file_based_local_timer.py

# Copyright (c) Meta Platforms, Inc. and its affiliates.
# All rights reserved.
#
# This source code is licensed under the BSD-style license found in the
# LICENSE file in the root directory of this source tree.

import io
import json
import logging
import os
import select
import signal
import sys
import threading
import time
from typing import Callable, Dict, List, Optional, Set, Tuple

from torch.distributed.elastic.timer.api import TimerClient, TimerRequest

__all__ = ["FileTimerClient", "FileTimerRequest", "FileTimerServer"]

class FileTimerRequest(TimerRequest):
    """
    Data object representing a countdown timer acquisition and release
    that is used between the ``FileTimerClient`` and ``FileTimerServer``.
    A negative ``expiration_time`` should be interpreted as a "release"
    request.
    ``signal`` is the signal to reap the worker process from the server
    process.
    """

    __slots__ = ["version", "worker_pid", "scope_id", "expiration_time", "signal"]

    def __init__(self, worker_pid: int, scope_id: str, expiration_time: float, signal: int = 0) -> None:
        self.version = 1
        self.worker_pid = worker_pid
        self.scope_id = scope_id
        self.expiration_time = expiration_time
        self.signal = signal

    def __eq__(self, other) -> bool:
        if isinstance(other, FileTimerRequest):
            return (
                self.version == other.version
                and self.worker_pid == other.worker_pid
                and self.scope_id == other.scope_id
                and self.expiration_time == other.expiration_time
                and self.signal == other.signal
            )
        return False

    def to_json(self) -> str:
        return json.dumps(
            {
                "version": self.version,
                "pid": self.worker_pid,
                "scope_id": self.scope_id,
                "expiration_time": self.expiration_time,
                "signal": self.signal
            },
        )


class FileTimerClient(TimerClient):
    """
    Client side of ``FileTimerServer``. This client is meant to be used
    on the same host that the ``FileTimerServer`` is running on and uses
    pid to uniquely identify a worker.
    This client uses a named_pipe to send timer requests to the
    ``FileTimerServer``. This client is a producer while the
    ``FileTimerServer`` is a consumer. Multiple clients can work with
    the same ``FileTimerServer``.

    Args:

        file_path: str, the path of a FIFO special file. ``FileTimerServer``
                        must have created it by calling os.mkfifo().

        signal: singal, the signal to use to kill the process. Using a
                        negative or zero signal will not kill the process.
    """
    def __init__(self, file_path: str, signal=(signal.SIGKILL if sys.platform != "win32" else
                                               signal.CTRL_C_EVENT)) -> None:  # type: ignore[attr-defined]
        super().__init__()
        self._file_path = file_path
        self.signal = signal

    def _open_non_blocking(self) -> Optional[io.TextIOWrapper]:
        try:
            fd = os.open(self._file_path, os.O_WRONLY | os.O_NONBLOCK)
            return os.fdopen(fd, "wt")
        except Exception:
            return None

    def _send_request(self, request: FileTimerRequest) -> None:
        # The server may have crashed or may haven't started yet.
        # In such case, calling open() in blocking model blocks the client.
        # To avoid such issue, open it in non-blocking mode, and an OSError will
        # be raised if the server is not there.
        file = self._open_non_blocking()
        if file is None:
            raise BrokenPipeError("Could not send the FileTimerRequest because FileTimerServer is not available.")
        with file:
            json_request = request.to_json()
            # Write request with no greater than select.PIPE_BUF is guarantee to be atomic.
            if len(json_request) > select.PIPE_BUF:
                raise RuntimeError(
                    f"FileTimerRequest larger than {select.PIPE_BUF} bytes "
                    f"is not supported: {json_request}"
                )
            file.write(json_request + "\n")

    def acquire(self, scope_id: str, expiration_time: float) -> None:
        self._send_request(
            request=FileTimerRequest(
                worker_pid=os.getpid(),
                scope_id=scope_id,
                expiration_time=expiration_time,
                signal=self.signal
            ),
        )

    def release(self, scope_id: str) -> None:
        self._send_request(
            request=FileTimerRequest(
                worker_pid=os.getpid(),
                scope_id=scope_id,
                expiration_time=-1,
                signal=0
            ),
        )


class FileTimerServer:
    """
    Server that works with ``FileTimerClient``. Clients are expected to be
    running on the same host as the process that is running this server.
    Each host in the job is expected to start its own timer server locally
    and each server instance manages timers for local workers (running on
    processes on the same host).

    Args:

        file_path: str, the path of a FIFO special file to be created.

        max_interval: float, max interval in seconds for each watchdog loop.

        daemon: bool, running the watchdog thread in daemon mode or not.
                      A daemon thread will not block a process to stop.
        log_event: Callable[[Dict[str, str]], None], an optional callback for
                logging the events in JSON format.
    """

    def __init__(
        self,
        file_path: str,
        max_interval: float = 10,
        daemon: bool = True,
        log_event: Callable[[str, Optional[FileTimerRequest]], None] = None
    ) -> None:
        self._file_path = file_path
        self._max_interval = max_interval
        self._daemon = daemon
        self._timers: Dict[Tuple[int, str], FileTimerRequest] = {}
        self._stop_signaled = False
        self._watchdog_thread: Optional[threading.Thread] = None
        if os.path.exists(self._file_path):
            os.remove(self._file_path)
        os.mkfifo(self._file_path)
        # For test only. Count the number of requests received.
        self._request_count = 0
        # For test only. Process all requests and stop the server.
        self._run_once = False
        self._log_event = log_event if log_event is not None else lambda name, request: None


    def start(self) -> None:
        logging.info(
            f"Starting {type(self).__name__}..."
            f" max_interval={self._max_interval},"
            f" daemon={self._daemon}"
        )
        self._watchdog_thread = threading.Thread(target=self._watchdog_loop, daemon=self._daemon)
        logging.info("Starting watchdog thread...")
        self._watchdog_thread.start()
        self._log_event("watchdog started", None)

    def stop(self) -> None:
        logging.info(f"Stopping {type(self).__name__}")
        self._stop_signaled = True
        if self._watchdog_thread:
            logging.info("Stopping watchdog thread...")
            self._watchdog_thread.join(self._max_interval)
            self._watchdog_thread = None
        else:
            logging.info("No watchdog thread running, doing nothing")
        if os.path.exists(self._file_path):
            os.remove(self._file_path)
        self._log_event("watchdog stopped", None)

    def run_once(self) -> None:
        self._run_once = True
        if self._watchdog_thread:
            logging.info("Stopping watchdog thread...")
            self._watchdog_thread.join()
            self._watchdog_thread = None
        else:
            logging.info("No watchdog thread running, doing nothing")
        if os.path.exists(self._file_path):
            os.remove(self._file_path)

    def _watchdog_loop(self) -> None:
        # Open the pipe in blocking mode blocks the server thread.
        # This is fine for the following reasons:
        #  1. No client case usually does not happen.
        #  2. We are running the watchdog loop in a separate daemon
        #     thread, which will not block the process to stop.
        with open(self._file_path, "rt") as fd:
            while not self._stop_signaled:
                try:
                    run_once = self._run_once
                    self._run_watchdog(fd)
                    if run_once:
                        break
                except Exception as e:
                    logging.error("Error running watchdog", exc_info=e)

    def _run_watchdog(self, fd: io.TextIOWrapper) -> None:
        timer_requests = self._get_requests(fd, self._max_interval)
        self.register_timers(timer_requests)
        now = time.time()
        reaped_worker_pids = set()
        for worker_pid, expired_timers in self.get_expired_timers(now).items():
            logging.info(f"Reaping worker_pid=[{worker_pid}]." f" Expired timers: {self._get_scopes(expired_timers)}")
            reaped_worker_pids.add(worker_pid)
            # In case we have multiple expired timers, we find the first timer
            # with a valid signal (>0) in the expiration time order.
            expired_timers.sort(key=lambda timer: timer.expiration_time)
            signal = 0
            expired_timer = None
            for timer in expired_timers:
                if timer.signal > 0:
                    signal = timer.signal
                    expired_timer = timer
                    break
            if signal <= 0:
                logging.info(f"No signal specified with worker=[{worker_pid}]. Do not reap it.")
                continue
            if self._reap_worker(worker_pid, signal):
                logging.info(f"Successfully reaped worker=[{worker_pid}] with signal={signal}")
                self._log_event("kill worker process", expired_timer)
            else:
                logging.error(f"Error reaping worker=[{worker_pid}]. Will retry on next watchdog.")
        self.clear_timers(reaped_worker_pids)

    def _get_scopes(self, timer_requests: List[FileTimerRequest]) -> List[str]:
        return [r.scope_id for r in timer_requests]

    def _get_requests(self, fd: io.TextIOWrapper, max_interval: float) -> List[FileTimerRequest]:
        start = time.time()
        requests = []
        while not self._stop_signaled or self._run_once:
            # For named pipe, readline() is blocking when at least one writer opens.
            # It returns only when flush() is called at the writer side.
            # Note that flush() is automatically called inside close().
            # After the last writer closes, readline() is not blocking.
            # It will return an empty string when it's at end-of-file.
            # Since the client side always opens the pipe, writes a message and closes
            # the pipe immediately, the readline() call below is not blocking for long.
            json_request = fd.readline()
            if len(json_request) == 0:
                if self._run_once:
                    break
                time.sleep(min(max_interval, 1))
            else:
                request = json.loads(json_request)
                pid = request["pid"]
                scope_id = request["scope_id"]
                expiration_time = request["expiration_time"]
                signal = request["signal"]
                requests.append(
                    FileTimerRequest(
                        worker_pid=pid, scope_id=scope_id, expiration_time=expiration_time, signal=signal
                    )
                )
            now = time.time()
            if now - start > max_interval:
                break
        return requests

    def register_timers(self, timer_requests: List[FileTimerRequest]) -> None:
        for request in timer_requests:
            pid = request.worker_pid
            scope_id = request.scope_id
            expiration_time = request.expiration_time
            self._request_count += 1

            key = (pid, scope_id)
            # negative expiration is a proxy for a release call
            if expiration_time < 0:
                if key in self._timers:
                    del self._timers[key]
                    self._log_event("clear timer", request)
            else:
                self._timers[key] = request
                self._log_event("set timer", request)

    def clear_timers(self, worker_pids: Set[int]) -> None:
        for (pid, scope_id) in list(self._timers.keys()):
            if pid in worker_pids:
                self._log_event("clear timer", self._timers[(pid, scope_id)])
                del self._timers[(pid, scope_id)]

    def get_expired_timers(self, deadline: float) -> Dict[int, List[FileTimerRequest]]:
        # pid -> [timer_requests...]
        expired_timers: Dict[int, List[FileTimerRequest]] = {}
        for request in self._timers.values():
            if request.expiration_time <= deadline:
                expired_scopes = expired_timers.setdefault(request.worker_pid, [])
                expired_scopes.append(request)
        return expired_timers

    def _reap_worker(self, worker_pid: int, signal: int) -> bool:
        try:
            os.kill(worker_pid, signal)
            return True
        except ProcessLookupError:
            logging.info(f"Process with pid={worker_pid} does not exist. Skipping")
            return True
        except Exception as e:
            logging.error(f"Error terminating pid={worker_pid}", exc_info=e)
        return False