Thanks Florian for tracing the error.

Also removed some unused imports.

The atomic block provides a clean rollback to a savepoint on failed writes.

The ticket reported a race condition which I don't know how to test.

Refs #9437.

Tests that require USE_I18N, LOGIN_URL or certain MIDDLEWARE_CLASSES
should be decorated appropriately.

Fixed #2227: `atomic` supports nesting.
Fixed #6623: `commit_manually` is deprecated and `atomic` doesn't suffer from this defect.
Fixed #8320: the problem wasn't identified, but the legacy transaction management is deprecated.
Fixed #10744: the problem wasn't identified, but the legacy transaction management is deprecated.
Fixed #10813: since autocommit is enabled, it isn't necessary to rollback after errors any more.
Fixed #13742: savepoints are now implemented for SQLite.
Fixed #13870: transaction management in long running processes isn't a problem any more, and it's documented.
Fixed #14970: while it digresses on transaction management, this ticket essentially asks for autocommit on PostgreSQL.
Fixed #15694: `atomic` supports nesting.
Fixed #17887: autocommit makes it impossible for a connection to stay "idle of transaction".

It isn't necessary to disable set_autocommit since its use is prohibited
inside an atomic block. It's still necessary to disable the legacy
transaction management methods for backwards compatibility, until
they're removed.

Since unmanaged == autocommit, there's nothing to commit or roll back.

There isn't much else to say, really.

Apparently django.db.transaction used to be an object.

Replaced them with per-database options, for proper multi-db support.

Also toned down the recommendation to tie transactions to HTTP requests.
Thanks Jeremy for sharing his experience.

It disables transaction management entirely when AUTOCOMMIT is False.

Thanks Florian again.

Currently it only works in autocommit mode.

Based on @xact by Christophe Pettus.

Technically speaking they aren't usable yet.

Prepared for using savepoints within transactions in autocommit mode.

These methods are only called when uses_savepoints = True anyway.

It's synchronized with the autocommit flag.

Since "unless managed" now means "if database-level autocommit",
committing or rolling back doesn't have any effect.

Restored transactional integrity in a few places that relied on
automatically-started transactions with a transitory API.

This is mostly a documentation change.

It has the same backwards-incompatibility consequences as those
described for PostgreSQL in a previous commit.

The goal is to make all databases share a common, autocommit-based,
implementation.

This API is useful because autocommit cannot be managed without an open
connection.