Fix broken data for time-zones with no transitions

While an invalid time-zone shall have no transitions, so may various
constant zones, like UTC.  The TZ data may include only the POSIX rule
for such a zone, in which case we should use it, even if there are no
transitions.

Broke out a piece of repeated code as a common method, in the process,
since I was complicating it further.

Added test for the case that revealed this; and made sure we see a
warning if any of the checkOffset() tests gets skipped because its
zone is unsupported.

Fixes: QTBUG-74614
Change-Id: Ic8e039a2a9b3f4e0f567585682a94f4b494b558d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 23 insertions, 24 deletions

diff --git a/src/corelib/tools/qtimezoneprivate_p.h b/src/corelib/tools/qtimezoneprivate_p.h
index c9a5726216..24a9a00f11 100644
--- a/src/corelib/tools/qtimezoneprivate_p.h
+++ b/src/corelib/tools/qtimezoneprivate_p.h
@@ -331,6 +331,7 @@ public:
 
 private:
     void init(const QByteArray &ianaId);
+    QVector<QTimeZonePrivate::Data> getPosixTransitions(qint64 msNear) const;
 
     Data dataForTzTransition(QTzTransitionTime tran) const;
     QVector<QTzTransitionTime> m_tranTimes;
diff --git a/src/corelib/tools/qtimezoneprivate_tz.cpp b/src/corelib/tools/qtimezoneprivate_tz.cpp
index fa4ec031fa..f75a61977d 100644
--- a/src/corelib/tools/qtimezoneprivate_tz.cpp
+++ b/src/corelib/tools/qtimezoneprivate_tz.cpp
@@ -943,19 +943,21 @@ QTimeZonePrivate::Data QTzTimeZonePrivate::dataForTzTransition(QTzTransitionTime
     return data;
 }
 
-QTimeZonePrivate::Data QTzTimeZonePrivate::data(qint64 forMSecsSinceEpoch) const
+QVector<QTimeZonePrivate::Data> QTzTimeZonePrivate::getPosixTransitions(qint64 msNear) const
 {
-    // If we have no rules (so probably an invalid tz), return invalid data:
-    if (!m_tranTimes.size())
-        return invalidData();
+    const int year = QDateTime::fromMSecsSinceEpoch(msNear, Qt::UTC).date().year();
+    // The Data::atMSecsSinceEpoch of the single entry if zone is constant:
+    qint64 atTime = m_tranTimes.isEmpty() ? msNear : m_tranTimes.last().atMSecsSinceEpoch;
+    return calculatePosixTransitions(m_posixRule, year - 1, year + 1, atTime);
+}
 
-    // If the required time is after the last transition and we have a POSIX rule then use it
-    if (m_tranTimes.last().atMSecsSinceEpoch < forMSecsSinceEpoch
+QTimeZonePrivate::Data QTzTimeZonePrivate::data(qint64 forMSecsSinceEpoch) const
+{
+    // If the required time is after the last transition (or there were none)
+    // and we have a POSIX rule then use it:
+    if ((m_tranTimes.isEmpty() || m_tranTimes.last().atMSecsSinceEpoch < forMSecsSinceEpoch)
         && !m_posixRule.isEmpty() && forMSecsSinceEpoch >= 0) {
-        const int year = QDateTime::fromMSecsSinceEpoch(forMSecsSinceEpoch, Qt::UTC).date().year();
-        QVector<QTimeZonePrivate::Data> posixTrans =
-            calculatePosixTransitions(m_posixRule, year - 1, year + 1,
-                                      m_tranTimes.last().atMSecsSinceEpoch);
+        QVector<QTimeZonePrivate::Data> posixTrans = getPosixTransitions(forMSecsSinceEpoch);
         auto it = std::partition_point(posixTrans.cbegin(), posixTrans.cend(),
                                        [forMSecsSinceEpoch] (const QTimeZonePrivate::Data &at) {
                                            return at.atMSecsSinceEpoch <= forMSecsSinceEpoch;
@@ -986,13 +988,11 @@ bool QTzTimeZonePrivate::hasTransitions() const
 
 QTimeZonePrivate::Data QTzTimeZonePrivate::nextTransition(qint64 afterMSecsSinceEpoch) const
 {
-    // If the required time is after the last transition and we have a POSIX rule then use it
-    if (m_tranTimes.size() > 0 && m_tranTimes.last().atMSecsSinceEpoch < afterMSecsSinceEpoch
+    // If the required time is after the last transition (or there were none)
+    // and we have a POSIX rule then use it:
+    if ((m_tranTimes.isEmpty() || m_tranTimes.last().atMSecsSinceEpoch < afterMSecsSinceEpoch)
         && !m_posixRule.isEmpty() && afterMSecsSinceEpoch >= 0) {
-        const int year = QDateTime::fromMSecsSinceEpoch(afterMSecsSinceEpoch, Qt::UTC).date().year();
-        QVector<QTimeZonePrivate::Data> posixTrans =
-            calculatePosixTransitions(m_posixRule, year - 1, year + 1,
-                                      m_tranTimes.last().atMSecsSinceEpoch);
+        QVector<QTimeZonePrivate::Data> posixTrans = getPosixTransitions(afterMSecsSinceEpoch);
         auto it = std::partition_point(posixTrans.cbegin(), posixTrans.cend(),
                                        [afterMSecsSinceEpoch] (const QTimeZonePrivate::Data &at) {
                                            return at.atMSecsSinceEpoch <= afterMSecsSinceEpoch;
@@ -1011,21 +1011,19 @@ QTimeZonePrivate::Data QTzTimeZonePrivate::nextTransition(qint64 afterMSecsSince
 
 QTimeZonePrivate::Data QTzTimeZonePrivate::previousTransition(qint64 beforeMSecsSinceEpoch) const
 {
-    // If the required time is after the last transition and we have a POSIX rule then use it
-    if (m_tranTimes.size() > 0 && m_tranTimes.last().atMSecsSinceEpoch < beforeMSecsSinceEpoch
+    // If the required time is after the last transition (or there were none)
+    // and we have a POSIX rule then use it:
+    if ((m_tranTimes.isEmpty() || m_tranTimes.last().atMSecsSinceEpoch < beforeMSecsSinceEpoch)
         && !m_posixRule.isEmpty() && beforeMSecsSinceEpoch > 0) {
-        const int year = QDateTime::fromMSecsSinceEpoch(beforeMSecsSinceEpoch, Qt::UTC).date().year();
-        QVector<QTimeZonePrivate::Data> posixTrans =
-            calculatePosixTransitions(m_posixRule, year - 1, year + 1,
-                                      m_tranTimes.last().atMSecsSinceEpoch);
+        QVector<QTimeZonePrivate::Data> posixTrans = getPosixTransitions(beforeMSecsSinceEpoch);
         auto it = std::partition_point(posixTrans.cbegin(), posixTrans.cend(),
                                        [beforeMSecsSinceEpoch] (const QTimeZonePrivate::Data &at) {
                                            return at.atMSecsSinceEpoch < beforeMSecsSinceEpoch;
                                        });
         if (it > posixTrans.cbegin())
             return *--it;
-        // else: it fell between the last transition and the first of the POSIX rule.
-        return dataForTzTransition(m_tranTimes.last());
+        // It fell between the last transition (if any) and the first of the POSIX rule:
+        return m_tranTimes.isEmpty() ? invalidData() : dataForTzTransition(m_tranTimes.last());
     }
 
     // Otherwise if we can find a valid tran then use its rule