the traceback as shown by py-bt of one of the hanging children

Traceback (most recent call first):
  <built-in method connect of module object at remote 0x7f9bbe096680>
  File "/.../venv/dbg/lib/python3.8/site-packages/diskcache/core.py", line 882, in _con
    
  File "/.../venv/dbg/lib/python3.8/site-packages/diskcache/core.py", line 651, in _sql
    return self._con.execute
  File "/.../venv/dbg/lib/python3.8/site-packages/diskcache/core.py", line 655, in _sql_retry
    sql = self._sql
  File "/.../venv/dbg/lib/python3.8/site-packages/diskcache/core.py", line 1227, in __init__
    else:
  File "spawn.py", line 12, in task
  File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.8/multiprocessing/process.py", line 571, in _bootstrap
  File "/usr/lib/python3.8/multiprocessing/popen_fork.py", line 75, in _launch
    code = process_obj._bootstrap(parent_sentinel=child_r)
  File "/usr/lib/python3.8/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/usr/lib/python3.8/multiprocessing/context.py", line 277, in _Popen
    return Popen(process_obj)
  File "/usr/lib/python3.8/multiprocessing/context.py", line 224, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "/usr/lib/python3.8/multiprocessing/process.py", line 633, in start

and the stacktrace beyond python:

#0  __lll_lock_wait (futex=futex@entry=0x7f9bbdfa3788, private=0) at lowlevellock.c:52
#1  0x00007f9bbf58d0a3 in __GI___pthread_mutex_lock (mutex=0x7f9bbdfa3788) at ../nptl/pthread_mutex_lock.c:80
#2  0x00007f9bbde9340c in ?? () from /lib/x86_64-linux-gnu/libsqlite3.so.0
#3  0x00007f9bbde9351a in ?? () from /lib/x86_64-linux-gnu/libsqlite3.so.0
#4  0x00007f9bbdf63dbe in ?? () from /lib/x86_64-linux-gnu/libsqlite3.so.0
#5  0x00007f9bbdfc80d5 in pysqlite_connection_init (self=self@entry=0x7f9bbcd4ed50, args=args@entry=('/tmp/par_test_p15/cache.db',), kwargs=kwargs@entry={'timeout': 0, 'isolation_level': None})
    at ./Modules/_sqlite/connection.c:119
#6  0x00000000005a6867 in type_call (type=<optimized out>, type@entry=0x7f9bbdfd1e20 <pysqlite_ConnectionType>, args=args@entry=('/tmp/par_test_p15/cache.db',), 
    kwds=kwds@entry={'timeout': 0, 'isolation_level': None}) at ../Objects/typeobject.c:994
#7  0x00000000005f2c0e in PyObject_Call (callable=<type at remote 0x7f9bbdfd1e20>, args=('/tmp/par_test_p15/cache.db',), kwargs={'timeout': 0, 'isolation_level': None}) at ../Objects/call.c:245
#8  0x00007f9bbdfc46a8 in module_connect (self=<optimized out>, args=('/tmp/par_test_p15/cache.db',), kwargs={'timeout': 0, 'isolation_level': None}) at ./Modules/_sqlite/module.c:92
#9  0x00000000005f2fb9 in cfunction_call_varargs (kwargs=<optimized out>, args=<optimized out>, func=<built-in method connect of module object at remote 0x7f9bbe096680>) at ../Objects/call.c:772
#10 PyCFunction_Call (func=<built-in method connect of module object at remote 0x7f9bbe096680>, args=<optimized out>, kwargs=<optimized out>) at ../Objects/call.c:772
#11 0x00000000005f3446 in _PyObject_MakeTpCall (callable=<built-in method connect of module object at remote 0x7f9bbe096680>, args=<optimized out>, nargs=<optimized out>, keywords=<optimized out>)
    at ../Include/internal/pycore_pyerrors.h:13
#12 0x000000000056fb02 in _PyObject_Vectorcall (kwnames=('timeout', 'isolation_level'), nargsf=<optimized out>, args=<optimized out>, 
    callable=<built-in method connect of module object at remote 0x7f9bbe096680>) at ../Include/cpython/abstract.h:125
#13 _PyObject_Vectorcall (kwnames=('timeout', 'isolation_level'), nargsf=<optimized out>, args=<optimized out>, callable=<built-in method connect of module object at remote 0x7f9bbe096680>)
    at ../Include/cpython/abstract.h:115
#14 call_function (kwnames=('timeout', 'isolation_level'), oparg=<optimized out>, pp_stack=<synthetic pointer>, tstate=<optimized out>) at ../Python/ceval.c:4963
#15 _PyEval_EvalFrameDefault (f=<optimized out>, throwflag=<optimized out>) at ../Python/ceval.c:3515
#16 0x0000000000502a72 in PyEval_EvalFrameEx (throwflag=0, 
    f=Frame 0x7f9bbcd77040, for file /.../venv/dbg/lib/python3.8/site-packages/diskcache/core.py, line 882, in _con (self=<Cache(_directory='/tmp/par_test_p15', _timeout=0, _local=<_thread._local at remote 0x7f9bbddbac20>, _txn_id=None) at remote 0x7f9bbe138b50>, local_pid=None, pid=1854794, con=None)) at ../Python/ceval.c:741