QDateTime - Fix round-trip of second occurrence times

At the Daylight Tme to Standard Time transition, the local time repeats
itself, i.e. 2am occurs twice. Qt's behavior when setting this using
the local time is ambiguous, as it depends on the system implementation
of mktime, which behaves differently on different platforms.  Currently
this behavior remains undefined.  When setting using an msecs or time_t
value however we can determine the correct instance to use and cache it
to ensure that any conversion back from local time to msecs is performed
consistantly on all platforms.

Note that caching this value will result in any calculations being wrong
should the system time zone change, or its rules change.  This will be
fixed in Qt 5.3 when the system time zone change signal is implemented
and QDateTime switches to using QTimeZone instead of mktime to provide
consistnt behavior across platforms.

The QTimeZone spec does not require this fix as it already caches the
correct offset in setMSecsFromEpoch().

Change-Id: I799588db474e744a6d81e80f6a0442920569ebd3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 44 insertions, 10 deletions

diff --git a/src/corelib/tools/qdatetime.cpp b/src/corelib/tools/qdatetime.cpp
index d64d929d5a..4839560c51 100644
--- a/src/corelib/tools/qdatetime.cpp
+++ b/src/corelib/tools/qdatetime.cpp
@@ -2224,7 +2224,7 @@ static QString qt_tzname(QDateTimePrivate::DaylightStatus daylightStatus)
 #endif // Q_OS_WINCE
 }
 
-// Calls the platform variant of mktime for the given date and time,
+// Calls the platform variant of mktime for the given date, time and daylightStatus,
 // and updates the date, time, daylightStatus and abbreviation with the returned values
 // If the date falls outside the 1970 to 2037 range supported by mktime / time_t
 // then null date/time will be returned, you should adjust the date first if
@@ -2288,7 +2288,10 @@ static qint64 qt_mktime(QDate *date, QTime *time, QDateTimePrivate::DaylightStat
     local.tm_year = yy - 1900;
     local.tm_wday = 0;
     local.tm_yday = 0;
-    local.tm_isdst = -1;
+    if (daylightStatus)
+        local.tm_isdst = int(*daylightStatus);
+    else
+        local.tm_isdst = -1;
 #if defined(Q_OS_WIN)
     int hh = local.tm_hour;
 #endif // Q_OS_WIN
@@ -2481,7 +2484,7 @@ static bool epochMSecsToLocalTime(qint64 msecs, QDate *localDate, QTime *localTi
 
 // Convert a LocalTime expressed in local msecs encoding into a UTC epoch msecs
 // Optionally populate the returned values from mktime for the adjusted local
-// date and time and daylight status
+// date and time and daylight status.  Uses daylightStatus in calculation if populated.
 static qint64 localMSecsToEpochMSecs(qint64 localMsecs, QDate *localDate = 0, QTime *localTime = 0,
                                      QDateTimePrivate::DaylightStatus *daylightStatus = 0,
                                      QString *abbreviation = 0, bool *ok = 0)
@@ -2614,6 +2617,7 @@ void QDateTimePrivate::setTimeSpec(Qt::TimeSpec spec, int offsetSeconds)
 {
     clearValidDateTime();
     clearTimeZoneCached();
+    clearSetToDaylightStatus();
 
 #ifndef QT_BOOTSTRAPPED
     m_timeZone = QTimeZone();
@@ -2688,6 +2692,30 @@ void QDateTimePrivate::getDateTime(QDate *date, QTime *time) const
         *time = QTime();
 }
 
+// Set the Daylight Status if LocalTime set via msecs
+void QDateTimePrivate::setDaylightStatus(QDateTimePrivate::DaylightStatus status)
+{
+    if (status == DaylightTime) {
+        m_status = m_status & ~SetToStandardTime;
+        m_status = m_status | SetToDaylightTime;
+    } else if (status == StandardTime) {
+        m_status = m_status & ~SetToDaylightTime;
+        m_status = m_status | SetToStandardTime;
+    } else {
+        clearSetToDaylightStatus();
+    }
+}
+
+// Get the Daylight Status if LocalTime set via msecs
+QDateTimePrivate::DaylightStatus QDateTimePrivate::daylightStatus() const
+{
+    if ((m_status & SetToDaylightTime) == SetToDaylightTime)
+        return DaylightTime;
+    if ((m_status & SetToStandardTime) == SetToStandardTime)
+        return StandardTime;
+    return UnknownDaylightTime;
+}
+
 // Check the UTC / offsetFromUTC validity
 void QDateTimePrivate::checkValidDateTime()
 {
@@ -2748,12 +2776,14 @@ void QDateTimePrivate::refreshDateTime()
     QDate testDate;
     QTime testTime;
     qint64 epochMSecs = 0;
+    if (m_spec == Qt::LocalTime) {
+        DaylightStatus status = daylightStatus();
+        epochMSecs = localMSecsToEpochMSecs(m_msecs, &testDate, &testTime, &status);
 #ifndef QT_BOOTSTRAPPED
-    if (m_spec == Qt::TimeZone)
+    } else {
         epochMSecs = zoneMSecsToEpochMSecs(m_msecs, m_timeZone, &testDate, &testTime);
-    else
 #endif // QT_BOOTSTRAPPED
-        epochMSecs = localMSecsToEpochMSecs(m_msecs, &testDate, &testTime);
+    }
     if (testDate == date && testTime == time) {
         setValidDateTime();
         // Cache the offset to use in toMSecsSinceEpoch()
@@ -3189,7 +3219,8 @@ QString QDateTime::timeZoneAbbreviation() const
 #endif // QT_BOOTSTRAPPED
     case Qt::LocalTime:  {
         QString abbrev;
-        localMSecsToEpochMSecs(d->m_msecs, 0, 0, 0, &abbrev);
+        QDateTimePrivate::DaylightStatus status = d->daylightStatus();
+        localMSecsToEpochMSecs(d->m_msecs, 0, 0, &status, &abbrev);
         return abbrev;
         }
     }
@@ -3218,8 +3249,9 @@ bool QDateTime::isDaylightTime() const
         return d->m_timeZone.d->isDaylightTime(toMSecsSinceEpoch());
 #endif // QT_BOOTSTRAPPED
     case Qt::LocalTime: {
-        QDateTimePrivate::DaylightStatus status;
-        localMSecsToEpochMSecs(d->m_msecs, 0, 0, &status, 0);
+        QDateTimePrivate::DaylightStatus status = d->daylightStatus();
+        if (status == QDateTimePrivate::UnknownDaylightTime)
+            localMSecsToEpochMSecs(d->m_msecs, 0, 0, &status, 0);
         return (status == QDateTimePrivate::DaylightTime);
         }
     }
@@ -3424,8 +3456,10 @@ void QDateTime::setMSecsSinceEpoch(qint64 msecs)
     case Qt::LocalTime: {
         QDate dt;
         QTime tm;
-        epochMSecsToLocalTime(msecs, &dt, &tm);
+        QDateTimePrivate::DaylightStatus status;
+        epochMSecsToLocalTime(msecs, &dt, &tm, &status);
         d->setDateTime(dt, tm);
+        d->setDaylightStatus(status);
         break;
         }
     }