QCborValue: make QDateTime constructor consistent with fromCbor

The QCborValue(const QDateTime &) constructor unconditionally set the
type to QCborValue::DateTime. For a null QDateTime, toString() yields an
empty string, so the value was encoded as tag 0 wrapping an empty text
string, 0(""). An empty string is not a valid date/time. RFC 3339's ABNF
requires both a date and a time:

    iso-date-time     = date "T" time

and QDateTime::fromString() agrees by returning an invalid QDateTime.

If we tried decoding that with fromCbor(), it would correctly fail,
leaving type() to Tag and breaking equality over toCbor()/fromCbor()
round-trip (though the tagged values still compared equal).

Rather than keep two notions of what a DateTime is, let's add a central
function to set it. A valid QDateTime is promoted to DateTime as before;
a null one stays a plain Tag(0, ""), matching what fromCbor() produces.

Fixes: QTBUG-149559
Pick-to: 6.12 6.11 6.8
Change-Id: I3502380cd23dbc867fdefffda35f95940c6e0091
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

Author: thiagomacieira

src/corelib/serialization/qcborvalue.cpp

@@ -752,6 +752,7 @@ Q_DECL_UNUSED static constexpr quint64 MaximumPreallocatedElementCount =
  */
 
 using namespace QtCbor;
+static QCborValue::Type setToExtendedDateTimeType(QCborContainerPrivate *d, QStringView dtString);
 
 static QCborContainerPrivate *assignContainer(QCborContainerPrivate *&d, QCborContainerPrivate *x)
 {
@@ -798,15 +799,8 @@ static QCborValue::Type convertToExtendedType(QCborContainerPrivate *d)
             if (ok)
                 dt = QDateTime::fromMSecsSinceEpoch(msecs, QTimeZone::UTC);
         }
-        if (dt.isValid()) {
-            QByteArray text = dt.toString(Qt::ISODateWithMs).toLatin1();
-            if (!text.isEmpty()) {
-                replaceByteData(text, text.size(), Element::StringIsAscii);
-                e.type = QCborValue::String;
-                d->elements[0].value = qint64(QCborKnownTags::DateTimeString);
-                return QCborValue::DateTime;
-            }
-        }
+        if (QString dtString = dt.toString(Qt::ISODateWithMs); !dtString.isEmpty())
+            return setToExtendedDateTimeType(d, dtString);
         break;
     }
 #endif
@@ -2086,6 +2080,18 @@ QCborValue::QCborValue(const QCborValue &other) noexcept
 }
 
 #if QT_CONFIG(datestring)
+static QCborValue::Type setToExtendedDateTimeType(QCborContainerPrivate *d, QStringView text)
+{
+    // when called from convertToExtendedType(), *d isn't pristine
+    d->data.resize(0);
+    d->elements.resize(0);
+    d->elements.reserve(2);
+
+    d->append(QCborTag(QCborKnownTags::DateTimeString));
+    d->appendAsciiString(text);
+    return text.isEmpty() ? QCborValue::Tag : QCborValue::DateTime;
+}
+
 /*!
     Creates a QCborValue object of the date/time extended type and containing
     the value represented by \a dt. The value can later be retrieved using
@@ -2100,11 +2106,10 @@ QCborValue::QCborValue(const QCborValue &other) noexcept
     \sa toDateTime(), isDateTime(), taggedValue()
  */
 QCborValue::QCborValue(const QDateTime &dt)
-    : QCborValue(QCborKnownTags::DateTimeString, dt.toString(Qt::ISODateWithMs).toLatin1())
+    : n(-1), container(new QCborContainerPrivate),
+      t(setToExtendedDateTimeType(container, dt.toString(Qt::ISODateWithMs)))
 {
-    // change types
-    t = DateTime;
-    container->elements[1].type = String;
+    container->ref.storeRelaxed(1);
 }
 #endif
 

tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp

@@ -2417,6 +2417,9 @@ static void addCommonCborData()
     QTest::newRow("String:Unicode") << QCborValue(QStringLiteral(u"éś α €"))
                                      << raw("\x6b\xc3\xa9\xc5\x9b \xce\xb1 \xe2\x82\xac") << noxfrm;
 
+    QTest::newRow("DateTime:Empty") << QCborValue(QDateTime())
+                                    << raw("\xc0\x60")
+                                    << noxfrm;
     QTest::newRow("DateTime") << QCborValue(dt)             // this is UTC
                               << "\xc0\x78\x18" + dt.toString(Qt::ISODateWithMs).toLatin1()
                               << noxfrm;
@@ -2771,7 +2774,7 @@ void tst_QCborValue::extendedTypeValidation_data()
     QTest::addColumn<QCborValue>("expected");
 
     // QDateTime currently stores time in milliseconds, so make sure
-    // we don't overflow
+    // we don't overflow and therefore keep the QCborValue as UnixTime_t.
     {
         quint64 limit = std::numeric_limits<quint64>::max() / 1000;
         QTest::newRow("UnixTime_t:integer-overflow-positive")