"""Real-fd regression canary.

Drives a real socket code path (the TCP reachability probe) in a tight loop and
asserts the process's open-fd count doesn't grow. If someone reintroduces a
socket leak in the probe path — the class of bug behind the Errno 24 lockups —
this fails in CI instead of in production. Uses real descriptors, not fakes, so
the assertion has teeth.
"""
import asyncio
import os

import pytest

from steward.monitors.ping import tcp_check

_ITERATIONS = 100


def _fd_count() -> int | None:
    try:
        return len(os.listdir("/proc/self/fd"))
    except OSError:
        return None


async def _hammer_tcp_check() -> None:
    # 127.0.0.1:1 has no listener → connection refused immediately. Each call must
    # fully release its socket; a leak would add ~one fd per iteration.
    for _ in range(_ITERATIONS):
        await tcp_check("127.0.0.1", 1)


def test_tcp_check_does_not_leak_fds():
    if _fd_count() is None:
        pytest.skip("/proc/self/fd unavailable on this platform")

    asyncio.run(_hammer_tcp_check())   # warm up (lazy imports, caches)
    before = _fd_count()
    asyncio.run(_hammer_tcp_check())
    after = _fd_count()

    # Small slack for interpreter-internal fds; a genuine leak over 100 iterations
    # would be far larger than this.
    assert after - before <= 5, (
        f"open fds grew {before}->{after} over {_ITERATIONS} tcp_check calls "
        "— possible socket leak")