QDateTime, QTimeZone: fix mappings from zone time to UTC

Such mappings are ill-defined in the presence of daylight-savings time
(DST); at its transitions, you need information about whether DST is
active or not to determine the correct UTC value.  Existing code did
not have a way to be told that hint, so could not be correct.

Fixing this required changing the (thankfully private) APIs by which
QDateTime accessed QTimeZone's information stipulated by zone time.
In QDateTime, this required propagating the needed hint, when DST
status was known.

QAndroidTimeZonePrivate overloaded QTimeZonePrivate::dataForLocalTime
with an implementation that works whenever !hasTransitions(); the base
implementation handled this case lamely, so I've moved the Android
implementation there, to have only one place for both re-writes.

Amended tst_QDateTime's expected failures; passing a date and time to
the constructor *is* ambiguous when the moment indicated is in a
transition.  I have changed which way we resolve that ambiguity.

Added round-trip test of QDateTime's fromMSecs/toMSecs (but as a
QTimeZone test, since that's what's actually getting tested), based on
a test-case from Marko Kangas.  Initially failed for various zones,
each at one hour-offset; and, on some platforms, for some zones, at
all offsets.  These last revealed that a platform may claim to have
zone information yet, for some zones, lack it (or have very incomplete
information).  In each case, despite this, the platform does give
offsetFromUtc().  (The test also found another pre-existing bug on
Linux; fixed in an earlier commit.)

To accommodate these gaps in transition data, the transition-based
code now falls back to the offsetFromUtc()-based code (used when there
are no transitions) if it can't find a previous transition (which, in
any case, it needs to do its job).

Task-number: QTBUG-56460
Task-number: QTBUG-56397
Task-number: QTBUG-52284
Change-Id: I2f7422a9e9d3767940b1901d887c6a2c1f36ac9f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 54 insertions, 2 deletions

diff --git a/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp b/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
index 4604e664b0..d4ae1b2a91 100644
--- a/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
+++ b/tests/auto/corelib/tools/qdatetime/tst_qdatetime.cpp
@@ -3145,17 +3145,17 @@ void tst_QDateTime::timeZones() const
     // Test local to MSecs
     // - Test first occurrence 02:00:00 = 1 hour before tran
     hourBeforeStd = QDateTime(QDate(2013, 10, 27), QTime(2, 0, 0), cet);
+    QEXPECT_FAIL("", "QDateTime doesn't properly support Daylight Transitions", Continue);
     QCOMPARE(hourBeforeStd.toMSecsSinceEpoch(), dstToStdMSecs - 3600000);
     // - Test first occurrence 02:59:59.999 = 1 msec before tran
     msecBeforeStd = QDateTime(QDate(2013, 10, 27), QTime(2, 59, 59, 999), cet);
+    QEXPECT_FAIL("", "QDateTime doesn't properly support Daylight Transitions", Continue);
     QCOMPARE(msecBeforeStd.toMSecsSinceEpoch(), dstToStdMSecs - 1);
     // - Test second occurrence 02:00:00 = at tran
     atStd = QDateTime(QDate(2013, 10, 27), QTime(2, 0, 0), cet);
-    QEXPECT_FAIL("", "QDateTime doesn't properly support Daylight Transitions", Continue);
     QCOMPARE(atStd.toMSecsSinceEpoch(), dstToStdMSecs);
     // - Test second occurrence 03:00:00 = 59 mins after tran
     afterStd = QDateTime(QDate(2013, 10, 27), QTime(2, 59, 59, 999), cet);
-    QEXPECT_FAIL("", "QDateTime doesn't properly support Daylight Transitions", Continue);
     QCOMPARE(afterStd.toMSecsSinceEpoch(), dstToStdMSecs + 3600000 - 1);
     // - Test 03:00:00 = 1 hour after tran
     hourAfterStd = QDateTime(QDate(2013, 10, 27), QTime(3, 0, 0), cet);
diff --git a/tests/auto/corelib/tools/qtimezone/tst_qtimezone.cpp b/tests/auto/corelib/tools/qtimezone/tst_qtimezone.cpp
index c631b395f8..593cb7821d 100644
--- a/tests/auto/corelib/tools/qtimezone/tst_qtimezone.cpp
+++ b/tests/auto/corelib/tools/qtimezone/tst_qtimezone.cpp
@@ -45,6 +45,8 @@ private slots:
     void dataStreamTest();
     void isTimeZoneIdAvailable();
     void availableTimeZoneIds();
+    void transitionEachZone_data();
+    void transitionEachZone();
     void stressTest();
     void windowsId();
     void isValidId_data();
@@ -367,6 +369,56 @@ void tst_QTimeZone::isTimeZoneIdAvailable()
     QCOMPARE(QTimeZonePrivate::isValidId("12345678901234/123456789012345"), false);
 }
 
+void tst_QTimeZone::transitionEachZone_data()
+{
+    QTest::addColumn<QByteArray>("zone");
+    QTest::addColumn<qint64>("secs");
+    QTest::addColumn<int>("start");
+    QTest::addColumn<int>("stop");
+
+    struct {
+        qint64 baseSecs;
+        int start, stop;
+        int year;
+    } table[] = {
+        { 25666200, 3, 12, 1970 },  // 1970-10-25 01:30 UTC; North America
+        { 1288488600, -4, 8, 2010 } // 2010-10-31 01:30 UTC; Europe, Russia
+    };
+
+    QString name;
+    for (int k = sizeof(table) / sizeof(table[0]); k-- > 0; ) {
+        foreach (QByteArray zone, QTimeZone::availableTimeZoneIds()) {
+            name.sprintf("%s@%d", zone.constData(), table[k].year);
+            QTest::newRow(name.toUtf8().constData())
+                << zone
+                << table[k].baseSecs
+                << table[k].start
+                << table[k].stop;
+        }
+    }
+}
+
+void tst_QTimeZone::transitionEachZone()
+{
+    // Regression test: round-trip fromMsecs/toMSecs should be idempotent; but
+    // various zones failed during fall-back transitions.
+    QFETCH(QByteArray, zone);
+    QFETCH(qint64, secs);
+    QFETCH(int, start);
+    QFETCH(int, stop);
+    QTimeZone named(zone);
+
+    for (int i = start; i < stop; i++) {
+        qint64 here = secs + i * 3600;
+        QDateTime when = QDateTime::fromMSecsSinceEpoch(here * 1000, named);
+        qint64 stamp = when.toMSecsSinceEpoch();
+        if (here * 1000 != stamp) // (The +1 is due to using *1*:30 as baseSecs.)
+            qDebug() << "Failing for" << zone << "at half past" << (i + 1) << "UTC";
+        QCOMPARE(stamp % 1000, 0);
+        QCOMPARE(here - stamp / 1000, 0);
+    }
+}
+
 void tst_QTimeZone::availableTimeZoneIds()
 {
     if (debug) {