utils/gatelock/gatelock/_preempt.py

# Copyright (c) 2026 Krzysztof Rudnicki
"""Diagnosing and clearing a blocked global input grab.

Split out of :mod:`gatelock._window`, which owns the *acquisition* loop; this
module owns what happens when that loop keeps failing -- naming the holder in
the log, and standing a weaker one down.

Both are free functions rather than methods because neither touches the Tk
window: they need only the arbiter, the config knobs and the set of pids
already signalled. That keeps the retry path in one file and the arbitration
policy in another, and lets this half be tested without a display.
"""

from __future__ import annotations

import logging
import os
import signal
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from gatelock._arbiter import Arbiter, Claim
    from gatelock._config import LockConfig

_logger = logging.getLogger(__name__)


def log_grab_blocked(
    attempt: int,
    *,
    arbiter: Arbiter | None,
    config: LockConfig,
    preempted_pids: set[int],
) -> None:
    """Say who is actually holding the grab, rather than guessing.

    v0.1.1 always blamed "a fullscreen game". On 2026-07-25 the holder was
    the other locker, and that guess sent the diagnosis in the wrong
    direction for the length of the outage.

    A holder that outranks us is left alone entirely -- retrying is the
    correct behavior there, matching :func:`gatelock.wait_for_turn`'s own
    rule for an app that has not armed yet. A *weaker* holder is preempted
    by :func:`maybe_preempt`, once per pid, rather than left to block us for
    as long as it takes to finish on its own (diet_guard held the grab for
    ~3 minutes against a higher-ranked screen_locker on 2026-08-21).

    Args:
        attempt: 1-based attempt counter, reported in the log line.
        arbiter: This app's arbiter, or None when arbitration is disabled.
        config: The window's configuration, consulted for the preempt opt-in.
        preempted_pids: Pids already signalled; mutated by the preempt path.
    """
    holder = arbiter.describe_holder() if arbiter else None
    if holder is None:
        _logger.warning(
            "global grab still blocked after %d attempts; no gatelock app "
            "holds it -- likely another X client (e.g. a fullscreen game)",
            attempt,
        )
        return
    _logger.warning(
        "global grab still blocked after %d attempts; held by gatelock app "
        "%r (rank %d, pid %d) since %s",
        attempt,
        holder.app,
        holder.rank,
        holder.pid,
        holder.started,
    )
    maybe_preempt(holder, arbiter=arbiter, config=config, preempted_pids=preempted_pids)


def maybe_preempt(
    holder: Claim,
    *,
    arbiter: Arbiter | None,
    config: LockConfig,
    preempted_pids: set[int],
) -> None:
    """SIGTERM a weaker holder once, so it stands down instead of blocking.

    SIGTERM (not SIGKILL): the holder's own signal handler
    (``LockWindow._install_signal_handlers``) raises ``SystemExit(0)``,
    which unwinds through its ``run()``'s ``finally`` into its own
    ``close()`` -- the identical teardown a clean dismiss gets (VT
    restored, arbiter claim released, app-specific ``on_close`` hook
    run), just triggered externally instead of by the user.

    Args:
        holder: The claim currently holding the grab.
        arbiter: This app's arbiter, or None when arbitration is disabled.
        config: The window's configuration; ``preempt_weaker_holder`` gates
            this entirely and defaults to off.
        preempted_pids: Pids already signalled, so each is signalled once.
    """
    if not config.preempt_weaker_holder or arbiter is None:
        return
    if holder.rank >= arbiter.claim.rank:
        return
    if holder.pid in preempted_pids:
        return
    preempted_pids.add(holder.pid)
    _logger.warning(
        "preempting weaker holder %r (rank %d, pid %d) so rank %d can arm",
        holder.app,
        holder.rank,
        holder.pid,
        arbiter.claim.rank,
    )
    try:
        os.kill(holder.pid, signal.SIGTERM)
    except ProcessLookupError:
        _logger.info(
            "holder pid %d was already gone by the time we signalled it",
            holder.pid,
        )
    except OSError:
        _logger.exception("could not signal holder pid %d", holder.pid)