gui: Show error if unrecognized command line args are present

Starting bitcoin-qt with non-dash ("-") arguments causes it to
silently ignore any later valid options. This change makes the
client exit with an error message if any such "loose" arguments
are encountered.

However, allow BIP-21 'bitcoin:' URIs only if no other options
follow.

src/qt/bitcoin.cpp

@@ -543,6 +543,34 @@ int GuiMain(int argc, char* argv[])
         return EXIT_FAILURE;
     }
 
+    // Error out when loose non-argument tokens are encountered on command line
+    // However, allow BIP-21 URIs only if no options follow
+    bool payment_server_token_seen = false;
+    for (int i = 1; i < argc; i++) {
+        QString arg(argv[i]);
+        bool invalid_token = !arg.startsWith("-");
+#ifdef ENABLE_WALLET
+        if (arg.startsWith(BITCOIN_IPC_PREFIX, Qt::CaseInsensitive)) {
+            invalid_token &= false;
+            payment_server_token_seen = true;
+        }
+#endif
+        if (payment_server_token_seen && arg.startsWith("-")) {
+            InitError(Untranslated(strprintf("Options ('%s') cannot follow a BIP-21 payment URI", argv[i])));
+            QMessageBox::critical(nullptr, PACKAGE_NAME,
+                                  // message cannot be translated because translations have not been initialized
+                                  QString::fromStdString("Options ('%1') cannot follow a BIP-21 payment URI").arg(QString::fromStdString(argv[i])));
+            return EXIT_FAILURE;
+        }
+        if (invalid_token) {
+            InitError(Untranslated(strprintf("Command line contains unexpected token '%s', see bitcoin-qt -h for a list of options.", argv[i])));
+            QMessageBox::critical(nullptr, PACKAGE_NAME,
+                                  // message cannot be translated because translations have not been initialized
+                                  QString::fromStdString("Command line contains unexpected token '%1', see bitcoin-qt -h for a list of options.").arg(QString::fromStdString(argv[i])));
+            return EXIT_FAILURE;
+        }
+    }
+
     // Now that the QApplication is setup and we have parsed our parameters, we can set the platform style
     app.setupPlatformStyle();
 

src/qt/paymentserver.h

@@ -54,6 +54,8 @@ class QLocalServer;
 class QUrl;
 QT_END_NAMESPACE
 
+extern const QString BITCOIN_IPC_PREFIX;
+
 class PaymentServer : public QObject
 {
     Q_OBJECT