allow empty lists for connection_overrides in simulation config (#266)

* due to a quirk with how nlohmann::json flatten()/unflatten() work,
  empty arrays aren't preserved, thus our datatype check was failing to
  allow empty arrays.  Work around this
* hat tip to anilbey for noticing: #262 (comment)

python/tests/test_config.py

@@ -528,6 +528,20 @@ def test_expanded_json(self):
  config = json.loads(self.config.expanded_json)
  self.assertEqual(config['output']['output_dir'], 'some/path/output')
 
+ def test_empty_connection_overrides(self):
+ contents = """
+ {
+ "manifest": {
+ "$CIRCUIT_DIR": "./circuit"
+ },
+ "network": "$CIRCUIT_DIR/circuit_config.json",
+ "run": { "random_seed": 12345, "dt": 0.05, "tstop": 1000 },
+ "connection_overrides": []
+ }
+ """
+ conf = SimulationConfig(contents, "./")
+ self.assertEqual(conf.connection_overrides(), [])
+
  def test_run(self):
  contents = """
  {
@@ -557,7 +571,7 @@ def test_simulation_config_failures(self):
  },
  "network": "$CIRCUIT_DIR/circuit_config.json",
  "run": { "random_seed": 12345, "dt": 0.05, "tstop": 1000 },
- "connection_overrides": { }
+ "connection_overrides": {"foo": "bar"}
  }
  """
  SimulationConfig(contents, "./")

src/config.cpp

@@ -1080,7 +1080,12 @@ class SimulationConfig::Parser
  std::vector<ConnectionOverride> result;
 
  const auto connIt = _json.find("connection_overrides");
- if (connIt == _json.end()) {
+ // nlohmann::json::flatten().unflatten() converts empty containers to `null`:
+ // https://json.nlohmann.me/api/basic_json/unflatten/#notes
+ // so we can't tell the difference between {} and []; however, since these are
+ // empty, we will assume the intent was to have no connection_overrides and forgo
+ // better error reporting
+ if (connIt == _json.end() || connIt->is_null()) {
  return result;
  }