qv4dateobject.cpp: USE_QTZ_SYSTEM_TIMEZONE on more platforms

The only platform where we may have a semi-adequate native
implementation of the time/date functions is windows. On all other
platforms we should use QTimeZone::systemTimeZone() if possible as that
at least gives us correct results when the system time zone doesn't
change at runtime.

Fixes: QTBUG-84474
Change-Id: I34b01e12a751ee1c9b17735810e62e2e847f8446
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
(cherry picked from commit 510253aa0996a89e6029d244002a08615b2fb65b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 35 insertions, 2 deletions

diff --git a/src/qml/jsruntime/qv4dateobject.cpp b/src/qml/jsruntime/qv4dateobject.cpp
index bebcbd7e44..4e445a73fa 100644
--- a/src/qml/jsruntime/qv4dateobject.cpp
+++ b/src/qml/jsruntime/qv4dateobject.cpp
@@ -54,11 +54,28 @@
 
 #include <wtf/MathExtras.h>
 
-#if defined(Q_OS_LINUX) && QT_CONFIG(timezone) && !defined(Q_OS_ANDROID)
+#if QT_CONFIG(timezone) && !defined(Q_OS_WIN)
 /*
   See QTBUG-56899.  Although we don't (yet) have a proper way to reset the
   system zone, the code below, that uses QTimeZone::systemTimeZone(), works
-  adequately on Linux, when the TZ environment variable is changed.
+  adequately on Linux.
+
+  QTimeZone::systemTimeZone() will automatically produce an updated value on
+  non-android Linux systems when the TZ environment variable or the relevant
+  files in /etc are changed. On other platforms it won't, and the information
+  produced here may be incorrect after changes to the system time zone.
+
+  We accept this defect for now because the localtime_r approach will
+  consistently produce incorrect results for some time zones, not only when
+  the system time zone changes. This is a worse problem, see also QTBUG-84474.
+
+  On windows we have a better implementation of getLocalTZA that hopefully
+  updates on time zone changes. However, we currently use the worse
+  implementation of DaylightSavingTA (returning either an hour or 0).
+
+  QTimeZone::systemTimeZone() on Linux is also slower than other approaches
+  because it has to poll the relevant files (if TZ is not set). See
+  QTBUG-75585 for an explanation and possible workarounds.
  */
 #define USE_QTZ_SYSTEM_ZONE
 #endif
diff --git a/tests/auto/qml/qqmllocale/tst_qqmllocale.cpp b/tests/auto/qml/qqmllocale/tst_qqmllocale.cpp
index e3094db708..7d0c10a702 100644
--- a/tests/auto/qml/qqmllocale/tst_qqmllocale.cpp
+++ b/tests/auto/qml/qqmllocale/tst_qqmllocale.cpp
@@ -34,6 +34,7 @@
 #include <QtCore/QDateTime>
 #include <QtCore/qscopeguard.h>
 #include <QtCore/qscopedpointer.h>
+#include <QtCore/qtimezone.h>
 #include <qcolor.h>
 #include "../../shared/util.h"
 
@@ -1297,6 +1298,13 @@ static void setTimeZone(const QByteArray &tz)
 
 void tst_qqmllocale::timeZoneUpdated()
 {
+    // Note: This test may not reliably hit the QEXPECT_FAIL clauses below if the initial
+    //       system time zone is equivalent to either Australia/Brisbane or Asia/Kalkota.
+
+    // Initialize the system time zone, so that we actually _change_ something below.
+    QVERIFY2(QTimeZone::systemTimeZone().isValid(),
+             "You know, Toto, I do believe we're not in Kansas any more.");
+
     QByteArray original(qgetenv("TZ"));
 
     // Set the timezone to Brisbane time, AEST-10:00
@@ -1318,12 +1326,20 @@ void tst_qqmllocale::timeZoneUpdated()
     QVERIFY2(!c.isError(), qPrintable(c.errorString()));
     obj.reset(c.create());
     QVERIFY(obj);
+
+#if !defined(Q_OS_WIN) && QT_CONFIG(timezone) && (!defined(Q_OS_LINUX) || defined(Q_OS_ANDROID))
+    QEXPECT_FAIL("", "Date.timeZoneUpdated() only works on non-Android Linux with QT_CONFIG(timezone).", Continue);
+#endif
     QVERIFY(obj->property("success").toBool());
 
     // Change to Indian time, IST-05:30
     setTimeZone(QByteArray("Asia/Kolkata"));
 
     QMetaObject::invokeMethod(obj.data(), "check");
+
+#if !defined(Q_OS_WIN) && QT_CONFIG(timezone) && (!defined(Q_OS_LINUX) || defined(Q_OS_ANDROID))
+    QEXPECT_FAIL("", "Date.timeZoneUpdated() only works on non-Android Linux with QT_CONFIG(timezone).", Continue);
+#endif
     QVERIFY(obj->property("success").toBool());
 }
 #endif