Ensured a connection is established when checking the database version.

Fixed a test broken by 21765c0a. Refs #18135.

django/db/backends/__init__.py

@@ -352,6 +352,18 @@ class BaseDatabaseWrapper(object):
     def make_debug_cursor(self, cursor):
         return util.CursorDebugWrapper(cursor, self)
 
+    @contextmanager
+    def temporary_connection(self):
+        # Ensure a connection is established, and avoid leaving a dangling
+        # connection, for operations outside of the request-response cycle.
+        must_close = self.connection is None
+        cursor = self.cursor()
+        try:
+            yield
+        finally:
+            cursor.close()
+            if must_close:
+                self.close()
 
 class BaseDatabaseFeatures(object):
     allows_group_by_pk = False

django/db/backends/mysql/base.py

@@ -453,6 +453,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
 
     @cached_property
     def mysql_version(self):
+        with self.temporary_connection():
             server_info = self.connection.get_server_info()
         match = server_version_re.match(server_info)
         if not match:

django/db/backends/oracle/base.py

@@ -623,8 +623,10 @@ class DatabaseWrapper(BaseDatabaseWrapper):
 
     @cached_property
     def oracle_version(self):
+        with self.temporary_connection():
+            version = self.connection.version
         try:
-            return int(self.connection.version.split('.')[0])
+            return int(version.split('.')[0])
         except ValueError:
             return None
 

django/db/backends/postgresql_psycopg2/base.py

@@ -152,6 +152,7 @@ class DatabaseWrapper(BaseDatabaseWrapper):
 
     @cached_property
     def pg_version(self):
+        with self.temporary_connection():
             return get_version(self.connection)
 
     def get_connection_params(self):

django/db/backends/postgresql_psycopg2/operations.py

@@ -195,8 +195,7 @@ class DatabaseOperations(BaseDatabaseOperations):
         NotImplementedError if this is the database in use.
         """
         if aggregate.sql_function in ('STDDEV_POP', 'VAR_POP'):
-            pg_version = self.connection.pg_version
+            if 80200 <= self.connection.pg_version <= 80204:
-            if pg_version >= 80200 and pg_version <= 80204:
                 raise NotImplementedError('PostgreSQL 8.2 to 8.2.4 is known to have a faulty implementation of %s. Please upgrade your version of PostgreSQL.' % aggregate.sql_function)
 
     def max_name_length(self):