qt: Backup former GUI settings on `-resetguisettings`

Writes the GUI settings to `guisettings.bak` in the data directory
before wiping them. This can be used to retroactively troubleshoot
issues (e.g. #11262) where `-resetguisettings` solves the problem.

doc/files.md

@@ -15,6 +15,7 @@
 * wallet.dat: personal wallet (BDB) with keys and transactions
 * .cookie: session RPC authentication cookie (written at start when cookie authentication is used, deleted on shutdown): since 0.12.0
 * onion_private_key: cached Tor hidden service private key for `-listenonion`: since 0.12.0
+* guisettings.ini.bak: backup of former GUI settings after `-resetguisettings` is used
 
 Only used in pre-0.8.0
 ---------------------

src/qt/optionsmodel.cpp

@@ -151,10 +151,32 @@ void OptionsModel::Init(bool resetSettings)
     language = settings.value("language").toString();
 }
 
+/** Helper function to copy contents from one QSettings to another.
+ * By using allKeys this also covers nested settings in a hierarchy.
+ */
+static void CopySettings(QSettings& dst, const QSettings& src)
+{
+    for (const QString& key : src.allKeys()) {
+        dst.setValue(key, src.value(key));
+    }
+}
+
+/** Back up a QSettings to an ini-formatted file. */
+static void BackupSettings(const fs::path& filename, const QSettings& src)
+{
+    qWarning() << "Backing up GUI settings to" << GUIUtil::boostPathToQString(filename);
+    QSettings dst(GUIUtil::boostPathToQString(filename), QSettings::IniFormat);
+    dst.clear();
+    CopySettings(dst, src);
+}
+
 void OptionsModel::Reset()
 {
     QSettings settings;
 
+    // Backup old settings to chain-specific datadir for troubleshooting
+    BackupSettings(GetDataDir(true) / "guisettings.ini.bak", settings);
+
     // Save the strDataDir setting
     QString dataDir = Intro::getDefaultDataDirectory();
     dataDir = settings.value("strDataDir", dataDir).toString();