"""Agent authentication: per-PC credentials, with the shared key as fallback.

Every agent-facing endpoints route authenticates through authenticate_agent()
and, when the route names a PC, binds the caller through bind_hostname().

  X-Endpoint-Credential  The PC's own secret. The server derives the asset from
                         it; a hostname or assetid in the request that names a
                         different asset is refused with ASSET_MISMATCH.
  X-API-Key              The shared fleet key (or a collector-scoped managed
                         token). Accepted only while
                         plugin.endpoints.sharedkeyallowed is on, and never for a
                         PC that holds an active credential. A PC blocked
                         from re-enrolling has no active credential, so an
                         older agent on it keeps working on the shared key.

Only the sha256 of a credential is stored. The full secret exists once, in the
enroll response.
"""

import hashlib
import hmac
import re
import secrets
from datetime import datetime, timedelta, timezone

from flask import g, request

from shopdb.api import (
    db, Setting, error_response, ErrorCodes, collector_authorized,
)

CREDENTIAL_HEADER = 'X-Endpoint-Credential'
CREDENTIAL_PREFIX = 'sdbe_'
# sdbe_ + 43 chars base64url (32 random bytes, unpadded).
_CREDENTIAL_PATTERN = re.compile(r'^sdbe_[A-Za-z0-9_-]{43}$')
DISPLAY_PREFIX_LENGTH = 12

SHARED_KEY_SETTING = 'plugin.endpoints.sharedkeyallowed'
AUTO_ENROLL_SETTING = 'plugin.endpoints.autoenroll'

CODE_CREDENTIAL_INVALID = 'CREDENTIAL_INVALID'
CODE_SHARED_KEY_REFUSED = 'SHARED_KEY_REFUSED'
CODE_ASSET_MISMATCH = 'ASSET_MISMATCH'

AUTH_CREDENTIAL = 'credential'
AUTH_SHARED_KEY = 'sharedkey'

# lastusedat is a display value, not an audit trail: one write per 5 minutes
# per PC keeps the long-poll from turning every claim into an UPDATE.
_TOUCH_INTERVAL = timedelta(minutes=5)


def _naive_utcnow():
    return datetime.now(timezone.utc).replace(tzinfo=None)


def generate_credential():
    return CREDENTIAL_PREFIX + secrets.token_urlsafe(32)


def hash_credential(secret):
    return hashlib.sha256(secret.encode('utf-8')).hexdigest()


def shared_key_allowed():
    return Setting.get(SHARED_KEY_SETTING, default=True) is not False


def auto_enroll_enabled():
    return Setting.get(AUTO_ENROLL_SETTING, default=True) is not False


def active_credential_for(assetid):
    from ..models import EndpointCredential

    if assetid is None:
        return None
    return (EndpointCredential.query
            .filter(EndpointCredential.assetid == assetid,
                    EndpointCredential.revokedat.is_(None))
            .first())


def reenroll_blocked(assetid):
    """True when an admin blocked this asset from enrolling again."""
    from ..models import EndpointCredential

    if assetid is None:
        return False
    return (EndpointCredential.query
            .filter(EndpointCredential.assetid == assetid,
                    EndpointCredential.reenrollblocked.is_(True))
            .first()) is not None


def resolve_credential(secret, touch=True):
    """Asset id for an active credential, else None."""
    from ..models import EndpointCredential

    if not secret or not _CREDENTIAL_PATTERN.match(secret):
        return None
    digest = hash_credential(secret)
    row = EndpointCredential.query.filter_by(credentialhash=digest).first()
    if row is None or row.revokedat is not None:
        return None
    if not hmac.compare_digest(row.credentialhash, digest):
        return None
    if touch:
        now = _naive_utcnow()
        if row.lastusedat is None or now - row.lastusedat >= _TOUCH_INTERVAL:
            row.lastusedat = now
            db.session.commit()
    return row.assetid


def resolve_hostname(hostname):
    """Asset id of the one computer with this hostname (case-insensitive)."""
    from plugins.computers.models import Computer

    hostname = (hostname or '').strip()
    if not hostname:
        return None
    computer = (Computer.query
                .filter(db.func.lower(Computer.hostname) == hostname.lower())
                .first())
    return computer.assetid if computer is not None else None


def shared_key_refused_for(hostname):
    """True when the shared key may not report for this hostname."""
    if not shared_key_allowed():
        return True
    assetid = resolve_hostname(hostname)
    return assetid is not None and active_credential_for(assetid) is not None


def credential_invalid_response():
    return error_response(
        CODE_CREDENTIAL_INVALID,
        'Endpoint credential is unknown or revoked; enroll again',
        http_code=401)


def shared_key_refused_response():
    return error_response(
        CODE_SHARED_KEY_REFUSED,
        'The shared key is not accepted for this endpoint; use its credential',
        http_code=401)


def asset_mismatch_response():
    return error_response(
        CODE_ASSET_MISMATCH,
        'The request names a different endpoint than the credential',
        http_code=403)


def authenticate_agent():
    """Authorize an agent request. Returns None when authorized, else a response.

    Sets g.agentauth ('credential' or 'sharedkey') and g.agentassetid (the
    credential's asset, or None until bind_hostname resolves one).
    """
    g.agentauth = None
    g.agentassetid = None

    secret = request.headers.get(CREDENTIAL_HEADER)
    if secret:
        assetid = resolve_credential(secret.strip())
        if assetid is None:
            return credential_invalid_response()
        g.agentauth = AUTH_CREDENTIAL
        g.agentassetid = assetid
        return None

    error = collector_authorized('endpoints')
    if error is not None:
        return error
    if not shared_key_allowed():
        return shared_key_refused_response()
    g.agentauth = AUTH_SHARED_KEY
    return None


def is_credential_caller():
    return getattr(g, 'agentauth', None) == AUTH_CREDENTIAL


def bind_hostname(hostname):
    """The asset this request acts for. Returns (assetid, error_response).

    Credential: the credential's asset; a hostname naming another asset is an
    ASSET_MISMATCH, and a missing hostname is fine.
    Shared key: the hostname's asset (None when absent or unknown, for the
    route to answer); an enrolled PC is refused.
    """
    hostname = (hostname or '').strip()
    if is_credential_caller():
        if hostname and resolve_hostname(hostname) != g.agentassetid:
            return None, asset_mismatch_response()
        return g.agentassetid, None

    if not hostname:
        return None, None
    assetid = resolve_hostname(hostname)
    if assetid is not None and active_credential_for(assetid) is not None:
        return None, shared_key_refused_response()
    g.agentassetid = assetid
    return assetid, None