 @@ public Q_SLOTS:
      * to not-editable, returns @c false. Otherwise, return @c true.
      * Calling this with an unknown field (one for which no presets
      * are accepted) will print a warning and return @c true.
+     *
+     * @see CONFIG_PREVENT_EDITING
+     *
+     * Most setters will call isEditable() to check if the field should
+     * be editable. Do not count on the setter not being called: the
+     * UI might not have set the field to fixed / constant / not-editable
+     * and then you can have the setter called by changes in the UI.
+     *
+     * To prevent the UI from changing **and** to make sure that the UI
+     * reflects the unchanged value (rather than the changed value it
+     * sent to the Config object), use CONFIG_PREVENT_EDITING, like so:
+     *
+     *  CONFIG_PREVENT_EDITING( type, "propertyName" );
+     *
+     * The ; is necessary. <type> is the type of the property; for instance
+     * QString. The name of the property is a (constant) string. The
+     * macro will return (out of the setter it is used in) if the field
+     * is not editable, and will send a notification event with the old
+     * value as soon as the event loop resumes.
      */
     bool isEditable( const QString& fieldName ) const;
 
@@ -104,4 +123,25 @@ private:
 }  // namespace ModuleSystem
 }  // namespace Calamares
 
+/// @see Config::isEditable()
+//
+// This needs to be a macro, because Q_ARG() is a macro that stringifies
+// the type name.
+#define CONFIG_PREVENT_EDITING( type, fieldName ) \
+    do \
+    { \
+        if ( !isEditable( QStringLiteral( fieldName ) ) ) \
+        { \
+            auto prop = property( fieldName ); \
+            const auto& metaobject = metaObject(); \
+            auto metaprop = metaobject->property( metaobject->indexOfProperty( fieldName ) ); \
+            if ( metaprop.hasNotifySignal() ) \
+            { \
+                metaprop.notifySignal().invoke( this, Qt::QueuedConnection, Q_ARG( type, prop.value< type >() ) ); \
+            } \
+            return; \
+        } \
+    } while ( 0 )
+
+
 #endif

src/modules/users/Config.cpp

@@ -22,6 +22,7 @@
 
 #include <QCoreApplication>
 #include <QFile>
+#include <QMetaProperty>
 #include <QRegExp>
 #include <QTimer>
 
@@ -184,12 +185,7 @@ Config::setSudoersGroup( const QString& group )
 void
 Config::setLoginName( const QString& login )
 {
-    if ( !isEditable( QStringLiteral( "loginName" ) ) )
-    {
-        // Should not have arrived here anyway
-        QTimer::singleShot( 0, this, [=]() { emit loginNameChanged( m_loginName ); } );
-        return;
-    }
+    CONFIG_PREVENT_EDITING( QString, "loginName" );
     if ( login != m_loginName )
     {
         m_customLoginName = !login.isEmpty();
@@ -400,12 +396,7 @@ makeHostnameSuggestion( const QStringList& parts )
 void
 Config::setFullName( const QString& name )
 {
-    if ( !isEditable( QStringLiteral( "fullName" ) ) )
-    {
-        // Should not have arrived here anyway
-        QTimer::singleShot( 0, this, [=]() { emit fullNameChanged( m_fullName ); } );
-        return;
-    }
+    CONFIG_PREVENT_EDITING( QString, "fullName" );
 
     if ( name.isEmpty() && !m_fullName.isEmpty() )
     {