Fix race condition in QDateTime::timeZone() and other methods

When timezone support for QDateTime was added, we decided it was a good
idea to delay creating the QTimeZone object and checking that the time
is valid in that timezone (including for local time) until the user
requested that information. Unfortunately, QExplicitlySharedDataPointer
returns a non-const T* in operator->(), which meant we were accidentally
modifying the d pointer's contents in const methods, which in turn means
those const methods were not thread-safe when operating on the same
object.

This commit changes the d pointer to QSharedDataPointer, which is safer
in this regard and pointed out where the issues with constness were
located. Since we can't lazily calculate QTimeZone anymore, we need to
do it whenever the date, time or offset changes.

Task-number: QTBUG-43703
Change-Id: Ic5d393bfd36e48a193fcffff13b9686ef4ef1454
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

4 files changed, 89 insertions, 35 deletions

diff --git a/src/corelib/tools/qdatetime.cpp b/src/corelib/tools/qdatetime.cpp
index eeefea8137..e5fbf5af5e 100644
--- a/src/corelib/tools/qdatetime.cpp
+++ b/src/corelib/tools/qdatetime.cpp
@@ -2607,7 +2607,6 @@ QDateTimePrivate::QDateTimePrivate(const QDate &toDate, const QTime &toTime,
 void QDateTimePrivate::setTimeSpec(Qt::TimeSpec spec, int offsetSeconds)
 {
     clearValidDateTime();
-    clearTimeZoneCached();
     clearSetToDaylightStatus();
 
 #ifndef QT_BOOTSTRAPPED
@@ -2707,27 +2706,44 @@ QDateTimePrivate::DaylightStatus QDateTimePrivate::daylightStatus() const
     return UnknownDaylightTime;
 }
 
+qint64 QDateTimePrivate::toMSecsSinceEpoch() const
+{
+    switch (m_spec) {
+    case Qt::OffsetFromUTC:
+    case Qt::UTC:
+        return (m_msecs - (m_offsetFromUtc * 1000));
+
+    case Qt::LocalTime:
+        // recalculate the local timezone
+        return localMSecsToEpochMSecs(m_msecs);
+
+    case Qt::TimeZone:
+#ifndef QT_BOOTSTRAPPED
+        return zoneMSecsToEpochMSecs(m_msecs, m_timeZone);
+#endif
+        break;
+    }
+    Q_UNREACHABLE();
+    return 0;
+}
+
 // Check the UTC / offsetFromUTC validity
 void QDateTimePrivate::checkValidDateTime()
 {
     switch (m_spec) {
     case Qt::OffsetFromUTC:
     case Qt::UTC:
+        // for these, a valid date and a valid time imply a valid QDateTime
         if (isValidDate() && isValidTime())
             setValidDateTime();
         else
             clearValidDateTime();
         break;
     case Qt::TimeZone:
-        // Defer checking until required as can be expensive
-        clearValidDateTime();
-        clearTimeZoneCached();
-        m_offsetFromUtc = 0;
-        break;
     case Qt::LocalTime:
-        // Defer checking until required as can be expensive
-        clearValidDateTime();
-        m_offsetFromUtc = 0;
+        // for these, we need to check whether the timezone is valid and whether
+        // the time is valid in that timezone. Expensive, but no other option.
+        refreshDateTime();
         break;
     }
 }
@@ -2741,12 +2757,6 @@ void QDateTimePrivate::refreshDateTime()
         // Always set by setDateTime so just return
         return;
     case Qt::TimeZone:
-        // If already cached then don't need to refresh as tz won't change
-        if (isTimeZoneCached())
-            return;
-        // Flag that will have a cached result after calculations
-        setTimeZoneCached();
-        break;
     case Qt::LocalTime:
         break;
     }
@@ -3082,7 +3092,6 @@ bool QDateTime::isNull() const
 
 bool QDateTime::isValid() const
 {
-    d->refreshDateTime();
     return (d->isValidDateTime());
 }
 
@@ -3143,13 +3152,11 @@ Qt::TimeSpec QDateTime::timeSpec() const
 QTimeZone QDateTime::timeZone() const
 {
     switch (d->m_spec) {
-    case Qt::OffsetFromUTC:
-        if (!d->m_timeZone.isValid())
-            d->m_timeZone = QTimeZone(d->m_offsetFromUtc);
-        return d->m_timeZone;
     case Qt::UTC:
         return QTimeZone::utc();
+    case Qt::OffsetFromUTC:
     case Qt::TimeZone:
+        Q_ASSERT(d->m_timeZone.isValid());
         return d->m_timeZone;
     case Qt::LocalTime:
         return QTimeZone::systemTimeZone();
@@ -3178,7 +3185,6 @@ QTimeZone QDateTime::timeZone() const
 
 int QDateTime::offsetFromUtc() const
 {
-    d->refreshDateTime();
     return d->m_offsetFromUtc;
 }
 
@@ -3350,7 +3356,7 @@ void QDateTime::setTimeZone(const QTimeZone &toZone)
     d->m_spec = Qt::TimeZone;
     d->m_offsetFromUtc = 0;
     d->m_timeZone = toZone;
-    d->m_status = d->m_status & ~QDateTimePrivate::ValidDateTime & ~QDateTimePrivate::TimeZoneCached;
+    d->refreshDateTime();
 }
 #endif // QT_BOOTSTRAPPED
 
@@ -3371,7 +3377,6 @@ void QDateTime::setTimeZone(const QTimeZone &toZone)
 */
 qint64 QDateTime::toMSecsSinceEpoch() const
 {
-    d->refreshDateTime();
     return d->toMSecsSinceEpoch();
 }
 
@@ -3453,8 +3458,8 @@ void QDateTime::setMSecsSinceEpoch(qint64 msecs)
         d->m_status = d->m_status
                     | QDateTimePrivate::ValidDate
                     | QDateTimePrivate::ValidTime
-                    | QDateTimePrivate::ValidDateTime
-                    | QDateTimePrivate::TimeZoneCached;
+                    | QDateTimePrivate::ValidDateTime;
+        d->refreshDateTime();
 #endif // QT_BOOTSTRAPPED
         break;
     case Qt::LocalTime: {
diff --git a/src/corelib/tools/qdatetime.h b/src/corelib/tools/qdatetime.h
index 784aced71a..88288872df 100644
--- a/src/corelib/tools/qdatetime.h
+++ b/src/corelib/tools/qdatetime.h
@@ -326,7 +326,7 @@ private:
     // ### Qt6: Using a private here has high impact on runtime
     // on users such as QFileInfo. In Qt 6, the data members
     // should be inlined.
-    QExplicitlySharedDataPointer<QDateTimePrivate> d;
+    QSharedDataPointer<QDateTimePrivate> d;
 
 #ifndef QT_NO_DATASTREAM
     friend Q_CORE_EXPORT QDataStream &operator<<(QDataStream &, const QDateTime &);
diff --git a/src/corelib/tools/qdatetime_p.h b/src/corelib/tools/qdatetime_p.h
index a139390a9d..b8934f7f70 100644
--- a/src/corelib/tools/qdatetime_p.h
+++ b/src/corelib/tools/qdatetime_p.h
@@ -79,10 +79,9 @@ public:
     enum StatusFlag {
         NullDate            = 0x01,
         NullTime            = 0x02,
-        ValidDate           = 0x04,
-        ValidTime           = 0x08,
-        ValidDateTime       = 0x10,
-        TimeZoneCached      = 0x20,
+        ValidDate           = 0x04, // just the date field
+        ValidTime           = 0x08, // just the time field
+        ValidDateTime       = 0x10, // the whole object (including timezone)
         SetToStandardTime   = 0x40,
         SetToDaylightTime   = 0x80
     };
@@ -120,7 +119,7 @@ public:
     DaylightStatus daylightStatus() const;
 
     // Returns msecs since epoch, assumes offset value is current
-    inline qint64 toMSecsSinceEpoch() const { return (m_msecs - (m_offsetFromUtc * 1000)); }
+    inline qint64 toMSecsSinceEpoch() const;
 
     void checkValidDateTime();
     void refreshDateTime();
@@ -133,14 +132,11 @@ public:
     inline bool isValidDateTime() const { return m_status & ValidDateTime; }
     inline void setValidDateTime() { m_status |= ValidDateTime; }
     inline void clearValidDateTime() { m_status &= ~ValidDateTime; }
-    inline bool isTimeZoneCached() const { return m_status & TimeZoneCached; }
-    inline void setTimeZoneCached() { m_status |= TimeZoneCached; }
-    inline void clearTimeZoneCached() { m_status &= ~TimeZoneCached; }
     inline void clearSetToDaylightStatus() { m_status &= ~(SetToStandardTime | SetToDaylightTime); }
 
 #ifndef QT_BOOTSTRAPPED
     static qint64 zoneMSecsToEpochMSecs(qint64 msecs, const QTimeZone &zone,
-                                        QDate *localDate, QTime *localTime);
+                                        QDate *localDate = 0, QTime *localTime = 0);
 #endif // QT_BOOTSTRAPPED
 
     static inline qint64 minJd() { return QDate::minJd(); }
diff --git a/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp b/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
index c6806086d3..446e56e936 100644
--- a/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
+++ b/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
@@ -142,6 +142,9 @@ private slots:
     void isDaylightTime() const;
     void daylightTransitions() const;
     void timeZones() const;
+#if defined(Q_OS_UNIX)
+    void systemTimeZoneChange() const;
+#endif
 
     void invalid() const;
 
@@ -2984,6 +2987,56 @@ void tst_QDateTime::timeZones() const
     QCOMPARE(future.offsetFromUtc(), 28800);
 }
 
+#if defined(Q_OS_UNIX)
+// Currently disabled on Windows as adjusting the timezone
+// requires additional privileges that aren't normally
+// enabled for a process. This can be achieved by calling
+// AdjustTokenPrivileges() and then SetTimeZoneInformation(),
+// which will require linking to a different library to access that API.
+static void setTimeZone(const QByteArray &tz)
+{
+    qputenv("TZ", tz);
+    ::tzset();
+
+// following left for future reference, see comment above
+// #if defined(Q_OS_WIN32)
+//     ::_tzset();
+// #endif
+}
+
+void tst_QDateTime::systemTimeZoneChange() const
+{
+    struct ResetTZ {
+        QByteArray original;
+        ResetTZ() : original(qgetenv("TZ")) {}
+        ~ResetTZ() { setTimeZone(original); }
+    } scopedReset;
+
+    // Set the timezone to Brisbane time
+    setTimeZone(QByteArray("AEST-10:00"));
+
+    QDateTime localDate = QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), Qt::LocalTime);
+    QDateTime utcDate = QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), Qt::UTC);
+    QDateTime tzDate = QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), QTimeZone("Australia/Brisbane"));
+    qint64 localMsecs = localDate.toMSecsSinceEpoch();
+    qint64 utcMsecs = utcDate.toMSecsSinceEpoch();
+    qint64 tzMsecs = tzDate.toMSecsSinceEpoch();
+
+    // check that Australia/Brisbane is known
+    QVERIFY(tzDate.timeZone().isValid());
+
+    // Change to Indian time
+    setTimeZone(QByteArray("IST-05:30"));
+
+    QCOMPARE(localDate, QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), Qt::LocalTime));
+    QVERIFY(localMsecs != localDate.toMSecsSinceEpoch());
+    QCOMPARE(utcDate, QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), Qt::UTC));
+    QCOMPARE(utcDate.toMSecsSinceEpoch(), utcMsecs);
+    QCOMPARE(tzDate, QDateTime(QDate(2012, 6, 1), QTime(2, 15, 30), QTimeZone("Australia/Brisbane")));
+    QCOMPARE(tzDate.toMSecsSinceEpoch(), tzMsecs);
+}
+#endif
+
 void tst_QDateTime::invalid() const
 {
     QDateTime invalidDate = QDateTime(QDate(0, 0, 0), QTime(-1, -1, -1));