thermograph/discord_interactions.py

"""Discord slash commands over the HTTP Interactions endpoint.

Discord POSTs each interaction (a slash command, a button, etc.) to a URL we
register in the developer portal, so commands are just an ordinary FastAPI route
(app.py wires POST {BASE}/discord/interactions) with no gateway connection and no
persistent bot process. Every request is signed; we verify the Ed25519 signature
over the raw body before trusting anything (Discord actively probes this with bad
signatures and disables the endpoint if we don't reject them).

Commands so far:
  /grade <city>  -> today's grade for a curated city, from the warm cache.

Register the command definitions with scripts/register_discord_commands.py.
"""
from __future__ import annotations

import datetime
import json
import os

from nacl.exceptions import BadSignatureError
from nacl.signing import VerifyKey

import cities
import discord
import homepage

# App's Ed25519 public key (Developer Portal -> General Information). Unset =>
# every request fails verification, which is the safe default for an unconfigured
# deployment.
PUBLIC_KEY = os.environ.get("THERMOGRAPH_DISCORD_PUBLIC_KEY", "").strip()

# Interaction types (Discord).
_PING = 1
_APPLICATION_COMMAND = 2
# Response types.
_PONG = 1
_CHANNEL_MESSAGE = 4
# Message flags: 64 = ephemeral (only the caller sees it).
_EPHEMERAL = 64


def verify(signature: str, timestamp: str, body: bytes, public_key: str | None = None) -> bool:
    """True iff `signature` is a valid Ed25519 signature of timestamp+body under the
    app public key. Any malformed input is a verification failure, never an error."""
    key = public_key if public_key is not None else PUBLIC_KEY
    if not key or not signature or not timestamp:
        return False
    try:
        VerifyKey(bytes.fromhex(key)).verify(timestamp.encode() + body, bytes.fromhex(signature))
        return True
    except (BadSignatureError, ValueError):
        return False


def _resolve_city(query: str) -> dict | None:
    """Best curated-city match for a free-text name: exact name first, then a
    prefix match, preferring the largest (cities.json is population-sorted)."""
    q = (query or "").strip().casefold()
    if not q:
        return None
    exact, prefix = None, None
    for c in cities.all_cities():          # already sorted by population, desc
        name = c["name"].casefold()
        if name == q:
            exact = exact or c
        elif prefix is None and name.startswith(q):
            prefix = c
    return exact or prefix


def _grade_message(query: str) -> dict:
    """The message data for /grade <city>: an embed with today's grade line, or a
    plain ephemeral note when the city is unknown or not yet warmed."""
    city = _resolve_city(query)
    if city is None:
        return {"content": f"I don't track a city called '{query}' yet. "
                           "Try a major city name.", "flags": _EPHEMERAL}
    card = homepage._grade_city(city, datetime.date.today())
    if card is None:
        return {"content": f"No recent reading is cached for {city['name']} yet. "
                           "It should appear once the data warms up.", "flags": _EPHEMERAL}
    color = discord._COLD if card.get("tail") == "cold" else discord._HOT
    return {"embeds": [{
        "title": f"{card.get('display') or city['name']} — today",
        "description": discord._line(card),
        "url": f"https://thermograph.org/climate/{city['slug']}",
        "color": color,
        "footer": {"text": "thermograph.org · graded against ~45 years of local history"},
    }]}


def _command_response(data: dict) -> dict:
    """Dispatch an application-command interaction to its handler."""
    name = (data.get("name") or "").lower()
    opts = {o.get("name"): o.get("value") for o in (data.get("options") or [])}
    if name == "grade":
        return {"type": _CHANNEL_MESSAGE, "data": _grade_message(opts.get("city", ""))}
    # Unknown command: acknowledge rather than error, so a stale registration can't
    # look broken to the user.
    return {"type": _CHANNEL_MESSAGE,
            "data": {"content": "Unknown command.", "flags": _EPHEMERAL}}


def handle(body: bytes, signature: str, timestamp: str) -> tuple[int, dict]:
    """Verify and route one interaction. Returns (http_status, json_body).

    401 on a bad/missing signature (what Discord's security probe expects); a PING
    is answered with a PONG; an application command is dispatched. The caller reads
    the RAW body for verification and passes it here unparsed."""
    if not verify(signature, timestamp, body):
        return 401, {"error": "bad signature"}
    try:
        payload = json.loads(body)
    except ValueError:
        return 400, {"error": "bad json"}
    itype = payload.get("type")
    if itype == _PING:
        return 200, {"type": _PONG}
    if itype == _APPLICATION_COMMAND:
        return 200, _command_response(payload.get("data") or {})
    # Components/modals/etc. aren't used yet; a PONG-shaped 200 is harmless.
    return 200, {"type": _CHANNEL_MESSAGE,
                 "data": {"content": "Unsupported interaction.", "flags": _EPHEMERAL}}