l2cache endpoint fails for segments with single level2id

[unidesigner]

As per discussion here and here. Needs a fix to return a single node graph (no edge).

[bdpedigo]

running into this issue also. is this a desired change to the endpoint or an edge case you dont want to support @fcollman @ceesem ?

[ceesem]

I think it's an edge case from not handling an empty list correctly and we should fix it in an upcoming version. However, would you mind pasting a more complete description of the error here just so that the only documentation isn't on a slack channel? Also, I think the issue is with the l2graph endpoint of the pychunkedgraph, not anything to do with the l2 cache service, but I would like to make sure we are all on the same page.

[bdpedigo]

Here's a minimal working example

import caveclient as cc

client = cc.CAVEclient("minnie65_phase3_v1")

root_id = 864691133037423712

print(len(client.chunkedgraph.get_leaves(root_id, stop_layer=2)))

client.chunkedgraph.level2_chunk_graph(root_id)

1

---------------------------------------------------------------------------
HTTPError                                 Traceback (most recent call last)
[/Users/bpedigo/aibs-code/cave-explorer/cave-explorer/experiments/edit_graph_edge_tracking.py](https://file+.vscode-resource.vscode-cdn.net/Users/bpedigo/aibs-code/cave-explorer/cave-explorer/experiments/edit_graph_edge_tracking.py) in line 11
      [220](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/experiments/edit_graph_edge_tracking.py?line=219) root_id = 864691133037423712
      [222](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/experiments/edit_graph_edge_tracking.py?line=221) print(len(client.chunkedgraph.get_leaves(root_id, stop_layer=2)))
---> [224](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/experiments/edit_graph_edge_tracking.py?line=223) client.chunkedgraph.level2_chunk_graph(root_id)

File [~/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/chunkedgraph.py:671](https://file+.vscode-resource.vscode-cdn.net/Users/bpedigo/aibs-code/cave-explorer/~/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/chunkedgraph.py:671), in ChunkedGraphClientV1.level2_chunk_graph(self, root_id)
    [669](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/chunkedgraph.py?line=668) endpoint_mapping["root_id"] = root_id
    [670](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/chunkedgraph.py?line=669) url = self._endpoints["lvl2_graph"].format_map(endpoint_mapping)
--> [671](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/chunkedgraph.py?line=670) r = handle_response(self.session.get(url))
    [672](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/chunkedgraph.py?line=671) return r["edge_graph"]

File [~/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py:88](https://file+.vscode-resource.vscode-cdn.net/Users/bpedigo/aibs-code/cave-explorer/~/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py:88), in handle_response(response, as_json, log_warning)
     [86](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=85) def handle_response(response, as_json=True, log_warning=True):
     [87](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=86)     """Deal with potential errors in endpoint response and return json for default case"""
---> [88](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=87)     _raise_for_status(response, log_warning=log_warning)
     [89](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=88)     _check_authorization_redirect(response)
     [90](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=89)     if as_json:

File [~/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py:79](https://file+.vscode-resource.vscode-cdn.net/Users/bpedigo/aibs-code/cave-explorer/~/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py:79), in _raise_for_status(r, log_warning)
     [71](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=70)     http_error_msg = "%s Server Error: %s for url: %s content:%s" % (
     [72](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=71)         r.status_code,
     [73](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=72)         reason,
     [74](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=73)         r.url,
     [75](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=74)         r.content,
     [76](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=75)     )
     [78](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=77) if http_error_msg:
---> [79](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=78)     raise requests.HTTPError(http_error_msg, response=r)
     [80](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=79) if log_warning:
     [81](file:///Users/bpedigo/aibs-code/cave-explorer/cave-explorer/.venv/lib/python3.11/site-packages/caveclient/base.py?line=80)     warning = r.headers.get("Warning")

HTTPError: 500 Server Error: need at least one array to concatenate for url: https://minnie.microns-daf.com/segmentation/api/v1/table/minnie3_v1/node/864691133037423712/lvl2_graph content:b'{\n  "code": 500,\n  "duration": 23.15545082092285,\n  "message": "need at least one array to concatenate",\n  "timestamp": "2023-10-12 18:28:05.642986",\n  "traceback": [\n    "Traceback (most recent call last):\\n",\n    "  File \\"/usr/local/lib/python3.7/site-packages/flask/app.py\\", line 1820, in full_dispatch_request\\n    rv = self.dispatch_request()\\n",\n    "  File \\"/usr/local/lib/python3.7/site-packages/flask/app.py\\", line 1796, in dispatch_request\\n    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)\\n",\n    "  File \\"/usr/local/lib/python3.7/site-packages/middle_auth_client/decorators.py\\", line 270, in decorated_function\\n    return f(*args, **kwargs)\\n",\n    "  File \\"/usr/local/lib/python3.7/site-packages/middle_auth_client/decorators.py\\", line 388, in decorated_function\\n    return f(*args, **kwargs)\\n",\n    "  File \\"./pychunkedgraph/app/app_utils.py\\", line 50, in decorated_function\\n    return f(*args, **kwargs)\\n",\n    "  File \\"./pychunkedgraph/app/segmentation/v1/routes.py\\", line 563, in handle_get_lvl2_graph\\n    resp = common.handle_get_layer2_graph(table_id, node_id)\\n",\n    "  File \\"./pychunkedgraph/app/segmentation/common.py\\", line 1047, in handle_get_layer2_graph\\n    edge_graph = pathing.get_lvl2_edge_list(cg, int(node_id))\\n",\n    "  File \\"./pychunkedgraph/graph/analysis/pathing.py\\", line 89, in get_lvl2_edge_list\\n    edge_array = np.concatenate(edge_array)\\n",\n    "  File \\"<__array_function__ internals>\\", line 6, in concatenate\\n",\n    "ValueError: need at least one array to concatenate\\n"\n  ]\n}\n'