xcb: Correctly report whether QInternalMimeData::retrieveData_sys() fails

QMimeData::retrieveData() checks whether the returned QVariant value is
valid, i.e. the variant contains some data. If the variant contains an
empty QByteArray, it will be considered valid.

So retrieveData_sys() should return QVariant() in case data with the
specified mime type cannot be retrieved.

Fixes: QTBUG-125531
Pick-to: 6.5 6.2 5.15
Change-Id: I0a668ca7ad5b2c5430335e8554cc597747287173
Reviewed-by: Liang Qi <liang.qi@qt.io>
(cherry picked from commit 59b7e143a98c1a91587f8cc87e92c0bc0650f55d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/xcb/qxcbclipboard.cpp

@@ -61,7 +61,7 @@ protected:
             // get the list of targets from the current clipboard owner - we do this
             // once so that multiple calls to this function don't require multiple
             // server round trips...
-            that->format_atoms = m_clipboard->getDataInFormat(modeAtom, m_clipboard->atom(QXcbAtom::AtomTARGETS));
+            that->format_atoms = m_clipboard->getDataInFormat(modeAtom, m_clipboard->atom(QXcbAtom::AtomTARGETS)).value_or(QByteArray());
 
             if (format_atoms.size() > 0) {
                 const xcb_atom_t *targets = (const xcb_atom_t *) format_atoms.data();
@@ -91,7 +91,7 @@ protected:
     {
         auto requestedType = type;
         if (fmt.isEmpty() || isEmpty())
-            return QByteArray();
+            return QVariant();
 
         (void)formats(); // trigger update of format list
 
@@ -108,7 +108,11 @@ protected:
         if (fmtatom == 0)
             return QVariant();
 
-        return mimeConvertToFormat(m_clipboard->connection(), fmtatom, m_clipboard->getDataInFormat(modeAtom, fmtatom), fmt, requestedType, hasUtf8);
+        const std::optional<QByteArray> result = m_clipboard->getDataInFormat(modeAtom, fmtatom);
+        if (!result.has_value())
+            return QVariant();
+
+        return mimeConvertToFormat(m_clipboard->connection(), fmtatom, result.value(), fmt, requestedType, hasUtf8);
     }
 private:
 
@@ -776,7 +780,7 @@ xcb_generic_event_t *QXcbClipboard::waitForClipboardEvent(xcb_window_t window, i
     return nullptr;
 }
 
-QByteArray QXcbClipboard::clipboardReadIncrementalProperty(xcb_window_t win, xcb_atom_t property, int nbytes, bool nullterm)
+std::optional<QByteArray> QXcbClipboard::clipboardReadIncrementalProperty(xcb_window_t win, xcb_atom_t property, int nbytes, bool nullterm)
 {
     QByteArray buf;
     QByteArray tmp_buf;
@@ -841,17 +845,16 @@ QByteArray QXcbClipboard::clipboardReadIncrementalProperty(xcb_window_t win, xcb
     // could consider next request to be still part of this timed out request
     setRequestor(0);
 
-    return QByteArray();
+    return std::nullopt;
 }
 
-QByteArray QXcbClipboard::getDataInFormat(xcb_atom_t modeAtom, xcb_atom_t fmtAtom)
+std::optional<QByteArray> QXcbClipboard::getDataInFormat(xcb_atom_t modeAtom, xcb_atom_t fmtAtom)
 {
     return getSelection(modeAtom, fmtAtom, atom(QXcbAtom::Atom_QT_SELECTION));
 }
 
-QByteArray QXcbClipboard::getSelection(xcb_atom_t selection, xcb_atom_t target, xcb_atom_t property, xcb_timestamp_t time)
+std::optional<QByteArray> QXcbClipboard::getSelection(xcb_atom_t selection, xcb_atom_t target, xcb_atom_t property, xcb_timestamp_t time)
 {
-    QByteArray buf;
     xcb_window_t win = requestor();
 
     if (time == 0) time = connection()->time();
@@ -866,17 +869,19 @@ QByteArray QXcbClipboard::getSelection(xcb_atom_t selection, xcb_atom_t target,
     free(ge);
 
     if (no_selection)
-        return buf;
+        return std::nullopt;
 
     xcb_atom_t type;
+    QByteArray buf;
     if (clipboardReadProperty(win, property, true, &buf, nullptr, &type, nullptr)) {
         if (type == atom(QXcbAtom::AtomINCR)) {
             int nbytes = buf.size() >= 4 ? *((int*)buf.data()) : 0;
-            buf = clipboardReadIncrementalProperty(win, property, nbytes, false);
+            return clipboardReadIncrementalProperty(win, property, nbytes, false);
         }
+        return buf;
     }
 
-    return buf;
+    return std::nullopt;
 }
 
 #endif // QT_NO_CLIPBOARD

src/plugins/platforms/xcb/qxcbclipboard.h

@@ -67,13 +67,13 @@ public:
     void handleXFixesSelectionRequest(xcb_xfixes_selection_notify_event_t *event);
 
     bool clipboardReadProperty(xcb_window_t win, xcb_atom_t property, bool deleteProperty, QByteArray *buffer, int *size, xcb_atom_t *type, int *format);
-    QByteArray clipboardReadIncrementalProperty(xcb_window_t win, xcb_atom_t property, int nbytes, bool nullterm);
+    std::optional<QByteArray> clipboardReadIncrementalProperty(xcb_window_t win, xcb_atom_t property, int nbytes, bool nullterm);
 
-    QByteArray getDataInFormat(xcb_atom_t modeAtom, xcb_atom_t fmtatom);
+    std::optional<QByteArray> getDataInFormat(xcb_atom_t modeAtom, xcb_atom_t fmtatom);
 
     bool handlePropertyNotify(const xcb_generic_event_t *event);
 
-    QByteArray getSelection(xcb_atom_t selection, xcb_atom_t target, xcb_atom_t property, xcb_timestamp_t t = 0);
+    std::optional<QByteArray> getSelection(xcb_atom_t selection, xcb_atom_t target, xcb_atom_t property, xcb_timestamp_t t = 0);
 
     int increment() const { return m_maxPropertyRequestDataBytes; }
     int clipboardTimeout() const { return clipboard_timeout; }

src/plugins/platforms/xcb/qxcbdrag.cpp

@@ -1306,32 +1306,33 @@ QVariant QXcbDropData::retrieveData_sys(const QString &mimetype, QMetaType reque
 
 QVariant QXcbDropData::xdndObtainData(const QByteArray &format, QMetaType requestedType) const
 {
-    QByteArray result;
-
     QXcbConnection *c = drag->connection();
     QXcbWindow *xcb_window = c->platformWindowFromId(drag->xdnd_dragsource);
     if (xcb_window && drag->currentDrag() && xcb_window->window()->type() != Qt::Desktop) {
         QMimeData *data = drag->currentDrag()->mimeData();
         if (data->hasFormat(QLatin1StringView(format)))
-            result = data->data(QLatin1StringView(format));
-        return result;
+            return data->data(QLatin1StringView(format));
+        return QVariant();
     }
 
     QList<xcb_atom_t> atoms = drag->xdnd_types;
     bool hasUtf8 = false;
     xcb_atom_t a = mimeAtomForFormat(c, QLatin1StringView(format), requestedType, atoms, &hasUtf8);
     if (a == XCB_NONE)
-        return result;
+        return QVariant();
 
 #ifndef QT_NO_CLIPBOARD
     if (c->selectionOwner(c->atom(QXcbAtom::AtomXdndSelection)) == XCB_NONE)
-        return result; // should never happen?
+        return QVariant(); // should never happen?
 
     xcb_atom_t xdnd_selection = c->atom(QXcbAtom::AtomXdndSelection);
-    result = c->clipboard()->getSelection(xdnd_selection, a, xdnd_selection, drag->targetTime());
+    const std::optional<QByteArray> result = c->clipboard()->getSelection(xdnd_selection, a, xdnd_selection, drag->targetTime());
+    if (!result.has_value())
+        return QVariant();
+    return mimeConvertToFormat(c, a, result.value(), QLatin1StringView(format), requestedType, hasUtf8);
+#else
+    return QVariant();
 #endif
-
-    return mimeConvertToFormat(c, a, result, QLatin1StringView(format), requestedType, hasUtf8);
 }
 
 bool QXcbDropData::hasFormat_sys(const QString &format) const