psycopg2.errors.DeadlockDetected error on PostgreSQL

[timobrembeck]

On a production environment where Django was deployed via Apache and mod_wsgi in 8 processes, we encountered a "deadlock detected" error during this statement:

Mar 14 12:43:53 ERROR   django.request - 500 Internal Server Error: /region/pages/de/14907/edit/
Traceback (most recent call last):
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
psycopg2.errors.DeadlockDetected: deadlock detected
DETAIL:  Process 1054249 waits for ShareLock on transaction 18359673; blocked by process 1054250.
Process 1054250 waits for ShareLock on transaction 18359675; blocked by process 1054249.
HINT:  See server log for query details.
CONTEXT:  while inserting index tuple (0,11) in relation "db_mutex_dbmutex_lock_id_key"


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/views/generic/base.py", line 70, in view
    return self.dispatch(request, *args, **kwargs)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/utils/decorators.py", line 43, in _wrapper
    return bound_method(*args, **kwargs)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/views/generic/base.py", line 98, in dispatch
    return handler(request, *args, **kwargs)
  File "/usr/lib/python3.9/contextlib.py", line 79, in inner
    return func(*args, **kwds)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/integreat_cms/cms/views/pages/page_form_view.py", line 324, in post
    page_translation_form.instance.page = page_form.save()
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/integreat_cms/cms/forms/custom_model_form.py", line 133, in save
    return super().save(commit=commit)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/treebeard/forms.py", line 153, in save
    self.instance.move(reference_node, pos=position_type)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/integreat_cms/cms/models/pages/page.py", line 327, in move
    super().move(target, pos)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/integreat_cms/cms/models/abstract_tree_node.py", line 270, in move
    with db_mutex(self.__class__.__name__):
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/db_mutex/db_mutex.py", line 93, in __enter__
    self.start()
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/db_mutex/db_mutex.py", line 107, in start
    self.lock = DBMutex.objects.create(lock_id=self.lock_id)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/query.py", line 453, in create
    obj.save(force_insert=True, using=self.db)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/base.py", line 739, in save
    self.save_base(using=using, force_insert=force_insert,
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/base.py", line 776, in save_base
    updated = self._save_table(
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/base.py", line 881, in _save_table
    results = self._do_insert(cls._base_manager, using, fields, returning_fields, raw)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/base.py", line 919, in _do_insert
    return manager._insert(
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/query.py", line 1270, in _insert
    return query.get_compiler(using=using).execute_sql(returning_fields)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/models/sql/compiler.py", line 1416, in execute_sql
    cursor.execute(sql, params)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/cacheops/transaction.py", line 98, in execute
    result = self._no_monkey.execute(self, sql, params)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/opt/integreat-cms/.venv/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
django.db.utils.OperationalError: deadlock detected