"""Regression test for #17758 — chained pending-message drains must not
grow the call stack.

Before the fix, ``_process_message_background`` finished a turn, found a
pending follow-up, and drained it via ``await
self._process_message_background(pending_event, session_key)``.  Each
queued follow-up added a frame to the call stack instead of starting
fresh, so under sustained pending-queue activity the C stack would
exhaust at ~2000 nested frames and the process would crash with
SIGSEGV.

After the fix, the in-band drain spawns a fresh task (mirroring the
late-arrival drain pattern), so the stack stays bounded regardless of
chain length.

We assert the invariant directly: count nested
``_process_message_background`` frames at handler entry across a chain
of N follow-ups.  Recursion makes depth grow linearly (1, 2, 3, …, N);
task spawning keeps it constant (1 every time).
"""

import asyncio
import sys

import pytest
from unittest.mock import AsyncMock

from gateway.config import Platform, PlatformConfig
from gateway.platforms.base import (
    BasePlatformAdapter,
    MessageEvent,
    MessageType,
)
from gateway.session import SessionSource, build_session_key


class _StubAdapter(BasePlatformAdapter):
    async def connect(self):
        pass

    async def disconnect(self):
        pass

    async def send(self, chat_id, text, **kwargs):
        return None

    async def get_chat_info(self, chat_id):
        return {}


def _make_adapter():
    adapter = _StubAdapter(PlatformConfig(enabled=True, token="t"), Platform.TELEGRAM)
    adapter._send_with_retry = AsyncMock(return_value=None)
    return adapter


def _make_event(text="hi", chat_id="42"):
    return MessageEvent(
        text=text,
        message_type=MessageType.TEXT,
        source=SessionSource(platform=Platform.TELEGRAM, chat_id=chat_id, chat_type="dm"),
    )


def _sk(chat_id="42"):
    return build_session_key(
        SessionSource(platform=Platform.TELEGRAM, chat_id=chat_id, chat_type="dm")
    )


def _count_pmb_frames() -> int:
    """Walk the current call stack and count nested
    ``_process_message_background`` frames.  Used to detect recursive
    in-band drains."""
    f = sys._getframe()
    n = 0
    while f is not None:
        if f.f_code.co_name == "_process_message_background":
            n += 1
        f = f.f_back
    return n


@pytest.mark.asyncio
async def test_in_band_drain_does_not_grow_stack():
    """Issue #17758: chained pending-message drains must not recurse.

    Queue a fresh pending message inside each handler invocation so the
    in-band drain block fires for every turn in the chain.  After N
    turns, the recorded stack depth at handler entry must stay bounded.
    Pre-fix, depths would be 1, 2, 3, …, N; post-fix, depths are 1
    every time because each drain runs in its own task.
    """
    N = 12
    adapter = _make_adapter()
    sk = _sk()

    depths: list[int] = []
    next_index = [1]

    async def handler(event):
        depths.append(_count_pmb_frames())
        if next_index[0] < N:
            adapter._pending_messages[sk] = _make_event(text=f"M{next_index[0]}")
            next_index[0] += 1
        return "ok"

    adapter._message_handler = handler

    await adapter.handle_message(_make_event(text="M0"))

    # Drain the chain.  Each turn schedules the next via the in-band
    # drain block, so we wait until N handler runs have completed and
    # the session has been released.
    for _ in range(400):
        if len(depths) >= N and sk not in adapter._active_sessions:
            break
        await asyncio.sleep(0.01)

    await adapter.cancel_background_tasks()

    assert len(depths) == N, (
        f"expected {N} handler runs in the chain, got {len(depths)}: depths={depths!r}"
    )
    max_depth = max(depths)
    assert max_depth <= 2, (
        f"in-band drain is recursing instead of spawning a fresh task — "
        f"stack depth grew with chain length: {depths!r}"
    )