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Question

I'm currently writing an application in python that concurrently downloads multiple files using multithreading. I'm not completely comfortable or knowledgeable about multithreading in Python, so I'm not sure how much or what information I should give.

When the downloader almost finishes downloading the file, it crashes, and I get this error message.

fatal Python error: _enter_buffered_busy: could not aquire lock for <_io.BufferedWriter name='<stdout>'> at interpreter shutdown, possibly due to daemon threads
Python runtime state: finalizing (tstate=000001EF6024D440)

Current thread 0x00002cf8 (most recent call first):
<no Python frame>

I've tested this quite a few times and have noticed the following:

• This error doesn't appear all the time. Most of the time, I have no issue with the app
• I have tried it and gotten errors in
  • Linux Ubuntu 18.04 (Laptop) running Python3.6
  • Linux Ubuntu 20.04 (Desktop) running Python3.6
  • Windows 10 (Desktop)
  • Docker container running Ubuntu / python3.6-slim-buster base image

I'm sorry that I have to dump all this code on SO; I really have no clue what this error means and don't know where to start troubleshooting. Our repository is located here

Here's the code that handles the multithreading:

dozent.py

import datetime
import json
import multiprocessing
import os
from pathlib import Path
from queue import Queue
from threading import Thread
from typing import List, Dict

try:
    from dozent.downloader_tools import DownloaderTools
    from dozent.progress_tracker import ProgressTracker
except ModuleNotFoundError:
    from downloader_tools import DownloaderTools
    from progress_tracker import ProgressTracker

CURRENT_FILE_PATH = Path(__file__)
DEFAULT_DATA_DIRECTORY = CURRENT_FILE_PATH.parent.parent / 'data'
TWITTER_ARCHIVE_STREAM_LINKS_PATH = CURRENT_FILE_PATH.parent / 'twitter-archive-stream-links.json'

FIRST_DAY_OF_SUPPORT = datetime.date(2017, 6, 1)
LAST_DAY_OF_SUPPORT = datetime.date(2020, 6, 30)

class _DownloadWorker(Thread):  # skip_tests

    def __init__(self, queue: Queue, download_dir: Path, tracker: ProgressTracker = None):
        Thread.__init__(self)
        self.queue = queue
        self.download_dir = download_dir
        self.tracker = tracker

    def run(self):
        while True:
            # Get the work from the queue and expand the tuple
            link = self.queue.get()
            try:
                DownloaderTools.download_with_pysmartdl(link, str(self.download_dir), tracker=self.tracker)
            finally:
                self.queue.task_done()

#
# Skipping non-multithreading code
#

    def download_timeframe(self, start_date: datetime.date, end_date: datetime.date, verbose: bool = True,
                           download_dir: Path = DEFAULT_DATA_DIRECTORY):
        """
        Download all tweet archives from self.start_date to self.end_date
        :return: None
        """

        # Create a queue to communicate with the worker threads
        queue = Queue()
        if verbose:
            tracker = ProgressTracker()
            tracker.daemon = True
            tracker.start()
        else:
            tracker = None

        os.makedirs(download_dir, exist_ok=True)

        for x in range(multiprocessing.cpu_count()):
            worker = _DownloadWorker(queue, download_dir, tracker=tracker)
            # worker.set_verbosity(verbose=verbosity)
            # Setting daemon to True will let the main thread exit even though the workers are blocking
            worker.daemon = True
            worker.start()

        for sample_date in self.get_links_for_days(start_date=start_date, end_date=end_date):
            print(f"Queueing tweets download for {sample_date['month']}-{sample_date['year']}")
            queue.put(sample_date['link'])

        queue.join()
        tracker.join()

catch_thread_execptions.py

import threading
import sys
import traceback
import os
import signal

"""
usage: install()
Once installed, all exceptions caught from within threads will cause the program to end.
"""


def sendKillSignal(etype, value, tb):
    lines = traceback.format_exception(etype, value, tb)
    for line in lines:
        print(line, flush=True)

    os.kill(os.getpid(), signal.SIGTERM)


original_init = threading.Thread.__init__


def patched_init(self, *args, **kwargs):
    original_init(self, *args, **kwargs)
    original_run = self.run

    def patched_run(*args, **kw):
        try:
            original_run(*args, **kw)
        except Exception:
            sys.excepthook(*sys.exc_info())
    self.run = patched_run


def install():
    sys.excepthook = sendKillSignal
    threading.Thread.__init__ = patched_init

progress_tracker.py

 
import sys
import os

import numpy as np
import threading
import shutil
import queue

from typing import Dict, Callable, Tuple
from collections import namedtuple

Task = namedtuple('Task', 'id progress_callback progress prefix suffix line_length')


class ProgressTracker(threading.Thread):
    """It is assumed that no other printing to stdout will occur while running this ProgressTracker thread.
    """

    def __init__(self):
        threading.Thread.__init__(self)

        self.tasks: Dict[int, Task] = {}
        self.message_queue = queue.Queue()
        self.sum_progress = 0
        self.next_task = 0

        self.terminal_size = None

        resized = self.refresh_terminal_size()
        assert(resized)  # This won't work if we couldn't read the terminal size

        self.lock = threading.Lock()
        self.tasks_pending = False
        self.running = True

    def refresh_terminal_size(self) -> bool:
        """
        Records any change in the terminal size.
        :returns: True if a change is detected, else False
        """

        old_size = self.terminal_size
        try:
            self.terminal_size = os.get_terminal_size()[0]
        except OSError:
            self.terminal_size = shutil.get_terminal_size()[0]

        return old_size != self.terminal_size

    def join(self):
        self.message_queue.join()
        self.stop()

    def stop(self):
        self.running = False
        self.update(0)

    def update(self, task_id: int):
        self.message_queue.put(task_id)

    def register_task(self, name: str, progress_callback: Callable[[], Tuple[float, str, str]]) -> int:
        """Register a new task with the given progress callback.
        :param name: A unique name for this task.
        :param progress_callback: A function, which will report a tuple of:
                                    a progress percentage in [0,100],
                                    a string prefix to be output before the progress bar,
                                    and a string suffix to be output after the progress bar.
        :return: The ID of the registered task.
        """

        task_id = self.next_task
        self.next_task += 1

        # Perform operation in helper function on own thread, so that this function does not block
        thread = threading.Thread(target=self._register_task_helper, args=(name, task_id, progress_callback))
        thread.daemon = True
        thread.start()

        return task_id

    def _register_task_helper(self, name: str, task_id: int, progress_callback: Callable[[], Tuple[float, str]]):
        with self.lock:
            # Create the task, with initial progress 0
            self.tasks[task_id] = (Task(id=task_id,
                                        progress_callback=progress_callback,
                                        progress=0,
                                        prefix=None,
                                        suffix=None,
                                        line_length=0))
            self.tasks_pending = True
            self._delete_progress_bars()
            print(f"Queueing tweet download {task_id}: {name}", flush=True)

    def _delete_progress_bars(self, starting_id: int = 0):
        if self.tasks:
            sys.stdout.write('\b' * (np.sum([self.tasks[i].line_length for i in range(starting_id, len(self.tasks))])))

    def _update_task_progress(self, task_id: int, draw: bool = True):
        """Update the progress bar for the given task
        :param task_id: Identifier of the task to update.
        :param draw: Whether to draw the updated progress bar, defaults to True
        """

        # Update the progress value
        task = self.tasks[task_id]
        assert(task.id == task_id)

        progress, prefix, suffix = task.progress_callback()
        assert(0 <= progress <= 100)

        self.sum_progress += progress - task.progress
        task = Task(id=task_id,
                    progress_callback=task.progress_callback,
                    progress=progress,
                    prefix=prefix,
                    suffix=suffix,
                    line_length=task.line_length)
        self.tasks[task_id] = task

        if draw:
            # Redraw all bars if the terminal size changes
            if self.refresh_terminal_size():
                task_id = 0

            # Return to start of the progress bar for task_id
            self._delete_progress_bars(starting_id=task_id)
            self._draw_task_progress(task_id)

    def _draw_task_progress(self, task_id: int):
        """Updates the progress bar for task_id and after.
        Assumes the console begins at the end of any previously drawn progress bars.
        """

        # We re-write the progress for all tasks, in order to finish at the end of all progress bars (our assumption)
        for task in [self.tasks[i] for i in range(task_id, len(self.tasks))]:
            if task.prefix is None or task.suffix is None:
                continue

            prefix = f"{task.id:3.0f}: "

            bar = ' '*len(prefix)
            bar_width = self.terminal_size - len(prefix) - 2
            assert(bar_width > 0)

            num_dashes = int(task.progress * bar_width / 100.0)
            bar += f"[{'-' * num_dashes}{' ' * (bar_width - num_dashes)}]"

            prefix += task.prefix[:int(self.terminal_size/2)]
            suffix = task.suffix[:int(self.terminal_size/3)]
            spaces = ' ' * (self.terminal_size - len(suffix) - len(prefix))

            line = f"{prefix}{spaces}{suffix}{bar}"

            assert(self.tasks[task.id] == task)
            self.tasks[task.id] = Task(id=task.id,
                                       progress_callback=task.progress_callback,
                                       progress=task.progress,
                                       prefix=task.prefix,
                                       suffix=task.suffix,
                                       line_length=len(line))

            sys.stdout.write(line)

        sys.stdout.flush()

    def _run_task(self):
        task_id = self.message_queue.get(True)

        try:
            with self.lock:
                if self.tasks_pending:
                    self._update_task_progress(0, draw=False)
                    self.tasks_pending = False

                else:
                    self._update_task_progress(task_id, draw=True)

        finally:
            self.message_queue.task_done()

    def run(self):
        while self.running:
            self._run_task()

        self._delete_progress_bars()
        print('')

Again, I'm not an expert with multithreading in Python and am not sure where to start troubleshooting. I would greatly appreciate any hints!

EDIT:

The examples I provided are not reproducible

How to reproduce

Pull Docker image:

docker pull socialmediapublicanalysis/dozent:latest

Download all tweets for a day (May 12th, 2020):

docker run -it socialmediapublicanalysis/dozent:latest python -m dozent -s 2020-05-12 -e 2020-05-12

Answer 1

There is a lot going on in your code and it is definitely not a minimal, reproducible example...

That said, and as per the error message, it looks like you are using daemon threads, and those are known to cause trouble if they try to print (to stdout ) at shutdown (in summary: it is likely that you are trying to write to a no-longer existing stdout with a daemon thread).

You should make sure you .join() all your threads (and, if they are expected to be long-running, equip them with a shutdown method that flips a flag, that in turns is checked by whatever loop you have going in them; or send a poison pill through the work queue).

A better approach for multithreading downloads would be to use ThreadPoolExecutor like in this example from the docs .

If you want progress reports, you may be interested in tqdm . In the end, your code will be much simpler and easier to maintain.

Here is a complete example that downloads a number (818, as of this writing) of urls from Wikipedia, using a ThreadPoolExecutor and showing progress as it goes:

import requests
from bs4 import BeautifulSoup
from urllib.parse import urljoin
from concurrent.futures import ThreadPoolExecutor
from tqdm import tqdm


tld = 'https://en.wikipedia.org'
url = '/wiki/Index_of_computing_articles'

soup = BeautifulSoup(requests.get(urljoin(tld, url)).content)
urllist = [urljoin(tld, a.get('href')) for a in soup.find_all(href=True)]

def load_url(url):
    return requests.get(url).content

def load_all(urllist):
    with ThreadPoolExecutor() as executor:
        results = list(tqdm(
            executor.map(load_url, urllist),
            total=len(urllist), unit=' pages'))
    return results

results = load_all(urllist)

While running, it shows a progress bar: