engine: use new visitor-callable from latest version

rhodecode/lib/dbmigrate/migrate/changeset/schema.py

@@ -138,7 +138,7 b' def alter_column(*p, **k):'
     delta = ColumnDelta(*p, **k)
 
     visitorcallable = get_engine_visitor(engine, 'schemachanger')
-    engine._run_visitor(visitorcallable, delta)
+    _run_visitor(engine, visitorcallable, delta)
 
     return delta
 
@@ -167,6 +167,20 b' def _to_index(index, table=None, engine='
     return ret
 
 
+def _run_visitor(
+    connectable, visitorcallable, element, connection=None, **kwargs
+):
+    if connection is not None:
+        visitorcallable(
+            connection.dialect, connection, **kwargs).traverse_single(element)
+    else:
+        conn = connectable.connect()
+        try:
+            visitorcallable(
+                conn.dialect, conn, **kwargs).traverse_single(element)
+        finally:
+            conn.close()
+
 
 # Python3: if we just use:
 #
@@ -568,7 +582,7 b' populated with defaults'
         self.add_to_table(table)
         engine = self.table.bind
         visitorcallable = get_engine_visitor(engine, 'columngenerator')
-        engine._run_visitor(visitorcallable, self, connection, **kwargs)
+        _run_visitor(engine, visitorcallable, self, connection, **kwargs)
 
         # TODO: reuse existing connection
         if self.populate_default and self.default is not None:
@@ -589,7 +603,7 b' populated with defaults'
             self.table = table
         engine = self.table.bind
         visitorcallable = get_engine_visitor(engine, 'columndropper')
-        engine._run_visitor(visitorcallable, self, connection, **kwargs)
+        _run_visitor(engine, visitorcallable, self, connection, **kwargs)
         self.remove_from_table(self.table, unset_table=False)
         self.table = None
         return self