config: fail fast on missing TempCtrl PID gains

Config access was unchecked dict[key] everywhere, so a missing/
misnamed gain (we've hit this once already: config.json.sim's gain/
Model nesting mismatch) surfaced as a bare KeyError from deep inside
some component's __init__ or first process() tick, with no indication
which config key was wrong.

utils/config_validate.py adds a small stdlib-only dotted-path checker
(no schema library - matches this project's stdlib-first leaning) and
validate_temp_ctrl(), scoped to the section that actually caused an
incident: TempCtrl.pid_type plus every kp/ki/kd/kt gain under
TempCtrl.Outer and TempCtrl.Inner.{Heat,Hold,Cool}. Called once in
server/brewpi.py right after json.load(), before any factory runs.
Optional-with-default keys (Outer.y_hold_min, Inner.Hold.yi_max,
Thresholds) stay optional here too.

Heater/Stirrer/TempSensor/Pot sections and unknown-key warnings are
deliberately left for later - see components/pid/TODO.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F4oEE99rkKVU8L8tkzXWdG

components/pid/TODO.md

@@ -55,17 +55,26 @@ git history for `temp_controller.py`/`temp_controller_smith.py`).
   at both call sites (`brewpi.py`, `components/sud_forecast.py`) were
   removed along with it.
 
-- [ ] **Config access is unchecked `dict[key]` everywhere.**
-  `params['Hold']`, `params['Td']`, etc., throughout, with no schema
-  validation at load time. We already hit this bug class once
-  (`config.json.sim`'s `gain`/`Model` nesting mismatch). A small
-  schema/dataclass validation layer at config-load would surface
-  errors immediately instead of mid-`__init__` — and would have caught
-  the dead `TempCtrl.Kalman` / `Model.kn` / `Plant.kn` keys that used
-  to sit unused in `config-sim.json.tpl`/`.sim` (since deleted), and the
-  `Model.gain`/`Plant.gain` keys that did the same in `config.json.sim`
-  before that file was removed entirely — `Pot` dropped `gain`
-  entirely, see `components/plant/TODO.md`.
+- [x] **Config access is unchecked `dict[key]` everywhere.** Partially
+  fixed, scoped to the section that actually caused an incident
+  (`config.json.sim`'s `gain`/`Model` nesting mismatch): `utils/
+  config_validate.py`'s `require()` is a small stdlib-only dotted-path
+  checker (no schema library added - matches this project's no-pytest/
+  stdlib-first preference), and `validate_temp_ctrl()` requires
+  `TempCtrl.pid_type` plus every `kp`/`ki`/`kd`/`kt` gain under
+  `TempCtrl.Outer` and `TempCtrl.Inner.{Heat,Hold,Cool}` - the ones every
+  `pid_type` reads unconditionally. Called once in `server/brewpi.py`
+  right after `json.load()`, before any factory runs, so a missing/
+  misnamed gain now fails fast with `ConfigError: missing required
+  config key: TempCtrl.Inner.Hold.kp` instead of a bare `KeyError` from
+  inside some component's `__init__` or first `process()` tick. Covered
+  by `tests/utils/test_config_validate.py`.
+  Deliberately left for later, since neither has actually crashed
+  anything yet: `Heater`/`Stirrer`/`TempSensor`/`Pot` config sections
+  (all already default gracefully via `.get(..., default)` in
+  `PlantFactory` etc.), and erroring (vs. just `log.warning()`) on
+  unknown/stale keys like the old `TempCtrl.Kalman`/`Model.kn`/
+  `Plant.kn`/`Model.gain` leftovers - see `components/plant/TODO.md`.
 
 - [x] **`pid_heat` can wind up during a `HOLD`-state disturbance with no
   anti-windup engagement.** A cold-water disturbance while holding drove

server/brewpi.py

@@ -10,6 +10,7 @@ import threading
 import time
 from http.server import SimpleHTTPRequestHandler, ThreadingHTTPServer
 from utils import ChangedFloat
+from utils.config_validate import validate_temp_ctrl
 from ws.message import MessageDispatcher
 from ws.server.ws_server_multi_user import WsServerMultiUser
 from components.pid import PidFactory
@@ -103,6 +104,7 @@ if __name__ == '__main__':
 	log.info("Logging to {}".format(log_path))
 
 	config = json.load(open(args.config))
+	validate_temp_ctrl(config)
 	dispatcher = MessageDispatcher()
 	server = WsServerMultiUser(listener=dispatcher)
 	taskmgr = TaskManager()

tests/utils/test_config_validate.py

@@ -0,0 +1,72 @@
+import unittest
+
+from utils.config_validate import ConfigError, require, validate_temp_ctrl
+
+
+def _valid_temp_ctrl_config():
+    gains = {"kp": 0.08, "ki": 0.02, "kd": 0.0, "kt": 1.5}
+    return {
+        "TempCtrl": {
+            "pid_type": "Smith",
+            "Outer": dict(gains),
+            "Inner": {
+                "Heat": dict(gains),
+                "Hold": dict(gains),
+                "Cool": dict(gains),
+            },
+        }
+    }
+
+
+class TestRequire(unittest.TestCase):
+    def test_returns_value_when_present(self):
+        self.assertEqual(require({"a": {"b": 1}}, "a.b"), 1)
+
+    def test_raises_on_missing_leaf(self):
+        with self.assertRaises(ConfigError):
+            require({"a": {}}, "a.b")
+
+    def test_raises_on_missing_intermediate(self):
+        with self.assertRaises(ConfigError):
+            require({}, "a.b")
+
+    def test_error_names_full_path_up_to_missing_key(self):
+        with self.assertRaisesRegex(ConfigError, r"a\.b"):
+            require({"a": {}}, "a.b.c")
+
+    def test_raises_on_wrong_type(self):
+        with self.assertRaises(ConfigError):
+            require({"a": "not a number"}, "a", type=(int, float))
+
+    def test_accepts_matching_type(self):
+        self.assertEqual(require({"a": 1.5}, "a", type=(int, float)), 1.5)
+
+
+class TestValidateTempCtrl(unittest.TestCase):
+    def test_valid_config_passes(self):
+        validate_temp_ctrl(_valid_temp_ctrl_config())
+
+    def test_missing_gain_raises(self):
+        config = _valid_temp_ctrl_config()
+        del config["TempCtrl"]["Inner"]["Hold"]["kp"]
+        with self.assertRaisesRegex(ConfigError, r"TempCtrl\.Inner\.Hold\.kp"):
+            validate_temp_ctrl(config)
+
+    def test_missing_pid_type_raises(self):
+        config = _valid_temp_ctrl_config()
+        del config["TempCtrl"]["pid_type"]
+        with self.assertRaises(ConfigError):
+            validate_temp_ctrl(config)
+
+    def test_optional_keys_not_required(self):
+        config = _valid_temp_ctrl_config()
+        # Outer.y_hold_min, Inner.Hold.yi_max, and the whole Thresholds
+        # section all have their own defaults elsewhere and must stay
+        # optional here too.
+        validate_temp_ctrl(config)
+        self.assertNotIn("y_hold_min", config["TempCtrl"]["Outer"])
+        self.assertNotIn("Thresholds", config["TempCtrl"])
+
+
+if __name__ == '__main__':
+    unittest.main()

utils/config_validate.py

@@ -0,0 +1,35 @@
+class ConfigError(Exception):
+    """Raised for a missing or wrong-typed config key, named by its full
+    dotted path (e.g. "TempCtrl.Inner.Hold.kp") so the error points straight
+    at the offending line in config.json instead of surfacing as a bare
+    KeyError/TypeError from wherever the value first gets used."""
+
+
+def require(config, path, type=None):
+    node = config
+    parts = path.split('.')
+    for i, key in enumerate(parts):
+        if not isinstance(node, dict) or key not in node:
+            raise ConfigError("missing required config key: {}".format('.'.join(parts[:i + 1])))
+        node = node[key]
+    if type is not None and not isinstance(node, type):
+        type_name = ' or '.join(t.__name__ for t in type) if isinstance(type, tuple) else type.__name__
+        raise ConfigError("config key {} must be {}, got {!r}".format(path, type_name, node))
+    return node
+
+
+def validate_temp_ctrl(config):
+    """Checked eagerly at startup so a missing/misnamed PID gain fails with
+    a clear message here instead of a bare KeyError mid-__init__ or on the
+    first process() tick (this bit us once already: config.json.sim's gain/
+    Model nesting mismatch). Only the gains every pid_type actually reads
+    unconditionally are required; anything with its own default elsewhere
+    (Outer.y_hold_min, Inner.Hold.yi_max, the whole Thresholds section -
+    see temp_controller_base.py/temp_controller_fsm.py's DEFAULT_THRESHOLDS)
+    stays optional here too, on purpose."""
+    require(config, 'TempCtrl', dict)
+    require(config, 'TempCtrl.pid_type', str)
+    for gain in ('kp', 'ki', 'kd', 'kt'):
+        require(config, 'TempCtrl.Outer.{}'.format(gain), (int, float))
+        for branch in ('Heat', 'Hold', 'Cool'):
+            require(config, 'TempCtrl.Inner.{}.{}'.format(branch, gain), (int, float))