Delay port binding to reduce wait at process start

With the new default behavior of eventlet 0.20 to set SO_REUSEPORT on listening
sockets, it becomes easy to have a zero-downtime reload.

By binding the listening socket as late as we can, we can reduce the
time an incoming connection will stay in queue, waiting for the process
to be ready to handle the connection.

This commit moves the socket binding after the load of the application.

On some benchmarks, it reduced the wait-time for incoming connections from
about 1.1s to about 350ms.

Change-Id: I045f4c9aa2c07cdc5ed95afd028540f1b3874637

swift/common/wsgi.py

@@ -892,12 +892,6 @@ def run_wsgi(conf_path, app_section, *args, **kwargs):
     else:
         strategy = WorkersStrategy(conf, logger)
 
-    error_msg = strategy.bind_ports()
-    if error_msg:
-        logger.error(error_msg)
-        print(error_msg)
-        return 1
-
     # Ensure the configuration and application can be loaded before proceeding.
     global_conf = {'log_name': log_name}
     if 'global_conf_callback' in kwargs:
@@ -911,6 +905,13 @@ def run_wsgi(conf_path, app_section, *args, **kwargs):
     # redirect errors to logger and close stdio
     capture_stdio(logger)
 
+    # Start listening on bind_addr/port
+    error_msg = strategy.bind_ports()
+    if error_msg:
+        logger.error(error_msg)
+        print(error_msg)
+        return 1
+
     no_fork_sock = strategy.no_fork_sock()
     if no_fork_sock:
         run_server(conf, logger, no_fork_sock, global_conf=global_conf)

test/unit/common/test_wsgi.py

@@ -730,7 +730,9 @@ def test_run_server_strategy_plumbing(self, mock_per_port, mock_workers,
             logger,
             'log_name',
         ]
-        with mock.patch.object(wsgi, '_initrp', return_value=stub__initrp):
+        with mock.patch.object(wsgi, '_initrp', return_value=stub__initrp), \
+                mock.patch.object(wsgi, 'loadapp'), \
+                mock.patch.object(wsgi, 'capture_stdio'):
             for server_type in ('account-server', 'container-server',
                                 'object-server'):
                 mock_per_port.reset_mock()