Make QCalendar's backend registration reentrant-safe

Previously, different threads instantiating the same back-end could
collide in the register, with various unwelcome results.

Give Registry an atomic status flag to track whether it has been
populated or is being destroyed; and protect it with a mutex to ensure
distinct threads do not collide during registration or attempt to
register while the registry is being destroyed.

Document the correct way to instantiate custom backends, and that no
code other than the QCalendar implementation should instantiate the
built-in ones. Instantiators that follow these rules should be safe
from failed registrations, provided they don't pick a name that
conflicts with some other backend. They can also use the recent change
to semantics of registerAlias() to verify that registration *has*
succeeded.

Done-with: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Pick-to: 6.0 5.15
Task-number: QTBUG-88815
Task-number: QTBUG-85692
Fixes: QTBUG-84575
Change-Id: Ie78e700e71d610454152c05cafb38f6f713649ad
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

0 files changed, 0 insertions, 0 deletions