Require that known-bad EC schemes be deprecated

We said we were going to do it, we've had two releases saying we'd do
it, we've even backported our saying it to Newton -- let's actually do
it.

Upgrade Consideration
=====================

Erasure-coded storage policies using isa_l_rs_vand and nparity >= 5 must
be configured as deprecated, preventing any new containers from being
created with such a policy. This configuration is known to harm data
durability. Any data in such policies should be migrated to a new
policy. See https://bugs.launchpad.net/swift/+bug/1639691 for more
information.

UpgradeImpact
Related-Change: I50159c9d19f2385d5f60112e9aaefa1a68098313
Change-Id: I8f9de0bec01032d9d9b58848e2a76ac92e65ab09
Closes-Bug: 1639691

doc/source/overview_erasure_code.rst

@@ -252,6 +252,15 @@ with the specified policy name and interacting as usual.
     case a change in the configuration is desired, you must create a new policy
     and migrate the data to a new container.
 
+.. warning::
+
+    Using ``isa_l_rs_vand`` with more than 4 parity fragments creates fragments
+    which may in some circumstances fail to reconstruct properly or (with
+    liberasurecode < 1.3.1) reconstruct corrupted data. New policies that need
+    large numbers of parity fragments should consider using ``isa_l_rs_cauchy``.
+    Any existing affected policies must be marked deprecated, and data in
+    containers with that policy should be migrated to a new policy.
+
 Migrating Between Policies
 ==========================
 

etc/swift.conf-sample

@@ -62,13 +62,14 @@ aliases = yellow, orange
 # EC configuration parameters 'ec_type', 'ec_num_data_fragments' and
 # 'ec_num_parity_fragments' must be specified.  'ec_type' is chosen from the
 # list of EC backends supported by PyECLib.  The ring configured for the
-# storage policy must have it's "replica" count configured to
+# storage policy must have its "replica" count configured to
 # 'ec_num_data_fragments' + 'ec_num_parity_fragments' - this requirement is
 # validated when services start.  'ec_object_segment_size' is the amount of
 # data that will be buffered up before feeding a segment into the
 # encoder/decoder.  More information about these configuration options and
-# supported `ec_type` schemes is available in the Swift documentation.  Please
-# refer to Swift documentation for details on how to configure EC policies.
+# supported 'ec_type' schemes is available in the Swift documentation.  See
+# https://docs.openstack.org/developer/swift/overview_erasure_code.html
+# for more information on how to configure EC policies.
 #
 # The example 'deepfreeze10-4' policy defined below is a _sample_
 # configuration with an alias of 'df10-4' as well as 10 'data' and 4 'parity'

swift/common/storage_policy.py

@@ -468,14 +468,10 @@ class ECStoragePolicy(BaseStoragePolicy):
                 'See https://bugs.launchpad.net/swift/+bug/1639691 for '
                 'more information.' % self.name)
             if not is_deprecated:
-                # TODO: To fully close bug 1639691, uncomment the raise and
-                # removing the warning below. This will be in the Pike release
-                # at the earliest.
-                logger.warning(
-                    'In a future release, this will prevent services from '
-                    'starting unless the policy is marked as deprecated.')
-                # raise PolicyError('Storage policy %s MUST be deprecated' %
-                #                   self.name)
+                raise PolicyError(
+                    'Storage policy %s uses an EC configuration known to harm '
+                    'data durability. This policy MUST be deprecated.'
+                    % self.name)
 
         # Initialize PyECLib EC backend
         try:

test/unit/common/test_storage_policy.py

@@ -676,18 +676,21 @@ class TestStoragePolicies(unittest.TestCase):
         ec_num_parity_fragments = 5
         """)
 
-        with capture_logging('swift.common.storage_policy') as records:
+        with capture_logging('swift.common.storage_policy') as records, \
+                self.assertRaises(PolicyError) as exc_mgr:
             parse_storage_policies(bad_conf)
-        mock_driver.assert_called_once()
+        self.assertEqual(exc_mgr.exception.message,
+                         'Storage policy bad-policy uses an EC '
+                         'configuration known to harm data durability. This '
+                         'policy MUST be deprecated.')
+        mock_driver.assert_not_called()
         mock_driver.reset_mock()
         self.assertEqual([r.levelname for r in records],
-                         ['WARNING', 'WARNING'])
+                         ['WARNING'])
         for msg in ('known to harm data durability',
                     'Any data in this policy should be migrated',
                     'https://bugs.launchpad.net/swift/+bug/1639691'):
             self.assertIn(msg, records[0].msg)
-        self.assertIn('In a future release, this will prevent services from '
-                      'starting', records[1].msg)
 
         slightly_less_bad_conf = self._conf("""
         [storage-policy:0]