dbm.sqlite breaks multi-threaded shelve usage

Bug report

Bug description:

dbm backends were previously thread safe, but I think dbm.sqlite introduced in https://github.com/python/cpython/pull/114481 is not.

I am not sure if the resolution should be to add a doc comment to shelve/dbm or some other way to fix it, say specifying a preferred backend or multithreading argument in shelve.open as an argument. (Or if there is a way to fix this in dbm.sqlite itself)

Example code:

from concurrent.futures import ThreadPoolExecutor
import shelve

CACHE = shelve.open('test')

def check_set_cache(value):
    if 'value' in CACHE:
        print(CACHE['value'])
    CACHE['value'] = value
    return CACHE['value']

jobs = list(range(1, 100))
executor = ThreadPoolExecutor(max_workers=5)
entries = list(executor.map(check_set_cache, jobs))
print(entries)

Log

Traceback (most recent call last):
  File "/usr/lib64/python3.13/dbm/sqlite3.py", line 79, in _execute
    return closing(self._cx.execute(*args, **kwargs))
                   ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
sqlite3.ProgrammingError: SQLite objects created in a thread can only be used in that same thread. The object was created in thread id 140036133836608 and this is thread id 140035884230336.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/megh/pybug/reproduce.py", line 14, in <module>
    entries = list(executor.map(check_set_cache, jobs))
  File "/usr/lib64/python3.13/concurrent/futures/_base.py", line 619, in result_iterator
    yield _result_or_cancel(fs.pop())
          ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/usr/lib64/python3.13/concurrent/futures/_base.py", line 317, in _result_or_cancel
    return fut.result(timeout)
           ~~~~~~~~~~^^^^^^^^^
  File "/usr/lib64/python3.13/concurrent/futures/_base.py", line 449, in result
    return self.__get_result()
           ~~~~~~~~~~~~~~~~~^^
  File "/usr/lib64/python3.13/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/usr/lib64/python3.13/concurrent/futures/thread.py", line 59, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/megh/pybug/reproduce.py", line 7, in check_set_cache
    if 'value' in CACHE:
       ^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/shelve.py", line 102, in __contains__
    return key.encode(self.keyencoding) in self.dict
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen _collections_abc>", line 817, in __contains__
  File "/usr/lib64/python3.13/dbm/sqlite3.py", line 89, in __getitem__
    with self._execute(LOOKUP_KEY, (key,)) as cu:
         ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.13/dbm/sqlite3.py", line 81, in _execute
    raise error(str(exc))
dbm.sqlite3.error: SQLite objects created in a thread can only be used in that same thread. The object was created in thread id 140036133836608 and this is thread id 140035884230336.

(Observed here https://github.com/beancount/beanprice/issues/91 )

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-131920

從 Lib/dbm/sqlite3.py 開始，尤其是 _execute，並查看 Lib/shelve.py，以了解後端如何選取及存取。在 Python 3.13 上重現執行緒範例，然後檢視連結的 PR gh-131920 和現有留言，再決定哪些行為和測試可以定義修正方案。