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QTzTimeZonePrivate: fix UB (data race on m_icu)

Andreas Sturmlechner requested to merge asturmlechner/qtbase:QtzTimeZonePrivate-UB into kde/5.15

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 (cherry picked from commit 11becbe9)

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