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[Bug 1706750] Re: FlushError for automatic IPAllocations when subnet create is retried

To: yahoo-eng-team@xxxxxxxxxxxxxxxxxxx
From: OpenStack Infra <1706750@xxxxxxxxxxxxxxxxxx>
Date: Thu, 21 Dec 2017 20:56:39 -0000
Reply-to: Bug 1706750 <1706750@xxxxxxxxxxxxxxxxxx>
Sender: bounces@xxxxxxxxxxxxx
Reviewed:  https://review.openstack.org/529169
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=d4c41071645d0c25485d93886fbc09ca671ea18b
Submitter: Zuul
Branch:    master

commit d4c41071645d0c25485d93886fbc09ca671ea18b
Author: Ihar Hrachyshka <ihrachys@xxxxxxxxxx>
Date:   Tue Dec 19 13:54:02 2017 -0800

    ipam: don't commit IPAllocation while IpamAllocation is rolled back
    
    Don't mix old and new session management styles. Mixing them is not
    guaranteed to work. The method is not switched to new enginefacade yet,
    so stick to the old style. Since OVO create() method already opens a
    nested transaction, do nothing in IPAM layer.
    
    This change fixes an issue where sometimes, whenever
    add_auto_addrs_on_network_ports raises a retriable exception like
    StaleDataError, corresponding IPAllocation is still persisted in the
    database without being rolled back and without the corresponding
    IpamAllocation model persisted. Later, this situation may break port and
    network deletion for resources affected by the issue, failing with the
    following error:
    
    Unable to find IP address 2003::f816:3eff:fed2:5006 on subnet
    eed623f4-cbad-488c-b230-7f4a8b1514eb
    
    It happens because whenever writer.using context manager exits, and it's
    top level manager, it commits the changes to database. Since
    writer.using doesn't know that it's being executed in scope of the old
    facade .begin() call, it doesn't postpone commitment as needed.
    
    The bug was introduced in Pike as part of the switch to the new
    enginefacade (that was never fully completed), specifically by:
    I50be115ea69f805b48b02aebe4259ec2c839830e
    
    For existing setups that already have IPAllocation objects without
    IpamAllocation counterparts, the solution is to remove those
    dangling IPAllocations manually from the database. (That's of course
    assuming they use Neutron DB IPAM plugin in the first place.)
    
    Note: If we ever decide to remove nested transactions from OVO base
    layer, we may need to revisit this code to keep ignore-on-failure
    semantics.
    
    Change-Id: Ic9e146f51d9a120011892828d3a67b0630f4c5ce
    Closes-Bug: #1706750


** Changed in: neutron
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1706750

Title:
  FlushError for automatic IPAllocations when subnet create is retried

Status in neutron:
  Fix Released

Bug description:
  The error is seen in Red Hat CI: https://bugzilla.redhat.com/show_bug.cgi?id=1470784
  There are hits in Kibana too: http://logstash.openstack.org/#dashboard/file/logstash.json?query=message%3A%5C%22FlushError%3A%20New%20instance%20%3CIPAllocation%20at%5C%22

  In server.log:

  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation [req-3c97d83e-3beb-4b28-a29f-697fc1ee8bc6 5f2e0d72ef8e4fb18b500759cfe5e503 205f2a3630a0426f8da20b4467ea003b - default default] POST failed.: FlushError: New instance <IPAllocation at 0x8b3f7d0> with identity key (<class 'neutron.db.models_v2.IPAllocation'>, ('2003::1:f816:3eff:fe3f:aa00', '8032a1cd-9236-4126-83fa-2f039ed2b890', 'c0542b84-bcd0-48e1-ace1-6da1f50246dc')) conflicts with persistent instance <IPAllocation at 0x89ac310>
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation Traceback (most recent call last):
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pecan/core.py", line 678, in __call__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.invoke_controller(controller, args, kwargs, state)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/pecan/core.py", line 569, in invoke_controller
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     result = controller(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 94, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     setattr(e, '_RETRY_EXCEEDED', True)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 90, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_db/api.py", line 150, in wrapper
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     ectxt.value = e.inner_exc
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_db/api.py", line 138, in wrapper
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 129, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     LOG.debug("Retry wrapper got retriable exception: %s", e)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 125, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*dup_args, **dup_kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/pecan_wsgi/controllers/utils.py", line 70, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/pecan_wsgi/controllers/resource.py", line 152, in post
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return self.create(resources)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/pecan_wsgi/controllers/resource.py", line 170, in create
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return {key: creator(*creator_args, **creator_kwargs)}
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/common/utils.py", line 665, in inner
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(self, context, *args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 164, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return method(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 94, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     setattr(e, '_RETRY_EXCEEDED', True)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 90, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_db/api.py", line 150, in wrapper
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     ectxt.value = e.inner_exc
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_db/api.py", line 138, in wrapper
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 129, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     LOG.debug("Retry wrapper got retriable exception: %s", e)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 125, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*dup_args, **dup_kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/plugin.py", line 985, in create_subnet
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     result, mech_context = self._create_subnet_db(context, subnet)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/plugins/ml2/plugin.py", line 977, in _create_subnet_db
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self._create_subnet_postcommit(context, result, net_db, ipam_sub)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 164, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return method(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 94, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     setattr(e, '_RETRY_EXCEEDED', True)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 90, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_db/api.py", line 150, in wrapper
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     ectxt.value = e.inner_exc
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_db/api.py", line 138, in wrapper
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 129, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     LOG.debug("Retry wrapper got retriable exception: %s", e)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.force_reraise()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     six.reraise(self.type_, self.value, self.tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 125, in wrapped
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     return f(*dup_args, **dup_kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/db_base_plugin_v2.py", line 675, in _create_subnet_postcommit
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     result, ipam_subnet)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/db/ipam_pluggable_backend.py", line 491, in add_auto_addrs_on_network_ports
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     allocated.create()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/objects/base.py", line 225, in decorator
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     res = func(self, *args, **kwargs)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/objects/base.py", line 632, in create
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.modify_fields_to_db(fields))
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib/python2.7/site-packages/neutron/objects/db/api.py", line 61, in create_object
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     context.session.add(db_obj)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 567, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.rollback()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 66, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     compat.reraise(exc_type, exc_value, exc_tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 564, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.commit()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 461, in commit
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self._prepare_impl()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 441, in _prepare_impl
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self.session.flush()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 2171, in flush
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     self._flush(objects)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 2291, in _flush
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     transaction.rollback(_capture_exception=True)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 66, in __exit__
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     compat.reraise(exc_type, exc_value, exc_tb)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 2255, in _flush
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     flush_context.execute()
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 389, in execute
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     rec.execute(self)
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 548, in execute
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     uow
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/persistence.py", line 156, in save_obj
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     base_mapper, states, uowtransaction
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation   File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/persistence.py", line 312, in _organize_states_for_save
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation     state_str(existing)))
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation FlushError: New instance <IPAllocation at 0x8b3f7d0> with identity key (<class 'neutron.db.models_v2.IPAllocation'>, ('2003::1:f816:3eff:fe3f:aa00', '8032a1cd-9236-4126-83fa-2f039ed2b890', 'c0542b84-bcd0-48e1-ace1-6da1f50246dc')) conflicts with persistent instance <IPAllocation at 0x89ac310>
  2017-07-13 15:20:51.064 85185 ERROR neutron.pecan_wsgi.hooks.translation

  Just before the traceback, we see retry triggered by StaleDataError:

  2017-07-13 15:20:50.668 85185 DEBUG neutron.db.api [req-3c97d83e-3beb-4b28-a29f-697fc1ee8bc6 5f2e0d72ef8e4fb18b500759cfe5e503 205f2a3630a0426f8da20b4467ea003b - default default] Retry wrapper got retriable exception: UPDATE statement on table 'standardattributes' expected to update 1 row(s); 0 were matched. wrapped /usr/lib/python2.7/site-packages/neutron/db/api.py:129
  2017-07-13 15:20:50.669 85185 DEBUG oslo_db.api [req-3c97d83e-3beb-4b28-a29f-697fc1ee8bc6 5f2e0d72ef8e4fb18b500759cfe5e503 205f2a3630a0426f8da20b4467ea003b - default default] Performing DB retry for function neutron.db.db_base_plugin_v2._create_subnet_postcommit wrapper /usr/lib/python2.7/site-packages/oslo_db/api.py:152

  This is probably related to https://review.openstack.org/#/c/396718/

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References

[Bug 1706750] [NEW] FlushError for automatic IPAllocations when subnet create is retried
From: Ihar Hrachyshka, 2017-07-26