Avoid eventlet_backdoor listing on same port

Oslo.service is binding to same port when we provide a port range
as eventlet is internally setting SO_REUSEPORT flag (starting from
eventlet version v0.20). And there is a flag (reuse_port)
introduced in v0.22 to give control to user to avoid SO_REUSEPORT.
In this patch, first we try passing reuse_port=False, if this fails
then directly open socket and listen instead of eventlist.listen.

Closes-Bug: #1810280
Change-Id: Idc842acc7e430199c76fe12785b0bf0e7a58e121

oslo_service/eventlet_backdoor.py

@@ -21,13 +21,13 @@ import gc
 import logging
 import os
 import pprint
-import socket
 import sys
 import traceback
 
 import eventlet.backdoor
 import greenlet
 
+from eventlet.green import socket
 from oslo_service._i18n import _
 from oslo_service import _options
 
@@ -121,11 +121,24 @@ def _parse_port_range(port_range):
             port_range, ex, _options.help_for_backdoor_port)
 
 
-def _listen(host, start_port, end_port, listen_func):
+def _listen_func(host, port):
+    # eventlet is setting SO_REUSEPORT by default from v0.20.
+    # But we can configure it by passing reuse_port argument
+    # from v0.22
+    try:
+        return eventlet.listen((host, port), reuse_port=False)
+    except TypeError:
+        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        sock.bind((host, port))
+        sock.listen(50)
+        return sock
+
+
+def _listen(host, start_port, end_port):
     try_port = start_port
     while True:
         try:
-            return listen_func((host, try_port))
+            return _listen_func(host, try_port)
         except socket.error as exc:
             if (exc.errno != errno.EADDRINUSE or
                try_port >= end_port):
@@ -169,7 +182,7 @@ def _initialize_if_enabled(conf):
 
     if conf.backdoor_socket is None:
         start_port, end_port = _parse_port_range(str(conf.backdoor_port))
-        sock = _listen('localhost', start_port, end_port, eventlet.listen)
+        sock = _listen('localhost', start_port, end_port)
         # In the case of backdoor port being zero, a port number is assigned by
         # listen().  In any case, pull the port number out here.
         where_running = sock.getsockname()[1]

oslo_service/tests/test_eventlet_backdoor.py

@@ -105,6 +105,14 @@ class BackdoorPortTest(base.ServiceBaseTestCase):
         self.assertRaises(socket.error,
                           eventlet_backdoor.initialize_if_enabled, self.conf)
 
+    @mock.patch.object(eventlet, 'spawn')
+    def test_backdoor_port_range_inuse(self, spawn_mock):
+        self.config(backdoor_port='8800:8801')
+        port = eventlet_backdoor.initialize_if_enabled(self.conf)
+        self.assertEqual(8800, port)
+        port = eventlet_backdoor.initialize_if_enabled(self.conf)
+        self.assertEqual(8801, port)
+
     @mock.patch.object(eventlet, 'spawn')
     @mock.patch.object(eventlet, 'listen')
     def test_backdoor_port_range(self, listen_mock, spawn_mock):