QDBusSignature: accept empty strings as valid

QDBusSignature holds a D-Bus value of type SIGNATURE, which is zero or
more signatures, not one or more. This changes the default constructor
to create a valid signature, which we denote by not being a null
QString. That means we need to use something other than the default
constructor in our tests for attempting to pass invalid signatures.

[ChangeLog][QtDBus][QDBusSignature] Fixed a bug that caused the class
not to accept an empty string as a valid D-Bus SIGNATURE value.

Fixes: QTBUG-124919
Pick-to: 6.5
Change-Id: I262c3499666e4f4fbcfbfffd17cb3793dcf2eae3
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit ed6d1fa71a79a70b7e6a20fbbc737ed9f6c287b1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/dbus/qdbusextratypes.cpp

@@ -25,6 +25,8 @@ void QDBusSignature::doCheck()
     if (!QDBusUtil::isValidSignature(m_signature)) {
         qWarning("QDBusSignature: invalid signature \"%s\"", qPrintable(m_signature));
         m_signature.clear();
+    } else if (m_signature.isEmpty()) {
+        m_signature.detach();       // we need it to not be null
     }
 }
 

src/dbus/qdbusextratypes.h

@@ -74,7 +74,10 @@ class Q_DBUS_EXPORT QDBusSignature
 {
     QString m_signature;
 public:
-    QDBusSignature() noexcept : m_signature() {}
+    QDBusSignature() noexcept
+    {
+        m_signature.detach(); // mark non-null (empty signatures are valid)
+    }
     // compiler-generated copy/move constructor/assignment operators are ok!
     // compiler-generated destructor is ok!
 

src/dbus/qdbusmarshaller.cpp

@@ -120,7 +120,7 @@ inline void QDBusMarshaller::append(const QDBusObjectPath &arg)
 inline void QDBusMarshaller::append(const QDBusSignature &arg)
 {
     QByteArray data = arg.signature().toUtf8();
-    if (!ba && data.isEmpty()) {
+    if (!ba && data.isNull()) {
         error("Invalid signature passed in arguments"_L1);
     } else {
         const char *cdata = data.constData();

src/dbus/qdbusutil.cpp

@@ -512,14 +512,14 @@ namespace QDBusUtil
     bool isValidSignature(const QString &signature)
     {
         QByteArray ba = signature.toLatin1();
-        const char *data = ba.constData();
-        while (true) {
+        const char *data = ba.constBegin();
+        const char *end = ba.constEnd();
+        while (data != end) {
             data = validateSingleType(data);
             if (!data)
                 return false;
-            if (*data == '\0')
-                return true;
         }
+        return true;
     }
 
     /*!

tests/auto/dbus/qdbusmarshall/tst_qdbusmarshall.cpp

@@ -161,7 +161,9 @@ void basicStringTypes_data()
 {
     QTest::newRow("string") << QVariant("ping") << "s" << "\"ping\"";
     QTest::newRow("objectpath") << QVariant::fromValue(QDBusObjectPath("/org/kde")) << "o" << "[ObjectPath: /org/kde]";
+    QTest::newRow("emptysignature") << QVariant::fromValue(QDBusSignature(QString())) << "g" << "[Signature: ]";
     QTest::newRow("signature") << QVariant::fromValue(QDBusSignature("g")) << "g" << "[Signature: g]";
+    QTest::newRow("multisignature") << QVariant::fromValue(QDBusSignature("bit")) << "g" << "[Signature: bit]";
     QTest::newRow("emptystring") << QVariant("") << "s" << "\"\"";
     QTest::newRow("nullstring") << QVariant(QString()) << "s" << "\"\"";
 }
@@ -907,7 +909,7 @@ void tst_QDBusMarshall::sendSignalErrors()
     QTest::ignoreMessage(QtWarningMsg, "QDBusConnection: error: could not send signal to service \"\" path \"/foo\" interface \"local.interfaceName\" member \"signalName\": Marshalling failed: Invalid object path passed in arguments");
     QVERIFY(!con.send(msg));
 
-    QDBusSignature sig;
+    QDBusSignature sig(QChar(0));
     msg.setArguments(QVariantList() << QVariant::fromValue(sig));
     QTest::ignoreMessage(QtWarningMsg, "QDBusConnection: error: could not send signal to service \"\" path \"/foo\" interface \"local.interfaceName\" member \"signalName\": Marshalling failed: Invalid signature passed in arguments");
     QVERIFY(!con.send(msg));
@@ -992,7 +994,7 @@ void tst_QDBusMarshall::sendCallErrors_data()
             << "";
 
     QTest::newRow("invalid-signature-arg") << serviceName << objectPath << interfaceName << "ping"
-            << (QVariantList() << QVariant::fromValue(QDBusSignature()))
+            << (QVariantList() << QVariant::fromValue(QDBusSignature(QChar(0))))
             << "org.freedesktop.DBus.Error.Failed"
             << "Marshalling failed: Invalid signature passed in arguments"
             << "";

tests/auto/dbus/qdbustype/tst_qdbustype.cpp

@@ -206,6 +206,7 @@ void tst_QDBusType::isValidBasicType()
 void tst_QDBusType::isValidSingleSignature_data()
 {
     addColumns();
+    QTest::newRow("empty") << "" << false;
     addSingleSignatures();
     addNakedDictEntry();
 }
@@ -222,6 +223,7 @@ void tst_QDBusType::isValidSingleSignature()
 void tst_QDBusType::isValidArray_data()
 {
     addColumns();
+    QTest::newRow("empty") << "" << false;
     addSingleSignatures();
 }
 
@@ -241,7 +243,10 @@ void tst_QDBusType::isValidArray()
 
 void tst_QDBusType::isValidSignature_data()
 {
-    isValidSingleSignature_data();
+    addColumns();
+    QTest::newRow("empty") << "" << true;
+    addSingleSignatures();
+    addNakedDictEntry();
 }
 
 void tst_QDBusType::isValidSignature()
@@ -250,8 +255,10 @@ void tst_QDBusType::isValidSignature()
     QFETCH(bool, result);
 
     data.append(data);
-    if (data.at(0).unicode())
-        QCOMPARE(bool(q_dbus_signature_validate(data.toLatin1(), 0)), result);
+    if (!data.isEmpty() && data.at(0).unicode()) {
+        // libdbus-1 API can't deal with string containing NULs
+        QCOMPARE(bool(q_dbus_signature_validate(data.toLatin1(), nullptr)), result);
+    }
     QCOMPARE(QDBusUtil::isValidSignature(data), result);
 }