Exception when deleting the discovery sub-system

[gbregman]

When enable_spdk_discovery_controller is set to True we see the discovery NQN "nqn.2014-08.org.nvmexpress.discovery" when running a get_subsystems CLI command. If we try to delete this sub-system we get an exception:

ceph-nvmeof-nvmeof-1  | INFO:control.grpc:delete_subsystem nqn.2014-08.org.nvmexpress.discovery: True
ceph-nvmeof-nvmeof-1  | DEBUG:control.state:omap_key removed: subsystem_nqn.2014-08.org.nvmexpress.discovery
ceph-nvmeof-nvmeof-1  | ERROR:control.grpc:Error persisting delete_subsystem nqn.2014-08.org.nvmexpress.discovery: 'subsystem_nqn.2014-08.org.nvmexpress.discovery'
ceph-nvmeof-nvmeof-1  | ERROR:grpc._server:Exception calling application: 'subsystem_nqn.2014-08.org.nvmexpress.discovery'
ceph-nvmeof-nvmeof-1  | Traceback (most recent call last):
ceph-nvmeof-nvmeof-1  |   File "/src/__pypackages__/3.9/lib/grpc/_server.py", line 494, in _call_behavior
ceph-nvmeof-nvmeof-1  |     response_or_iterator = behavior(argument, context)
ceph-nvmeof-nvmeof-1  |   File "/src/control/grpc.py", line 324, in delete_subsystem
ceph-nvmeof-nvmeof-1  |     return self.delete_subsystem_safe(request, context)
ceph-nvmeof-nvmeof-1  |   File "/src/control/grpc.py", line 314, in delete_subsystem_safe
ceph-nvmeof-nvmeof-1  |     self.gateway_state.remove_subsystem(request.subsystem_nqn)
ceph-nvmeof-nvmeof-1  |   File "/src/control/state.py", line 400, in remove_subsystem
ceph-nvmeof-nvmeof-1  |     self.local.remove_subsystem(subsystem_nqn)
ceph-nvmeof-nvmeof-1  |   File "/src/control/state.py", line 75, in remove_subsystem
ceph-nvmeof-nvmeof-1  |     self._remove_key(key)
ceph-nvmeof-nvmeof-1  |   File "/src/control/state.py", line 135, in _remove_key
ceph-nvmeof-nvmeof-1  |     self.state.pop(key)
ceph-nvmeof-nvmeof-1  | KeyError: 'subsystem_nqn.2014-08.org.nvmexpress.discovery'

We need to decide how do we want to handle such a deletion. Should we allow it? Should we check for this value and fail the CLI? And if we allow it, should we allow creating a sub-system with this NQN?

[gbregman]

When we use the discovery python script we delete subsystem "nqn.2014-08.org.nvmexpress.discovery" from SPDK. Now we can use a create_subsystem CLI command to create a subsystem with that NQN. This subsystem still has a type of NVMe and not discovery but its NQN is one of a discovery subsystem. This key is added to OMAP. Could it clash with other gateways? Wouldn't it be simpler to not allow is in the CLI? The NQN should be a unique value which identifies a subsystem. In this case it is not.