Revert QDateTime serialisation to pre-Qt 5 behaviour.

In Qt 5, I managed to break the guarantee that a deserialised local
datetime is the same time of day (potentially different UTC time),
regardless of which timezone it was serialised in. This happened after
I fixed QTBUG-4057 with If650e7960dca7b6ab44b8233410a6369c41df73a,
which serialised datetimes as UTC.

This patch reverts QDateTime serialisation to pre-Qt 5 behaviour to
restore the guarantee and consequently re-opens QTBUG-4057.

Change-Id: Iea877f7ed886f530b928067789b53534e89fe8cb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

dist/changes-5.1.0

@@ -41,6 +41,14 @@ Third party components
   * [QTBUG-8836] QAbstractItemView now allows manual deselect in
     SingleSelection mode (with control modifier)
 
+- QtCore
+
+  * The serialization behavior for QDateTime has reverted to pre-Qt 5,
+    due to the issue mentioned in section 4 here:
+    http://lists.qt-project.org/pipermail/development/2013-March/010559.html
+
+    This means that the QDataStream version will be 14 with Qt 5.1.
+
 ****************************************************************************
 *                          Library                                         *
 ****************************************************************************

src/corelib/io/qdatastream.cpp

@@ -539,7 +539,7 @@ void QDataStream::setByteOrder(ByteOrder bo)
     \value Qt_4_8 Same as Qt_4_6.
     \value Qt_4_9 Same as Qt_4_6.
     \value Qt_5_0 Version 13 (Qt 5.0)
-    \value Qt_5_1 Same as Qt_5_0.
+    \value Qt_5_1 Version 14 (Qt 5.1)
 
     \sa setVersion(), version()
 */
@@ -571,6 +571,7 @@ void QDataStream::setByteOrder(ByteOrder bo)
 
     \table
     \header \li Qt Version       \li QDataStream Version
+    \row \li Qt 5.1                  \li 14
     \row \li Qt 5.0                  \li 13
     \row \li Qt 4.6                  \li 12
     \row \li Qt 4.5                  \li 11

src/corelib/io/qdatastream.h

@@ -86,7 +86,7 @@ public:
         Qt_4_8 = Qt_4_7,
         Qt_4_9 = Qt_4_8,
         Qt_5_0 = 13,
-        Qt_5_1 = Qt_5_0
+        Qt_5_1 = 14
 #if QT_VERSION >= 0x050200
 #error Add the datastream version for this Qt version
 #endif

src/corelib/tools/qdatetime.cpp

@@ -3691,8 +3691,11 @@ QDataStream &operator>>(QDataStream &in, QTime &time)
 */
 QDataStream &operator<<(QDataStream &out, const QDateTime &dateTime)
 {
-    if (out.version() >= 13) {
+    if (out.version() == 13) {
         if (dateTime.isValid()) {
+            // This approach is wrong and should not be used again; it breaks
+            // the guarantee that a deserialised local datetime is the same time
+            // of day, regardless of which timezone it was serialised in.
             QDateTime asUTC = dateTime.toUTC();
             out << asUTC.d->date << asUTC.d->time;
         } else {
@@ -3721,7 +3724,7 @@ QDataStream &operator>>(QDataStream &in, QDateTime &dateTime)
 
     in >> dateTime.d->date >> dateTime.d->time;
 
-    if (in.version() >= 13) {
+    if (in.version() == 13) {
         qint8 ts = 0;
         in >> ts;
         if (dateTime.isValid()) {

tests/auto/corelib/io/qdatastream/tst_qdatastream.cpp

@@ -268,7 +268,8 @@ static int NColorRoles[] = {
     QPalette::ToolTipText + 1,     // Qt_4_5
     QPalette::ToolTipText + 1,     // Qt_4_6
     QPalette::ToolTipText + 1,     // Qt_5_0
-    0                              // add the correct value for Qt_5_1 here later
+    QPalette::ToolTipText + 1,     // Qt_5_1
+    0                              // add the correct value for Qt_5_2 here later
 };
 
 // Testing get/set functions

tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp

@@ -1398,13 +1398,19 @@ void tst_QDateTime::operator_insert_extract()
     {
         QDataStream dataStream(&byteArray, QIODevice::WriteOnly);
         dataStream.setVersion(dataStreamVersion);
-        if (dataStreamVersion >= QDataStream::Qt_5_0) {
+        if (dataStreamVersion == QDataStream::Qt_5_0) {
             // Qt 5 serialises as UTC and converts back to the stored timeSpec when
             // deserialising; we don't need to do it ourselves...
             dataStream << dateTime << dateTime;
         } else {
-            // ... but lower versions don't, so we have to here.
+            // ... but other versions don't, so we have to here.
             dataStream << dateTimeAsUTC << dateTimeAsUTC;
+            // We'll also make sure that a deserialised local datetime is the same
+            // time of day (potentially different UTC time), regardless of which
+            // timezone it was serialised in. E.g.: Tue Aug 14 08:00:00 2012
+            // serialised in WST should be deserialised as Tue Aug 14 08:00:00 2012
+            // HST.
+            dataStream << dateTime;
         }
     }
 
@@ -1420,7 +1426,7 @@ void tst_QDateTime::operator_insert_extract()
         QDateTime deserialised;
         dataStream >> deserialised;
 
-        if (dataStreamVersion >= QDataStream::Qt_5_0) {
+        if (dataStreamVersion == QDataStream::Qt_5_0) {
             // Ensure local time is still correct. Again, Qt 5 handles the timeSpec
             // conversion (in this case, UTC => LocalTime) for us when deserialising.
             QCOMPARE(deserialised, expectedLocalTime);
@@ -1453,6 +1459,14 @@ void tst_QDateTime::operator_insert_extract()
         QCOMPARE(deserialised, expectedLocalTime);
         // Sanity check UTC times.
         QCOMPARE(deserialised.toUTC(), expectedLocalTime.toUTC());
+
+        if (dataStreamVersion != QDataStream::Qt_5_0) {
+            // Deserialised local datetime should be the same time of day,
+            // regardless of which timezone it was serialised in.
+            QDateTime localDeserialized;
+            dataStream >> localDeserialized;
+            QCOMPARE(localDeserialized, dateTime);
+        }
     }
 
     qputenv("TZ", previousTimeZone.toLocal8Bit().constData());