QDBusConnectionPrivate: Close connection after failed q_dbus_bus_register()

The connection should be closed before executing
q_dbus_connection_unref(). Failing to do so results in an assertion
inside libdbus:

    dbus[1573958]: The last reference on a connection was dropped
    without closing the connection. This is a bug in an application.
    See dbus_connection_unref() documentation for details.
    Most likely, the application was supposed to call
    dbus_connection_close(), since this is a private connection.

The q_dbus_bus_register() may fail if maximum number of active
connections for a bus was reached. This can be tested by creating
a custom bus with "max_completed_connections" parameter set to 0.
Add such a test to tst_qdbusconnection.

Fixes: QTBUG-34613
Change-Id: I6ae7df19bf8b6546c2a504931ba852dc15d35f78
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/dbus/qdbusconnectionmanager.cpp

@@ -250,6 +250,7 @@ QDBusConnectionPrivate *QDBusConnectionManager::doConnectToBus(const QString &ad
     if (c) {
         // register on the bus
         if (!q_dbus_bus_register(c, error)) {
+            q_dbus_connection_close(c);
             q_dbus_connection_unref(c);
             c = nullptr;
         }

tests/auto/dbus/qdbusconnection/CMakeLists.txt

@@ -16,4 +16,6 @@ qt_internal_add_test(tst_qdbusconnection
         tst_qdbusconnection.cpp tst_qdbusconnection.h
     LIBRARIES
         Qt::DBus
+    TESTDATA
+        tst_qdbusconnection.conf
 )

tests/auto/dbus/qdbusconnection/tst_qdbusconnection.conf

@@ -0,0 +1,23 @@
+<!-- This configuration file for tst_qdbusconnection.
+-->
+
+<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-Bus Bus Configuration 1.0//EN"
+ "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
+<busconfig>
+  <type>tst_qdbusconnection</type>
+
+  <listen>unix:tmpdir=/tmp</listen>
+
+  <auth>EXTERNAL</auth>
+
+  <policy context="default">
+    <!-- Allow everything to be sent -->
+    <allow send_destination="*" eavesdrop="true"/>
+    <!-- Allow everything to be received -->
+    <allow eavesdrop="true"/>
+    <!-- Allow anyone to own anything -->
+    <allow own="*"/>
+  </policy>
+
+  <limit name="max_completed_connections">0</limit>
+</busconfig>

tests/auto/dbus/qdbusconnection/tst_qdbusconnection.cpp

@@ -1412,6 +1412,34 @@ void tst_QDBusConnection::pendingCallWhenDisconnected()
 #endif
 }
 
+void tst_QDBusConnection::connectionLimit()
+{
+#if !QT_CONFIG(process)
+    QSKIP("Test requires QProcess");
+#else
+    if (!QCoreApplication::instance())
+        QSKIP("Test requires a QCoreApplication");
+
+    QProcess daemon;
+    daemon.start("dbus-daemon",
+                 QStringList() << "--config-file" << QFINDTESTDATA("tst_qdbusconnection.conf")
+                               << "--nofork"
+                               << "--print-address");
+    QVERIFY2(daemon.waitForReadyRead(2000),
+             "Daemon didn't print its address in time; error: \"" + daemon.errorString().toLocal8Bit() +
+             "\"; stderr:\n" + daemon.readAllStandardError());
+
+    QString address = QString::fromLocal8Bit(daemon.readAll().trimmed());
+    QDBusConnection con = QDBusConnection::connectToBus(address, "connectionLimit");
+    QVERIFY2(!con.isConnected(), "Unexpected successful connection");
+    QCOMPARE(con.lastError().type(), QDBusError::LimitsExceeded);
+
+    // kill the bus
+    daemon.terminate();
+    daemon.waitForFinished();
+#endif
+}
+
 void tst_QDBusConnection::emptyServerAddress()
 {
     QDBusServer server({}, nullptr);

tests/auto/dbus/qdbusconnection/tst_qdbusconnection.h

@@ -120,6 +120,7 @@ private slots:
     void callVirtualObject();
     void callVirtualObjectLocal();
     void pendingCallWhenDisconnected();
+    void connectionLimit();
 
     void emptyServerAddress();