"""ICMP / TCP reachability checks — pure probes returning a result dict.

A failed check says why in `error_msg` (#5430): a down monitor that can't
say whether nothing answered or something refused leaves the operator to
guess.
"""
from __future__ import annotations
import asyncio
import logging
import socket
import time

logger = logging.getLogger(__name__)

TCP_TIMEOUT = 5.0
DEFAULT_TCP_PORT = 80
PING_WAIT_S = 2
PING_TIMEOUT = 5.0


def tcp_failure(exc: BaseException, address: str, port: int) -> str:
    """Why a TCP connect failed, in a phrase."""
    if isinstance(exc, (asyncio.TimeoutError, TimeoutError)):
        return f"No answer on port {port} within {TCP_TIMEOUT:g} s"
    if isinstance(exc, ConnectionRefusedError):
        return f"Connection refused on port {port}"
    if isinstance(exc, socket.gaierror):
        return f"Can't resolve {address}: {exc.strerror or exc}"
    if isinstance(exc, OSError):
        return f"Can't connect to port {port}: {exc.strerror or exc}"
    return str(exc) or type(exc).__name__


async def tcp_check(address: str, port: int) -> dict:
    """TCP connect probe. Returns {is_up, response_ms, error_msg}."""
    start = time.monotonic()
    try:
        reader, writer = await asyncio.wait_for(
            asyncio.open_connection(address, port),
            timeout=TCP_TIMEOUT,
        )
        writer.close()
        await writer.wait_closed()
        return {"is_up": True, "response_ms": round((time.monotonic() - start) * 1000, 2),
                "error_msg": None}
    except Exception as exc:
        return {"is_up": False, "response_ms": None, "error_msg": tcp_failure(exc, address, port)}


def ping_failure(returncode: int, stderr: str) -> str:
    """Why `ping -c 1` failed: exit 1 is no reply; anything else is ping's
    own complaint (an unknown name, no route), first line of it."""
    if returncode == 1:
        return f"No reply within {PING_WAIT_S} s"
    said = stderr.strip().splitlines()[0] if stderr.strip() else ""
    return said or f"ping failed (exit {returncode})"


async def icmp_check(address: str) -> dict:
    """ICMP echo via the system ping binary (setuid; no raw-socket privilege).

    Falls back to a TCP connect on the default port if the ping binary is
    unavailable or errors, so a host with ICMP filtered still gets a signal;
    a failure then says the TCP check stood in.
    """
    start = time.monotonic()
    try:
        proc = await asyncio.create_subprocess_exec(
            "ping", "-c", "1", "-W", str(PING_WAIT_S), address,
            stdout=asyncio.subprocess.DEVNULL,
            stderr=asyncio.subprocess.PIPE,
        )
    except Exception as exc:
        logger.warning("ICMP ping failed for %s: %s, falling back to TCP", address, exc)
        result = await tcp_check(address, DEFAULT_TCP_PORT)
        if not result["is_up"]:
            result["error_msg"] = f"ping unavailable, TCP check instead: {result['error_msg']}"
        return result
    try:
        _, stderr = await asyncio.wait_for(proc.communicate(), timeout=PING_TIMEOUT)
    except asyncio.TimeoutError:
        proc.kill()
        try:
            await asyncio.wait_for(proc.wait(), timeout=PING_TIMEOUT)
        except asyncio.TimeoutError:
            logger.warning("ping for %s did not exit after kill", address)
        return {"is_up": False, "response_ms": None,
                "error_msg": f"No reply within {PING_TIMEOUT:g} s"}
    if proc.returncode == 0:
        return {"is_up": True, "response_ms": round((time.monotonic() - start) * 1000, 2),
                "error_msg": None}
    return {"is_up": False, "response_ms": None,
            "error_msg": ping_failure(proc.returncode, (stderr or b"").decode(errors="replace"))}