Show a warning on a cache access when a CachedSession context manager has been exited

[alessio-locatelli]

Closes #241

MRE

import asyncio
from aiohttp_client_cache import CachedSession, SQLiteBackend

cache = SQLiteBackend(cache_name="./test_cache.sqlite")

async def task(n: int):
    url = f"https://httpbin.org/ip?count={n}"
    async with CachedSession(cache=cache) as session:
        return await session.get(url, ssl=False)
    
async def tasks(arr: list):
    tasks = [task(i) for i in arr]
    return await asyncio.gather(*tasks)


asyncio.run(tasks(list(range(10))))

1st run (there is no cache file yet)

We can see the warning inside get_connection() after the yield:

❯ python .gather_sessions.py
/usr/lib64/python3.13/contextlib.py:221: UserWarning: Cache access after closing the `Cachedsession` context manager is discouraged and can be forbidden in the future to prevent errors related to a closed database connection.
  await anext(self.gen)
Traceback (most recent call last):
[...]
sqlite3.ProgrammingError: Cannot operate on a closed database.

2nd run (a cache file already exists)

We can see the warning inside the def read():

/usr/lib64/python3.13/contextlib.py:221: UserWarning: Cache access after closing the `Cachedsession` context manager is discouraged and can be forbidden in the future to prevent errors related to a closed database connection.
  await anext(self.gen)
Traceback (most recent call last):
[...]
sqlite3.ProgrammingError: Cannot operate on a closed database.

Notes

Not related to this PR and the mentioned issue, but Redis and MongoDB must have the def close() method too because the connections must be closed.

[codecov]

Codecov Report

Attention: Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.17%. Comparing base (26bf273) to head (fb00280).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
aiohttp_client_cache/backends/sqlite.py	50.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
- Coverage   96.51%   96.17%   -0.35%     
==========================================
  Files          10       10              
  Lines        1063     1072       +9     
  Branches      186      188       +2     
==========================================
+ Hits         1026     1031       +5     
- Misses         28       30       +2     
- Partials        9       11       +2     

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[JWCook]

This looks like a nice improvement to me. Thanks!

My only thought for further improvement was whether we could use an existing method/attribute on the connection object to determine if it's closed instead of adding our own _closed flag. But it's an internal variable (aiosqlite.Connection._connection), so it's probably best to not rely on that.