Migrate from threading.Lock() to django-db-mutex

Avoid race conditions between processes

.github/workflows/test.yml

@@ -32,6 +32,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           python -m pip install --upgrade django~=${{ matrix.django-version }}
+          python -m pip install --upgrade django-db-mutex
           python -m pip install --upgrade requests
 
       - name: Run tests

CHANGELOG

@@ -1,5 +1,8 @@
 Unreleased
 
+* Breaking change: Migrated from `threading.Lock()` to `django-db-mutex`
+  to avoid race conditions between multiple processes.
+  Please add `'db_mutex'` to your `INSTALLED_APPS` setting.
 * Enable re-checking after rate limit was hit (Timo Ludwig, #153)
 * Ignore raw `post_save` signal (Timo Ludwig, #106)
 * Retry with fallback user agent on forbidden response (Timo Ludwig, #159)

README.rst

@@ -35,7 +35,7 @@ Basic usage
 #. Install app to somewhere on your Python path (e.g. ``pip install
    django-linkcheck``).
 
-#. Add ``'linkcheck'`` to your ``settings.INSTALLED_APPS``.
+#. Add ``'linkcheck'`` and ``'db_mutex'`` to your ``settings.INSTALLED_APPS``.
 
 #. Add a file named ``linklists.py`` to every app (see an example in ``examples/linklists.py``) that either:
 

linkcheck/__init__.py

@@ -1,9 +1,5 @@
-import threading
 from html.parser import HTMLParser
 
-# A global lock, showing whether linkcheck is busy
-update_lock = threading.Lock()
-
 
 class Lister(HTMLParser):
 

linkcheck/apps.py

@@ -1,6 +1,7 @@
 import importlib
 
 from django.apps import AppConfig, apps
+from django.core.exceptions import ImproperlyConfigured
 from django.db.models.signals import post_delete
 
 
@@ -17,6 +18,11 @@ class BaseLinkcheckConfig(AppConfig):
     all_linklists = {}
 
     def ready(self):
+        if not apps.is_installed('db_mutex'):
+            raise ImproperlyConfigured(
+                'This library depends on django-db-mutex, '
+                'please add "db_mutex" to your INSTALLED_APPS setting.'
+            )
         self.build_linklists()
 
     def build_linklists(self):

linkcheck/listeners.py

@@ -5,12 +5,14 @@
 from queue import Empty, LifoQueue
 from threading import Thread
 
+from db_mutex import DBMutexError
+from db_mutex.db_mutex import db_mutex
 from django.apps import apps
 from django.db.models import signals as model_signals
 
 from linkcheck.models import Link, Url
 
-from . import filebrowser, update_lock
+from . import filebrowser
 from .linkcheck_settings import MAX_URL_LENGTH
 
 logger = logging.getLogger(__name__)
@@ -31,6 +33,11 @@ def linkcheck_worker(block=True):
         # An error in any task should not stop the worker from continuing with the queue
         try:
             task['target'](*task['args'], **task['kwargs'])
+        except DBMutexError:
+            # Wait and reschedule the task
+            logger.debug("Lock is busy, waiting and rescheduling the task...")
+            time.sleep(0.1)
+            tasks_queue.put(task)
         except Exception as e:
             logger.exception(
                 "%s while running %s with args=%r and kwargs=%r: %s",
@@ -74,7 +81,7 @@ def do_check_instance_links(sender, instance, wait=False):
 
         if wait:
             time.sleep(0.1)
-        with update_lock:
+        with db_mutex('linkcheck'):
             content_type = linklist_cls.content_type()
             new_links = []
             old_links = Link.objects.filter(content_type=content_type, object_id=instance.pk)

linkcheck/tests/test_linkcheck.py

@@ -3,6 +3,7 @@
 from io import StringIO
 from unittest.mock import patch
 
+from db_mutex import DBMutexError
 from django.apps import apps
 from django.conf import settings
 from django.contrib.auth.models import User
@@ -902,6 +903,28 @@ def passing():
             'args=() and kwargs={}: Failing task'
         )
 
+    def test_reschedule_busy_lock(self):
+        global attempt
+        attempt = 0
+
+        def busy():
+            global attempt
+            if attempt < 5:
+                attempt += 1
+                raise DBMutexError("Could not acquire lock")
+
+        tasks_queue.put({
+            'target': busy,
+            'args': (),
+            'kwargs': {},
+        })
+        with self.assertLogs(level='DEBUG') as cm:
+            linkcheck_worker(block=False)
+        self.assertEqual(
+            cm.output,
+            ['DEBUG:linkcheck.listeners:Lock is busy, waiting and rescheduling the task...'] * 5
+        )
+
 
 class ViewTestCase(TestCase):
     def setUp(self):

linkcheck/views.py

@@ -1,6 +1,8 @@
 from itertools import groupby
 from operator import itemgetter
 
+from db_mutex import DBMutexError
+from db_mutex.db_mutex import db_mutex
 from django import forms
 from django.contrib.admin.views.decorators import staff_member_required
 from django.contrib.contenttypes.models import ContentType
@@ -12,7 +14,6 @@
 from django.urls import NoReverseMatch, reverse
 from django.utils.translation import gettext as _
 
-from linkcheck import update_lock
 from linkcheck.linkcheck_settings import RESULTS_PER_PAGE
 from linkcheck.models import Link
 from linkcheck.utils import get_coverage_data
@@ -163,21 +164,22 @@ def get_jquery_min_js():
 
 
 def get_status_message():
-    if update_lock.locked():
-        return "Still checking. Please refresh this page in a short while. "
-    else:
-        broken_links = Link.objects.filter(ignore=False, url__status=False).count()
-        if broken_links:
-            return (
-                "<span style='color: red;'>We've found {} broken link{}.</span><br>"
-                "<a href='{}'>View/fix broken links</a>".format(
-                    broken_links,
-                    "s" if broken_links > 1 else "",
-                    reverse('linkcheck_report'),
+    try:
+        with db_mutex('linkcheck'):
+            broken_links = Link.objects.filter(ignore=False, url__status=False).count()
+            if broken_links:
+                return (
+                    "<span style='color: red;'>We've found {} broken link{}.</span><br>"
+                    "<a href='{}'>View/fix broken links</a>".format(
+                        broken_links,
+                        "s" if broken_links > 1 else "",
+                        reverse('linkcheck_report'),
+                    )
                 )
-            )
-        else:
-            return ''
+            else:
+                return ''
+    except DBMutexError:
+        return 'Still checking. Please refresh this page in a short while.'
 
 
 def is_ajax(request):

runtests.py

@@ -13,7 +13,7 @@
         'INSTALLED_APPS': [
             'django.contrib.admin', 'django.contrib.auth',
             'django.contrib.sessions', 'django.contrib.contenttypes',
-            'django.contrib.messages',
+            'django.contrib.messages', 'db_mutex',
             'linkcheck', 'linkcheck.tests.sampleapp',
         ],
         'ROOT_URLCONF': "linkcheck.tests.urls",

setup.py

@@ -6,6 +6,7 @@
 def read(fname):
     return open(os.path.join(os.path.dirname(__file__), fname)).read()
 
+
 setup(
     name='django-linkcheck',
     version='2.1.0',
@@ -18,7 +19,7 @@ def read(fname):
     url='https://github.com/DjangoAdminHackers/django-linkcheck',
     packages=find_packages(),
     include_package_data=True,
-    install_requires=['django', 'requests'],
+    install_requires=['django', 'django-db-mutex', 'requests'],
     extras_require={
         "dev": ["flake8", "isort", "pre-commit"],
     },