QXmlStreamWriter: fix a thinko in doWriteToDevice(QSV)

Check for state.remainingChars to signal an encoding error only after
the last chunk has been processed. Splitting surrogates at chunk
boundaries is normal operation, not an error. Only if this happens at
the end of the whole input should we raise an error.

Amends fa2153bd10057d7adbc5f5ededa1fd97c4a68161.

Change-Id: Id92e37becaed25bbc11e0c22dedc4d41fb23f92a
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 4ccb10dc918c3d4d01afa1ad718a24eb5fa49a3c)
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/serialization/qxmlstream.cpp

@@ -3069,13 +3069,11 @@ void QXmlStreamWriterPrivate::doWriteToDevice(QStringView s)
     while (!s.isEmpty()) {
         const qsizetype chunkSize = std::min(s.size(), MaxChunkSize);
         char *end = QUtf8::convertFromUnicode(buffer, s.first(chunkSize), &state);
-        if (state.remainingChars > 0) {
-            hasEncodingError = true;
-            return;
-        }
         doWriteToDevice(QUtf8StringView{buffer, end});
         s = s.sliced(chunkSize);
     }
+    if (state.remainingChars > 0)
+        hasEncodingError = true;
 }
 
 void QXmlStreamWriterPrivate::doWriteToDevice(QUtf8StringView s)

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

@@ -1695,16 +1695,23 @@ void tst_QXmlStream::readBack() const
 {
     QFETCH(const int, plane);
 
+    constexpr qsizetype MaxChunkSizeWhenEncoding = 512; // from qxmlstream.cpp
     QBuffer buffer;
+    QString text = QString(513, 'a'); // one longer than the internal conversion buffer
 
     for (char16_t i = 0; i < (std::numeric_limits<char16_t>::max)(); ++i) {
 
         const char32_t c = (plane << 16) + i;
 
+        // end chunk in invalid character, split surrogates:
+        const auto pair = QChar::fromUcs4(c);
+        text.resize(MaxChunkSizeWhenEncoding + 1 - pair.size());
+        text += pair;
+
         QVERIFY(buffer.open(QIODevice::WriteOnly|QIODevice::Truncate));
         QXmlStreamWriter writer(&buffer);
         writer.writeStartDocument();
-        writer.writeTextElement("a", c);
+        writer.writeTextElement("a", text);
         writer.writeEndDocument();
         buffer.close();