Fixed #21202 -- Maintained atomicity when the server disconnects.

Thanks intgr for the report.

This commit doesn't include a test because I don't know how to emulate a
database disconnection in a cross-database compatible way.

Also simplified a 'backends' test that was constrained by this problem.

django/db/transaction.py

@@ -2,7 +2,7 @@ from functools import wraps
 
 from django.db import (
     connections, DEFAULT_DB_ALIAS,
-    DatabaseError, ProgrammingError)
+    DatabaseError, Error, ProgrammingError)
 from django.utils.decorators import available_attrs
 
 
@@ -224,7 +224,12 @@ class Atomic(object):
                     try:
                         connection.commit()
                     except DatabaseError:
+                        try:
                             connection.rollback()
+                        except Error:
+                            # Error during rollback means the connection was
+                            # closed. Clean up in case the server dropped it.
+                            connection.close()
                         raise
             else:
                 # This flag will be set to True again if there isn't a savepoint
@@ -245,7 +250,12 @@ class Atomic(object):
                             connection.needs_rollback = True
                 else:
                     # Roll back transaction
+                    try:
                         connection.rollback()
+                    except Error:
+                        # Error during rollback means the connection was
+                        # closed. Clean up in case the server dropped it.
+                        connection.close()
 
         finally:
             # Outermost block exit when autocommit was enabled.

tests/backends/tests.py

@@ -664,13 +664,6 @@ class BackendTestCase(TestCase):
             self.assertIsInstance(cursor, CursorWrapper)
         self.assertTrue(cursor.closed)
 
-
-class IsUsableTests(TransactionTestCase):
-    # Avoid using a regular TestCase because Django really dislikes closing
-    # the database connection inside a transaction at this point (#21202).
-
-    available_apps = []
-
     # Unfortunately with sqlite3 the in-memory test database cannot be closed.
     @skipUnlessDBFeature('test_db_allows_multiple_connections')
     def test_is_usable_after_database_disconnects(self):