Make QDBusPendingReply behave like QDBusReply when dealing with errors

QDBusReply allows one to extract a QVariant and the type reply from an
error reply and getting a default-constructed value. This is useful when
a valid reply can never contain the default-constructed value (0, false,
empty strings, empty arrays, etc.), so it simplifies error checking.

More importantly, qdbusxml2cpp was changed a while ago from generating
QDBusReply to generating QDBusPendingReply, so we need to have the same
behavior.

Task-number: QTBUG-29046
Change-Id: Ia873b9fd4311c0d4e94f0ef623ba405c20bc0e8c
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/dbus/qdbuspendingreply.cpp

@@ -185,6 +185,11 @@
     function's return value is undefined (will probably cause an
     assertion failure), so it is important to verify that the
     processing is finished and the reply is valid.
+
+    If the reply does not contain an argument at position \a index or if the
+    reply was an error, this function returns an invalid QVariant. Since D-Bus
+    messages can never contain invalid QVariants, this return can be used to
+    detect an error condition.
 */
 
 /*!
@@ -197,6 +202,11 @@
 
     Note that, if the reply hasn't arrived, this function causes the
     calling thread to block until the reply is processed.
+
+    If the reply does not contain an argument at position \c Index or if the
+    reply was an error, this function returns a \c Type object that is default
+    constructed, which may be indistinguishable from a valid value. To reliably
+    determine whether the message was an error, use isError().
 */
 
 /*!
@@ -211,6 +221,10 @@
 
     Note that, if the reply hasn't arrived, this function causes the
     calling thread to block until the reply is processed.
+
+    If the reply is an error reply, this function returns a default-constructed
+    \c T1 object, which may be indistinguishable from a valid value. To
+    reliably determine whether the message was an error, use isError().
 */
 
 /*!
@@ -225,6 +239,10 @@
 
     Note that, if the reply hasn't arrived, this function causes the
     calling thread to block until the reply is processed.
+
+    If the reply is an error reply, this function returns a default-constructed
+    \c T1 object, which may be indistinguishable from a valid value. To
+    reliably determine whether the message was an error, use isError().
 */
 
 /*!
@@ -260,14 +278,12 @@ void QDBusPendingReplyData::assign(const QDBusMessage &message)
 
 QVariant QDBusPendingReplyData::argumentAt(int index) const
 {
-    if (d)
-        d->waitForFinished();   // bypasses "const"
+    if (!d)
+        return QVariant();
 
-    Q_ASSERT_X(d && index >= 0 && index < d->replyMessage.arguments().count(),
-               "QDBusPendingReply::argumentAt",
-               "Index out of bounds");
+    d->waitForFinished();   // bypasses "const"
 
-    return d->replyMessage.arguments().at(index);
+    return d->replyMessage.arguments().value(index);
 }
 
 void QDBusPendingReplyData::setMetaTypes(int count, const int *types)

src/dbus/qdbuspendingreply.h

@@ -168,9 +168,7 @@ public:
     template<int Index> inline
     const typename Select<Index>::Type argumentAt() const
     {
-        // static assert?
-        Q_ASSERT_X(Index < count() && Index >= 0, "QDBusPendingReply::argumentAt",
-                   "Index out of bounds");
+        Q_STATIC_ASSERT_X(Index >= 0 && Index < Count, "Index out of bounds");
         typedef typename Select<Index>::Type ResultType;
         return qdbus_cast<ResultType>(argumentAt(Index), 0);
     }

tests/auto/dbus/qdbuspendingreply/tst_qdbuspendingreply.cpp

@@ -572,6 +572,8 @@ void tst_QDBusPendingReply::errors()
     QVERIFY(rint.isError());
     error = rint.error();
     VERIFY_ERROR(error);
+    int dummyint = rint;
+    QCOMPARE(dummyint, int());
 
     QDBusPendingReply<int,int> rintint(iface->asyncCall("sendError"));
     rintint.waitForFinished();
@@ -579,6 +581,9 @@ void tst_QDBusPendingReply::errors()
     QVERIFY(rintint.isError());
     error = rintint.error();
     VERIFY_ERROR(error);
+    dummyint = rintint;
+    QCOMPARE(dummyint, int());
+    QCOMPARE(rintint.argumentAt<1>(), int());
 
     QDBusPendingReply<QString> rstring(iface->asyncCall("sendError"));
     rstring.waitForFinished();
@@ -586,6 +591,8 @@ void tst_QDBusPendingReply::errors()
     QVERIFY(rstring.isError());
     error = rstring.error();
     VERIFY_ERROR(error);
+    QString dummystring = rstring;
+    QCOMPARE(dummystring, QString());
 }
 
 QTEST_MAIN(tst_QDBusPendingReply)