Quantum Server failed to start due to empty api_extensions_path

[Edward]

The devstack script failed at creating subnet by enabling Quantum V2 with OVS plugin. The quantum service should be started at $Q_HOST:$Q_PORT (localhost:9696) but not which refused the CLI to create subnet via service API.

==============================Devstack.sh===========================================
# Create a small network
        NET_ID=$(quantum net-create --os_token $Q_ADMIN_USERNAME --os_url http://$Q_HOST:$Q_PORT --tenant_id $TENANT_ID net1 | grep ' id ' | get_field 2)

        # Create a subnet
        quantum subnet-create --os_token $Q_ADMIN_USERNAME --os_url http://$Q_HOST:$Q_PORT --tenant_id $TENANT_ID --ip_version 4 --gateway $NETWORK_GATEWAY $NET_ID $FIXED_RANGE

===================================================================================

=====================================Devstack Log====================================
++ quantum net-create --os_token quantum --os_url http://localhost:9696 --tenant_id 53d12248bbd147f2854faf49d6f78a34 net1
++ grep ' id '
++ get_field 2
++ read data
ERROR: cliff.app [Errno 111] Connection refused
+ NET_ID=
+ quantum subnet-create --os_token quantum --os_url http://localhost:9696 --tenant_id 53d12248bbd147f2854faf49d6f78a34 --ip_version 4 --gateway 10.0.0.1 10.0.0.0/24
usage: quantum subnet-create [-h] [-f {html,json,shell,table,yaml}]
                             [-c COLUMN] [--variable VARIABLE]
                             [--prefix PREFIX] [--request_format {json,xml}]
                             [--tenant_id tenant_id] [--ip_version {4,6}]
                             [--gateway gateway]
                             [--allocation_pool ALLOCATION_POOL]
                             network_id cidr ...
quantum subnet-create: error: too few arguments
++ failed
++ local r=2
++ set +o xtrace
===================================================================================

[Edward (zhang-hare)]

Quantum server failed to start at reading config file. This is the root cause of devstack failure.
================================quantum server log=========================
 cd /opt/stack/quantum && python /opt/stack/quantum/bin/quantum-server --config-file /etc/quantum/quantum.conf
Traceback (most recent call last):
  File "/opt/stack/quantum/bin/quantum-server", line 24, in <module>
    server()
  File "/opt/stack/quantum/quantum/server/__init__.py", line 34, in main
    config.parse(sys.argv)
  File "/opt/stack/quantum/quantum/common/config.py", line 53, in parse
    version='%%prog %s' % version_string())
  File "/opt/stack/quantum/quantum/openstack/common/cfg.py", line 1020, in __call__
    self._parse_config_files()
  File "/opt/stack/quantum/quantum/openstack/common/cfg.py", line 1436, in _parse_config_files
    read_ok = self._cparser.read(config_files)
  File "/opt/stack/quantum/quantum/openstack/common/cfg.py", line 885, in read
    parser.parse()
  File "/opt/stack/quantum/quantum/openstack/common/cfg.py", line 852, in parse
    return super(ConfigParser, self).parse(f)
  File "/opt/stack/quantum/quantum/openstack/common/iniparser.py", line 93, in parse
    key, value = self._split_key_value(line)
  File "/opt/stack/quantum/quantum/openstack/common/iniparser.py", line 56, in _split_key_value
    if value[0] == value[-1] and value[0] == "\"" or value[0] == "'":
IndexError: string index out of range
openstack@edward-ThinkPad-T61p:/opt/stack/quantum$
==============================================================================

[Edward (zhang-hare)]

A work around to resolve this problem is comment out the following setting:
# api_extensions_path =