Emit resource warning if sqlite3 fails to close the database

[erlend-aasland]

See #103837 (comment) and #103837 (comment):

EAA:

I still think we should consider changing the underlying API so we can emit a resource warning if close failed.

VS:

IMO sqlite3 should be modified to emit a ResourceWarning.

Currently, no ResourceWarning is emitted when a connection is not closed explicitly:

Linked PRs

• gh-105539: Emit ResourceWarning if sqlite3 database is not closed explicitly #108015
• gh-105539: Explict resource management for connection objects in sqlite3 tests #108017
• gh-105539: Fix ResourceWarning from unclosed SQLite connection in ClosedCurTests. #108360
• gh-64662: Fix NEWS entry and remove What's New entry #108565

[vstinner]

What is the consequences if a database is not closed explicitly and fails to be closed at Python exit? Is there a risk of losing data?

[erlend-aasland]

What is the consequences if a database is not closed explicitly and fails to be closed at Python exit? Is there a risk of losing data?

If sqlite3_close() or sqlite3_close_v2() fails, there is of course a risk of losing data. However, we do a pretty good job at cleaning up the connection and its debris (cursors, statements, registered callbacks, etc.), so that risk is almost non-existent :)

[vstinner]

By "fail to be closed", I was thinking about a database connection involved in a reference cycle which is impossible to break, and so close() is never called and the object is never destroyed.

[erlend-aasland]

By "fail to be closed", I was thinking about a database connection involved in a reference cycle which is impossible to break, and so close() is never called and the object is never destroyed.

If the connection object is never destroyed (dealloc or finalize are never called), there is no way to detect that in the extension module that I know about. The proposed change involves both the finalizer and the dealloc callbacks, so I'm relying on them actually being called at some point.

[vstinner]

Congrats :-) IMO it's a nice enhancement.

[erlend-aasland]

Thanks for bringing the idea up in the first place, Victor! :)

[erlend-aasland]

Apparently, I did not cover all cases in #108017; the following now emit resource warnings:

 ./python.exe -m test -R : test_sqlite3 -m ClosedCurTests