#!/usr/bin/env python3
from __future__ import annotations
"""
Shared RFC 5424 syslog helper for DECNET service templates.

Provides two functions consumed by every service's server.py:
  - syslog_line(service, hostname, event_type, severity, **fields) -> str
  - write_syslog_file(line: str) -> None
  - forward_syslog(line: str, log_target: str) -> None

RFC 5424 structure:
  <PRI>1 TIMESTAMP HOSTNAME APP-NAME PROCID MSGID [SD-ELEMENT] MSG

Facility: local0 (16), PEN for SD element ID: decnet@55555
"""

import logging
import logging.handlers
import os
import socket
from datetime import datetime, timezone
from pathlib import Path
from typing import Any

# ─── Constants ────────────────────────────────────────────────────────────────

_FACILITY_LOCAL0 = 16
_SD_ID = "decnet@55555"
_NILVALUE = "-"

SEVERITY_EMERG   = 0
SEVERITY_ALERT   = 1
SEVERITY_CRIT    = 2
SEVERITY_ERROR   = 3
SEVERITY_WARNING = 4
SEVERITY_NOTICE  = 5
SEVERITY_INFO    = 6
SEVERITY_DEBUG   = 7

_MAX_HOSTNAME = 255
_MAX_APPNAME  = 48
_MAX_MSGID    = 32

_LOG_FILE_ENV     = "DECNET_LOG_FILE"
_DEFAULT_LOG_FILE = "/var/log/decnet/decnet.log"
_MAX_BYTES        = 10 * 1024 * 1024  # 10 MB
_BACKUP_COUNT     = 5

# ─── Formatter ────────────────────────────────────────────────────────────────

def _sd_escape(value: str) -> str:
    """Escape SD-PARAM-VALUE per RFC 5424 §6.3.3."""
    return value.replace("\\", "\\\\").replace('"', '\\"').replace("]", "\\]")


def _sd_element(fields: dict[str, Any]) -> str:
    if not fields:
        return _NILVALUE
    params = " ".join(f'{k}="{_sd_escape(str(v))}"' for k, v in fields.items())
    return f"[{_SD_ID} {params}]"


def syslog_line(
    service: str,
    hostname: str,
    event_type: str,
    severity: int = SEVERITY_INFO,
    timestamp: datetime | None = None,
    msg: str | None = None,
    **fields: Any,
) -> str:
    """
    Return a single RFC 5424-compliant syslog line (no trailing newline).

    Args:
        service:    APP-NAME (e.g. "http", "mysql")
        hostname:   HOSTNAME (decky node name)
        event_type: MSGID    (e.g. "request", "login_attempt")
        severity:   Syslog severity integer (default: INFO=6)
        timestamp:  UTC datetime; defaults to now
        msg:        Optional free-text MSG
        **fields:   Encoded as structured data params
    """
    pri     = f"<{_FACILITY_LOCAL0 * 8 + severity}>"
    ts      = (timestamp or datetime.now(timezone.utc)).isoformat()
    host    = (hostname or _NILVALUE)[:_MAX_HOSTNAME]
    appname = (service  or _NILVALUE)[:_MAX_APPNAME]
    msgid   = (event_type or _NILVALUE)[:_MAX_MSGID]
    sd      = _sd_element(fields)
    message = f" {msg}" if msg else ""
    return f"{pri}1 {ts} {host} {appname} {_NILVALUE} {msgid} {sd}{message}"


# ─── File handler ─────────────────────────────────────────────────────────────

_file_logger: logging.Logger | None = None


def _get_file_logger() -> logging.Logger:
    global _file_logger
    if _file_logger is not None:
        return _file_logger

    log_path = Path(os.environ.get(_LOG_FILE_ENV, _DEFAULT_LOG_FILE))
    try:
        log_path.parent.mkdir(parents=True, exist_ok=True)
        handler = logging.handlers.RotatingFileHandler(
            log_path,
            maxBytes=_MAX_BYTES,
            backupCount=_BACKUP_COUNT,
            encoding="utf-8",
        )
    except OSError:
        handler = logging.StreamHandler()

    handler.setFormatter(logging.Formatter("%(message)s"))
    _file_logger = logging.getLogger("decnet.syslog")
    _file_logger.setLevel(logging.DEBUG)
    _file_logger.propagate = False
    _file_logger.addHandler(handler)
    return _file_logger


def write_syslog_file(line: str) -> None:
    """Append a syslog line to the rotating log file."""
    try:
        _get_file_logger().info(line)
    except Exception:
        pass


# ─── TCP forwarding ───────────────────────────────────────────────────────────

def forward_syslog(line: str, log_target: str) -> None:
    """Forward a syslog line over TCP to log_target (ip:port)."""
    if not log_target:
        return
    try:
        host, port = log_target.rsplit(":", 1)
        with socket.create_connection((host, int(port)), timeout=3) as s:
            s.sendall((line + "\n").encode())
    except Exception:
        pass