Fixed #22491 -- documented how select_for_update() should be tested.

``nowait`` argument. Obviously, users of external third-party backends should

check with their backend's documentation for specifics in those cases.

Passing ``nowait=True`` to ``select_for_update`` using database backends that

Passing ``nowait=True`` to ``select_for_update()`` using database backends that

do not support ``nowait``, such as MySQL, will cause a

:exc:`~django.db.DatabaseError` to be raised. This is in order to prevent code

unexpectedly blocking.

Evaluating a queryset with ``select_for_update`` in autocommit mode is

an error because the rows are then not locked. If allowed, this would

facilitate data corruption, and could easily be caused by calling,

outside of any transaction, code that expects to be run in one.

Evaluating a queryset with ``select_for_update()`` in autocommit mode is

a :exc:`~django.db.transaction.TransactionManagementError` error because the

rows are not locked in that case. If allowed, this would facilitate data

corruption and could easily be caused by calling code that expects to be run in

a transaction outside of one.

Using ``select_for_update`` on backends which do not support

Using ``select_for_update()`` on backends which do not support

``SELECT ... FOR UPDATE`` (such as SQLite) will have no effect.

.. versionchanged:: 1.6.3

    It is now an error to execute a query with ``select_for_update()`` in

    autocommit mode. With earlier releases in the 1.6 series it was a no-op.

.. warning::

    Although ``select_for_update()`` normally fails in autocommit mode, since

    :class:`~django.test.TestCase` automatically wraps each test in a

    transaction, calling ``select_for_update()`` in a ``TestCase`` even outside

    an :func:`~django.db.transaction.atomic()` block will (perhaps unexpectedly)

    pass without raising a ``TransactionManagementError``. To properly test

    ``select_for_update()`` you should use

    :class:`~django.test.TransactionTestCase`.

raw

~~~

    While ``commit`` and ``rollback`` operations still *appear* to work when

    used in ``TestCase``, no actual commit or rollback will be performed by the

    database. This can cause your tests to pass or fail unexpectedly. Always

    use ``TransactionTestCase`` when testing transactional behavior.

    use ``TransactionTestCase`` when testing transactional behavior or any code

    that can't normally be excuted in autocommit mode

    (:meth:`~django.db.models.query.QuerySet.select_for_update()` is an

    example).

``TransactionTestCase`` inherits from :class:`~django.test.SimpleTestCase`.