gh-135386: Fix "unable to open database file" errors on readonly DB

[GeneralK1ng]

What

This PR adds the SQLite URI parameter immutable=1 when opening a database in read-only mode in Lib/dbm/sqlite3.py. This explicitly informs SQLite that the database is read-only and avoids attempts to create or write to auxiliary WAL/SHM files.

Why

Without this flag, opening a read-only SQLite database may fail with errors such as "unable to open database file" when the WAL or SHM files cannot be created due to filesystem permissions or other restrictions. This is especially common when the database file is read-only and the environment prevents creation of additional files.

Adding immutable=1 allows SQLite to operate correctly without needing to create WAL/SHM files, thus preventing these errors.

Where

The change is made in the _Database.__init__ constructor in Lib/dbm/sqlite3.py. The URI used to open the database connection is appended with &immutable=1 only if the flag is "ro" (read-only).

Related issue

#135386

---

This is my first contribution to CPython. I appreciate the opportunity and look forward to feedback from the community. Thank you!

• Issue: Unable to read sqlite3 database keys in read-only directory with Python 3.13 with shelve #135386

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[ZeroIntensity]

Please add a test case. You can refer to the devguide for how to do that.

[ZeroIntensity]

No need to force-push, we squash at the end.

[GeneralK1ng]

No need to force-push, we squash at the end.

Got it, thanks for the reminder!

[ZeroIntensity]

FYI, tests are failing on Windows.

[GeneralK1ng]

FYI, tests are failing on Windows.

Thanks for the suggestion. I've skipped the test on Windows using @unittest.skipIf(sys.platform.startswith("win"), ...) because of platform differences in file permissions and locking behavior that caused the test to fail in CI. Should be good now!