Add test for time zone transitions near the epoch

This needs a zone with transitions near the epoch; and the only CET
with DST that winter was Italy (copied by Malta), for which the Olson
database had a recent (2016) correction to its data, for that winter.
That means we get inconsistent results on O/Sen of different ages.

So add a separate testEpochTranPrivate(), alongside testCetPrivate(),
and test it with America/Toronto.  (Unfortunately, MS-Win gets the
date wrong on the first transition after the epoch, so we have to code
round that.)

Since information before the epoch isn't reliably available, only test
the search backwards if nextTransition does find something before it.
(We can safely assume all real transitions happened since 1601;
non-celestial time-keeping wasn't accurate enough, before that, for
anyone to synchronize with anything but celestial time.)

Change-Id: I984b46938a2805b93bb2afd6855e317b5d66b386
Reviewed-by: Thiago Macieira <[email protected]>

tests/auto/corelib/tools/qtimezone/tst_qtimezone.cpp

@@ -62,7 +62,9 @@ private slots:
 private:
     void printTimeZone(const QTimeZone &tz);
 #ifdef QT_BUILD_INTERNAL
+    // Generic tests of privates, called by implementation-specific private tests:
     void testCetPrivate(const QTimeZonePrivate &tzp);
+    void testEpochTranPrivate(const QTimeZonePrivate &tzp);
 #endif // QT_BUILD_INTERNAL
     const bool debug;
 };
@@ -757,6 +759,7 @@ void tst_QTimeZone::icuTest()
     }
 
     testCetPrivate(tzp);
+    testEpochTranPrivate(QIcuTimeZonePrivate("America/Toronto"));
 #endif // QT_USE_ICU
 }
 
@@ -830,6 +833,7 @@ void tst_QTimeZone::tzTest()
     }
 
     testCetPrivate(tzp);
+    testEpochTranPrivate(QTzTimeZonePrivate("America/Toronto"));
 
     // Test first and last transition rule
     // Warning: This could vary depending on age of TZ file!
@@ -982,6 +986,7 @@ void tst_QTimeZone::macTest()
     }
 
     testCetPrivate(tzp);
+    testEpochTranPrivate(QMacTimeZonePrivate("America/Toronto"));
 #endif // QT_BUILD_INTERNAL && Q_OS_DARWIN
 }
 
@@ -1047,6 +1052,7 @@ void tst_QTimeZone::winTest()
     }
 
     testCetPrivate(tzp);
+    testEpochTranPrivate(QWinTimeZonePrivate("America/Toronto"));
 #endif // Q_OS_WIN
 }
 
@@ -1148,6 +1154,44 @@ void tst_QTimeZone::testCetPrivate(const QTimeZonePrivate &tzp)
         }
     }
 }
+
+// Needs a zone with DST around the epoch; currently America/Toronto (EST5EDT)
+void tst_QTimeZone::testEpochTranPrivate(const QTimeZonePrivate &tzp)
+{
+    if (!tzp.hasTransitions())
+        return; // test only viable for transitions
+
+    QTimeZonePrivate::Data tran = tzp.nextTransition(0); // i.e. first after epoch
+    // 1970-04-26 02:00 EST, -5 -> -4
+    const QDateTime after = QDateTime(QDate(1970, 4, 26), QTime(2, 0), Qt::OffsetFromUTC, -5 * 3600);
+    const QDateTime found = QDateTime::fromMSecsSinceEpoch(tran.atMSecsSinceEpoch, Qt::UTC);
+#ifdef Q_OS_WIN // MS gets the date wrong: 5th April instead of 26th.
+    QCOMPARE(found.toOffsetFromUtc(-5 * 3600).time(), after.time());
+#else
+    QCOMPARE(found, after);
+#endif
+    QCOMPARE(tran.offsetFromUtc, -4 * 3600);
+    QCOMPARE(tran.standardTimeOffset, -5 * 3600);
+    QCOMPARE(tran.daylightTimeOffset, 3600);
+
+    // Pre-epoch time-zones might not be supported at all:
+    tran = tzp.nextTransition(QDateTime(QDate(1601, 1, 1), QTime(0, 0),
+                                        Qt::UTC).toMSecsSinceEpoch());
+    if (tran.atMSecsSinceEpoch != QTimeZonePrivate::invalidSeconds()
+        && tran.atMSecsSinceEpoch < 0) {
+        // ... but, if they are, we should be able to search back to them:
+        tran = tzp.previousTransition(0); // i.e. last before epoch
+        // 1969-10-26 02:00 EDT, -4 -> -5
+        QCOMPARE(QDateTime::fromMSecsSinceEpoch(tran.atMSecsSinceEpoch, Qt::UTC),
+                 QDateTime(QDate(1969, 10, 26), QTime(2, 0), Qt::OffsetFromUTC, -4 * 3600));
+        QCOMPARE(tran.offsetFromUtc, -5 * 3600);
+        QCOMPARE(tran.standardTimeOffset, -5 * 3600);
+        QCOMPARE(tran.daylightTimeOffset, 0);
+    } else {
+        // Do not use QSKIP(): that would discard the rest of this sub-test's caller.
+        qDebug() << "No support for pre-epoch time-zone transitions";
+    }
+}
 #endif // QT_BUILD_INTERNAL
 
 QTEST_APPLESS_MAIN(tst_QTimeZone)