QXmlStreamReader: fix addData() unnecessary conversion to UTF-8

The addData(QASV) overload was unconditionally converting UTF-16 and
Latin1 data to UTF-8.
However, if we already started reading the XML document, and we know
that its encoding is UTF-16 or Latin1, then we know for sure that the
new data has to be added as-is.

Amends 6bc227a06a0d1392d220aa79ddb1cdc145d4f76e.

[ChangeLog][QtCore][QXmlStreamReader] Fixed a bug when
addData(QAnyStringView) was incorrectly recoding UTF-16 and Latin1
data to UTF-8, thus potentially mangling it.

Fixes: QTBUG-135129
Pick-to: 6.8 6.5
Change-Id: Ie1171a5e5596b72a6f160031a4c5a9df3baae4fd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit b6b725aef59390f403a1a39f49d1318c48f13c07)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/serialization/qxmlstream.cpp

@@ -553,6 +553,15 @@ QIODevice *QXmlStreamReader::device() const
     \sa readNext(), clear()
 */
 
+static bool isDecoderForEncoding(const QStringDecoder &dec, QStringDecoder::Encoding enc)
+{
+    if (!dec.isValid())
+        return false;
+
+    const QAnyStringView nameView{dec.name()};
+    return !nameView.empty() && nameView == QStringDecoder::nameForEncoding(enc);
+}
+
 /*!
     Adds more \a data for the reader to read. This function does
     nothing if the reader has a device().
@@ -567,11 +576,25 @@ void QXmlStreamReader::addData(QAnyStringView data)
     Q_D(QXmlStreamReader);
     data.visit([this, d](auto data) {
         if constexpr (std::is_same_v<decltype(data), QStringView>) {
+            if (d->lockEncoding && isDecoderForEncoding(d->decoder, QStringDecoder::Utf16)) {
+                // We already expect the data in the proper encoding, no need
+                // to recode the data.
+                addDataImpl(QByteArray{reinterpret_cast<const char *>(data.utf16()),
+                                       data.size() * 2});
+                return;
+            }
+            // keep the pre-existing behavior
             d->lockEncoding = true;
             if (!d->decoder.isValid())
                 d->decoder = QStringDecoder(QStringDecoder::Utf8);
             addDataImpl(data.toUtf8());
         } else if constexpr (std::is_same_v<decltype(data), QLatin1StringView>) {
+            if (d->lockEncoding && isDecoderForEncoding(d->decoder, QStringDecoder::Latin1)) {
+                // We already expect the data in the proper encoding, no need
+                // to recode the data.
+                addDataImpl(QByteArray{data.data(), data.size()});
+                return;
+            }
             // Conversion to a QString is required, to avoid breaking
             // pre-existing (before porting to QAnyStringView) behavior.
             d->lockEncoding = true;

tests/auto/corelib/serialization/qxmlstream/tst_qxmlstream.cpp

@@ -1275,7 +1275,6 @@ void tst_QXmlStream::appendToRawDocumentWithNonUtf8Encoding_data()
         QStringConverter::Utf16, u"M\u00E5rten"_s);
     row("l1+utf8", "iso-8859-1"_ba, "M\xE5rten"_ba, QString::fromLatin1("M\xE5rten"),
         QStringConverter::Utf8, QString::fromUtf8("M\xC3\xA5rten"));
-    // Even this fails, because we internally convert the second L1 to UTF-8!
     row("l1+l1", "iso-8859-1"_ba, "M\xE5rten"_ba, QString::fromLatin1("M\xE5rten"),
         QStringConverter::Latin1, QString::fromLatin1("M\xE5rten"));
 
@@ -1317,11 +1316,14 @@ void tst_QXmlStream::appendToRawDocumentWithNonUtf8Encoding()
     default:
         Q_UNREACHABLE();
     }
-    QEXPECT_FAIL("utf16+utf16", "QTBUG-135129: Parser expected UTF-16, but got UTF-8", Abort);
     QVERIFY(reader.readNextStartElement()); // a
     text = reader.readElementText();
 
-    QEXPECT_FAIL("", "Parser expects the data in the initial encoding, but we convert to UTF-8",
+    QEXPECT_FAIL("l1+utf16",
+                 "Parser expects the data in the initial encoding, but we convert to UTF-8",
+                 Continue);
+    QEXPECT_FAIL("l1+utf8",
+                 "Parser expects the data in the initial encoding, but we convert to UTF-8",
                  Continue);
     QCOMPARE(text, expectedNextElementText);
 }