Since position is a sequential sequence starting at 1 ，and the query for a listener from the database, the result of the return is sorted by position. So from l7polices.pop (position-1) is to find the policy. The exception is the position for the 2147483647 policy, which is the maximum position, it does not and other policy position composed of a continuous sequence.
The neutron-server log is as follows：
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource [req-2bf4ea84-89de-4eee-95c8-7ec715797c0e 142d8663efce464c89811c63e45bd82e f21a9c86d7114bf99c711f4874d80474 - - -] update failed: No details.
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource Traceback (most recent call last):
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/neutron/api/v2/resource.py", line 79, in resource
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource result = method(request=request, **args)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/neutron/api/v2/base.py", line 604, in update
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource return self._update(request, id, body, **kwargs)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 88, in wrapped
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource setattr(e, '_RETRY_EXCEEDED', True)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource self.force_reraise()
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource six.reraise(self.type_, self.value, self.tb)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 84, in wrapped
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource return f(*args, **kwargs)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/oslo_db/api.py", line 151, in wrapper
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource ectxt.value = e.inner_exc
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource self.force_reraise()
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource six.reraise(self.type_, self.value, self.tb)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/oslo_db/api.py", line 139, in wrapper
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource return f(*args, **kwargs)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 124, in wrapped
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource traceback.format_exc())
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource self.force_reraise()
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource six.reraise(self.type_, self.value, self.tb)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/neutron/db/api.py", line 119, in wrapped
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/neutron/api/v2/base.py", line 652, in _update
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource obj = obj_updater(request.context, id, **kwargs)
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource File "/usr/lib/python2.7/site-packages/neutron_lbaas/services/loadbalancer/plugin.py", line 915, in update_l7policy
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource raise exc
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource IndexError: pop index out of range
2017-07-19 10:55:02.324 44136 ERROR neutron.api.v2.resource
Can not repro on master.