sim_config.py

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"""JSON-file sim configuration loader. Replaces the old per-variable
os.environ reads (RBC_WEST_HOST, RBC_WEST_TRAIN_PORT, RBC_TRAIN_INDEX,
...) that used to be set directly in docker-compose.yml's own
`environment:` block for each service.
 
One JSON file per sim INSTANCE, not per sim type - train-west and
train-east need different values (trainIndex, which RBC ports) even
though they run the exact same Docker image (safeapi-sim:latest, see
docker-compose.yml's own header on why one image is reused for every
instance) - so the file is supplied at container-start time via a
bind-mounted path (SIM_CONFIG_PATH env var, default
DEFAULT_CONFIG_PATH), not baked into the image. See
sims/config/*.json for the actual per-instance files and
docker-compose.yml's own `volumes:` entries for how each gets mounted.
 
Schema (fields present depend on sim type - train/il have `trainIndex`,
ctc does not; see each sim's own header for its exact expected keys):
  {
    "siteTag": "WEST",                          # display tag only, log prefix
    "trainIndex": 0,                             # train/il only - 0 or 1
    "rbcWest": {"host": "c-west", "port": 15004},
    "rbcEast": {"host": "c-east", "port": 15014},
    "controlServer": {"bindHost": "0.0.0.0", "port": 9100},
    "simPeriodSeconds": 2.0                       # train only
  }
"""
 
import json
import os
 
DEFAULT_CONFIG_PATH = "/app/config.json"
 
 
def load():
    """@return the parsed JSON config dict from SIM_CONFIG_PATH (env,
    default DEFAULT_CONFIG_PATH). Raises with the path included on a
    missing file or invalid JSON, so a bad config fails loudly and
    specifically at startup rather than as a generic KeyError somewhere
    inside main() once a sim tries to use a field that was never
    there."""
    path = os.environ.get("SIM_CONFIG_PATH", DEFAULT_CONFIG_PATH)
    try:
        with open(path, "r", encoding="utf-8") as f:
            text = f.read()
    except OSError as exc:
        raise FileNotFoundError(
            f"sim config file not found/readable: {path} ({exc}) - set SIM_CONFIG_PATH to override"
        ) from None
    try:
        return json.loads(text)
    except json.JSONDecodeError as exc:
        raise ValueError(f"sim config file {path} is not valid JSON: {exc}") from None
 
 
def require(config, *keys):
    """@return config[keys[0]][keys[1]]... - raises a clear KeyError
    (naming the full dotted path and the config file it came from) if
    any level is missing, rather than Python's own bare KeyError(key)
    for just the last, most confusing level."""
    value = config
    for i, key in enumerate(keys):
        if not isinstance(value, dict) or key not in value:
            dotted = ".".join(keys[: i + 1])
            raise KeyError(f"sim config is missing required field {dotted!r}")
        value = value[key]
    return value