app/backend/fluksio/cloud/config.py

"""What this instance knows about the portal it is enrolled with.

One file on the data volume, beside the OAuth keypair and for the same reason:
it is a credential, it must survive a rebuild, and it must never be in the
flows repository. Deleting it is the local, unilateral way to sever the
connection — the portal's tokens stop verifying immediately, whatever the
portal still believes.

The portal's public keys are stored here rather than fetched per request. They
are pinned at enrolment, when a person was holding a claim code they had just
read off the portal's own screen, and replaced by the set the hub sends each
time the tunnel attaches, so the hub can publish a new key beside the old one.
That makes the tunnel's TLS what a later key rests on: only a hub that answers
at the enrolled address, to this instance's own credential, can hand it one.
"""

from __future__ import annotations

import json
import logging
import os
import secrets
from collections.abc import Mapping
from dataclasses import dataclass
from typing import Any

import jwt
from jwt.exceptions import InvalidTokenError

from fluksio.core.config import settings

logger = logging.getLogger(__name__)

#: Marks a request the connector replayed off the tunnel. The value is minted
#: per process and never leaves it, because the header itself is not evidence:
#: anything that can reach this API directly can set one, and the difference
#: decides whether a pairing device is handed a credential the whole internet
#: can present. The connector overwrites it on every frame, so a browser
#: sending its own gets nowhere from either direction.
VIA_HEADER = "x-fluksio-via"
VIA_PORTAL = secrets.token_urlsafe(16)


@dataclass(frozen=True)
class CloudConfig:
    portal_url: str
    ws_url: str
    instance_id: str
    token: str
    issuer: str
    jwks: dict[str, Any]
    #: The local account this instance acts as on its own behalf: what the
    #: health summary is collected as, and what a screen paired through the
    #: portal borrows for want of a person. Recorded at enrolment from whoever
    #: performed it. Portal *sessions* no longer come through here — they name
    #: a person, and are resolved to the local account mapped to them.
    local_user_id: str
    enrolled_at: str
    portal_account: str | None = None


_cache: tuple[float, CloudConfig | None] | None = None


def _read() -> CloudConfig | None:
    """Load the config, re-reading only when the file has changed."""
    global _cache
    path = settings.CLOUD_CONFIG_FILE
    try:
        mtime = path.stat().st_mtime
    except OSError:
        _cache = (0.0, None)
        return None
    if _cache is not None and _cache[0] == mtime:
        return _cache[1]
    try:
        data = json.loads(path.read_text())
        # A file written before the installation/instance rename. Adopt it
        # rather than failing to parse, which would read as "never enrolled".
        if "installation_id" in data:
            data["instance_id"] = data.pop("installation_id")
        config = CloudConfig(**data)
    except (OSError, ValueError, TypeError):
        logger.exception("Could not read %s; remote access stays off", path)
        _cache = (mtime, None)
        return None
    _cache = (mtime, config)
    return config


def load() -> CloudConfig | None:
    return _read()


def exists() -> bool:
    return settings.CLOUD_CONFIG_FILE.exists()


def save(config: CloudConfig) -> None:
    path = settings.CLOUD_CONFIG_FILE
    path.parent.mkdir(parents=True, exist_ok=True)
    # Written through a temporary file so a crash mid-write cannot leave a
    # half-parsed credential behind, and never group- or world-readable.
    tmp = path.with_suffix(".tmp")
    tmp.write_text(json.dumps(config.__dict__, indent=2))
    os.chmod(tmp, 0o600)
    tmp.replace(path)
    global _cache
    _cache = None


def delete() -> None:
    settings.CLOUD_CONFIG_FILE.unlink(missing_ok=True)
    global _cache
    _cache = None


def via_portal(headers: Mapping[str, str]) -> bool:
    """Whether the connector replayed this request off the tunnel."""
    return secrets.compare_digest(headers.get(VIA_HEADER, ""), VIA_PORTAL)


def portal_base(headers: Mapping[str, str]) -> str | None:
    """Where this instance answers at the hub, for a request that came through it.

    The hub serves an instance under ``/i/{id}`` on the host it signs as, the
    issuer enrolment pinned. ``None`` for a request made here.
    """
    config = _read()
    if config is None or not via_portal(headers):
        return None
    return f"{config.issuer.rstrip('/')}/i/{config.instance_id}"


def decode_portal_token(token: str) -> dict[str, Any]:
    """Validate a token the portal minted for this instance.

    Raises ``InvalidTokenError`` for everything else, including the ordinary
    case of not being enrolled at all — which is what makes this branch free
    for an instance nobody connected.
    """
    config = _read()
    if config is None:
        raise InvalidTokenError("this instance is not enrolled with a portal")

    # By `kid`, so the hub can publish the next key beside the current one and
    # tokens signed with either verify while it rotates.
    kid = jwt.get_unverified_header(token).get("kid")
    try:
        keys = jwt.PyJWKSet.from_dict(config.jwks).keys
    except (jwt.PyJWKError, jwt.PyJWKSetError) as exc:
        raise InvalidTokenError(f"the pinned portal keys are unusable: {exc}") from exc
    key = next((candidate for candidate in keys if candidate.key_id == kid), None)
    if key is None:
        raise InvalidTokenError("signed with a key the portal has not published")

    claims: dict[str, Any] = jwt.decode(
        token,
        key,
        algorithms=["RS256"],
        # The audience is this instance's own id, so a token the portal
        # minted for somebody else's machine fails here.
        audience=config.instance_id,
        issuer=config.issuer,
    )
    scope = claims.get("scope")
    if scope == "panel":
        # A screen that paired through the portal. The portal named the panel
        # and the nonce this instance gave it when asking, nothing else; what
        # that panel may read is decided here, by the same checks a panel
        # paired on this network passes, the nonce included. A token of this
        # scope that names no panel is refused.
        panel = str(claims.get("sub") or "")
        if not panel:
            raise InvalidTokenError("a panel token must name its panel")
        return {"panel": panel, "pnc": claims.get("pnc")}
    if scope != "proxy":
        raise InvalidTokenError("not a proxy token")
    # A portal session names the person holding it, and that name is the whole
    # of their identity here: the caller resolves it to the local account it
    # was mapped to, and a portal identity nobody mapped resolves to nothing.
    # Deliberately no local account by default — the failure of a mapping must
    # be a refusal, not a fallback onto whoever enrolled.
    portal_sub = str(claims.get("sub") or "")
    if not portal_sub:
        raise InvalidTokenError("a proxy token must name its user")
    return {"portal_sub": portal_sub}