Fixed #27683 -- Made MySQL default to the read committed isolation level.

Thanks Shai Berger for test help and Adam Johnson for review.

docs/ref/databases.txt

@@ -449,8 +449,14 @@ this entry are the four standard isolation levels:
 * ``'serializable'``
 
 or ``None`` to use the server's configured isolation level. However, Django
-works best with read committed rather than MySQL's default, repeatable read.
-Data loss is possible with repeatable read.
+works best with and defaults to read committed rather than MySQL's default,
+repeatable read. Data loss is possible with repeatable read.
+
+.. versionchanged:: 2.0
+
+    In older versions, the MySQL database backend defaults to using the
+    database's isolation level (which defaults to repeatable read) rather
+    than read committed.
 
 .. _transaction isolation level: https://dev.mysql.com/doc/refman/en/innodb-transaction-isolation-levels.html
 

docs/releases/2.0.txt

@@ -227,6 +227,15 @@ The end of upstream support for Oracle 11.2 is Dec. 2020. Django 1.11 will be
 supported until April 2020 which almost reaches this date. Django 2.0
 officially supports Oracle 12.1+.
 
+Default MySQL isolation level is read committed
+-----------------------------------------------
+
+MySQL's default isolation level, repeatable read, may cause data loss in
+typical Django usage. To prevent that and for consistency with other databases,
+the default isolation level is now read committed. You can use the
+:setting:`DATABASES` setting to :ref:`use a different isolation level
+<mysql-isolation-level>`, if needed.
+
 :attr:`AbstractUser.last_name <django.contrib.auth.models.User.last_name>` ``max_length`` increased to 150
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