[1.3.x] Fixed #18135 -- Close connection used for db version checking

On MySQL when checking the server version, a new connection could be
created but never closed. This could result in open connections on
server startup.

Backport of 4423757c0c.

django/db/backends/mysql/base.py

@@ -334,10 +334,19 @@ class DatabaseWrapper(BaseDatabaseWrapper):
 
     def get_server_version(self):
         if not self.server_version:
+            new_connection = False
             if not self._valid_connection():
-                self.cursor()
-            m = server_version_re.match(self.connection.get_server_info())
+                # Ensure we have a connection with the DB by using a temporary
+                # cursor
+                new_connection = True
+                self.cursor().close()
+            server_info = self.connection.get_server_info()
+            if new_connection:
+                # Make sure we close the connection
+                self.connection.close()
+                self.connection = None
+            m = server_version_re.match(server_info)
             if not m:
-                raise Exception('Unable to determine MySQL version from version string %r' % self.connection.get_server_info())
+                raise Exception('Unable to determine MySQL version from version string %r' % server_info)
             self.server_version = tuple([int(x) for x in m.groups()])
         return self.server_version

tests/regressiontests/backends/tests.py

@@ -56,6 +56,14 @@ class OracleChecks(unittest.TestCase):
         self.assertEqual(connection.connection.encoding, "UTF-8")
         self.assertEqual(connection.connection.nencoding, "UTF-8")
 
+class MySQLTests(TestCase):
+    @unittest.skipUnless(connection.vendor == 'mysql',
+                        "Test valid only for MySQL")
+    def test_server_version_connections(self):
+        connection.close()
+        connection.get_server_version()
+        self.assertTrue(connection.connection is None)
+
 class DateQuotingTest(TestCase):
 
     def test_django_date_trunc(self):