The fallback m_icu QIcuTimeZonePrivate is lazily constructed, which
means that two threads each with their own copy of a QTimeZone with a
shared QTzTimeZonePrivate will race over who gets to set m_icu,
e.g. when concurrently calling QTimeZone::displayName().
Fix by protecting m_icu with a mutex. For simplicity, use a static
mutex, not a per-instance one (which would delete the
QTzTimeZonePrivate copy constructor, which clone() relies on). This is
sufficient for 5.15. For Qt 6, going forward, we could make this
lock-less, too.
[ChangeLog][QtCore][QTimeZone] Fixed a data race on Unix platforms when
implicitly-shared copies of QTimeZone objects were used in certain ways
(e.g. calling displayName()) from different threads and Qt was
configured with ICU support.
Pick-to: 6.3 6.2 5.15
Change-Id: I7e57aef3dd44a90289ad86d0578ece1e54920730
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
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