Fix reconstructer to be able to run non ec policy environment

Since the related change, object-reconstructor gathers the local devices
for ec policy via get_local_devices method but the method causes TypeError when
attempting *reduce* for empty set list. the list can be empty when no EC config
found in swift.conf.

This patch fixes the get_local_devices to return empty set even when no ec config
in swift.conf without errors.

Co-Authored-By: Kirill Zaitsev <k.zaitsev@me.com>
Change-Id: Ic121fb547966787a43f9eae83c91bb2bf640c4be
Related-Change: 701a172afac37229b85ea762f20428f6f422d29b
Closes-Bug: #1707595

swift/obj/reconstructor.py

@@ -1034,7 +1034,7 @@ class ObjectReconstructor(Daemon):
         policy2devices = self.get_policy2devices()
         return reduce(set.union, (
             set(d['device'] for d in devices)
-            for devices in policy2devices.values()))
+            for devices in policy2devices.values()), set())
 
     def collect_parts(self, override_devices=None, override_partitions=None):
         """

test/unit/obj/test_reconstructor.py

@@ -3856,6 +3856,20 @@ class TestObjectReconstructor(BaseTestObjectReconstructor):
         # hashpath is still there, but it's empty
         self.assertEqual([], os.listdir(df._datadir))
 
+    def test_get_local_devices(self):
+        local_devs = self.reconstructor.get_local_devices()
+        self.assertEqual({'sda'}, local_devs)
+
+    @patch_policies(legacy_only=True)
+    def test_get_local_devices_with_no_ec_policy_env(self):
+        # even no ec_policy found on the server, it runs just like as
+        # no ec device found
+        self.policy = POLICIES.default
+        self._configure_reconstructor()
+        self.assertEqual([], self.reconstructor.policies)
+        local_devs = self.reconstructor.get_local_devices()
+        self.assertEqual(set(), local_devs)
+
 
 class TestReconstructFragmentArchive(BaseTestObjectReconstructor):
     obj_path = '/a/c/o'  # subclass overrides this