Use eventlet.patcher.original to get Python select module in get_hub

get_hub function was added in commit b155da42 with the idea to bypass
eventlet automatic hub selection that prefers epoll if available by default.

Since version 0.20.0 eventlet removed select.poll() function in its patched
select module (eventlet.green.select), see:
   - https://github.com/eventlet/eventlet/commit/614a20462

So if eventlet monkey patching is done before a get_hub() call (as now in
wsgi.py since commit c9410c7d) if we use 'import select' we get the eventlet
version that don't have poll attribute.

To prevent that we use eventlet.patcher.original function to get python select
module to test if poll() is available on current platform.

Change-Id: I69b3db3951b3d3b6583845978deb2883492e7f0f
Closes-Bug: 1804627
(cherry picked from commit 4809884d9f)

swift/common/utils.py

@@ -2015,9 +2015,18 @@ def get_hub():
 
     In contrast, both poll() and select() specify the set of interesting
     file descriptors with each call, so there's no problem with forking.
+
+    As eventlet monkey patching is now done before call get_hub() in wsgi.py
+    if we use 'import select' we get the eventlet version, but since version
+    0.20.0 eventlet removed select.poll() function in patched select (see:
+    http://eventlet.net/doc/changelog.html and
+    https://github.com/eventlet/eventlet/commit/614a20462).
+
+    We use eventlet.patcher.original function to get python select module
+    to test if poll() is available on platform.
     """
     try:
-        import select
+        select = eventlet.patcher.original('select')
         if hasattr(select, "poll"):
             return "poll"
         return "selects"

test/unit/common/test_utils.py

@@ -4542,6 +4542,38 @@ class TestFileLikeIter(unittest.TestCase):
         iter_file.close()
         self.assertTrue(iter_file.closed)
 
+    def test_get_hub(self):
+        # This test mock the eventlet.green.select module without poll
+        # as in eventlet > 0.20
+        # https://github.com/eventlet/eventlet/commit/614a20462
+        # We add __original_module_select to sys.modules to mock usage
+        # of eventlet.patcher.original
+
+        class SelectWithPoll(object):
+            def poll():
+                pass
+
+        class SelectWithoutPoll(object):
+            pass
+
+        # Platform with poll() that call get_hub before eventlet patching
+        with mock.patch.dict('sys.modules',
+                             {'select': SelectWithPoll,
+                              '__original_module_select': SelectWithPoll}):
+            self.assertEqual(utils.get_hub(), 'poll')
+
+        # Platform with poll() that call get_hub after eventlet patching
+        with mock.patch.dict('sys.modules',
+                             {'select': SelectWithoutPoll,
+                              '__original_module_select': SelectWithPoll}):
+            self.assertEqual(utils.get_hub(), 'poll')
+
+        # Platform without poll() -- before or after patching doesn't matter
+        with mock.patch.dict('sys.modules',
+                             {'select': SelectWithoutPoll,
+                              '__original_module_select': SelectWithoutPoll}):
+            self.assertEqual(utils.get_hub(), 'selects')
+
 
 class TestStatsdLogging(unittest.TestCase):
     def setUp(self):