PersistentDict: thread safety

[matthiasdiener]

Workaround for #233.

cc: illinois-ceesd/logpyle#106 illinois-ceesd/logpyle#107 illinois-ceesd/logpyle#108

Based partly on https://ricardoanderegg.com/posts/python-sqlite-thread-safety/.

Notes:

• Full thread safety ("serialized mode" in SQLite) is the default mode of SQLite (https://sqlite.org/threadsafe.html)
• There appears to be no way thread safety can be set from the Python interface

The only other alternative I can see is to use a fully multithreaded implementation such as https://github.com/piskvorky/sqlitedict

[matthiasdiener]

This is ready for a first look @inducer @vincefn

[inducer]

I'm not sure I'd like to document these, since it encodes things that feel like implementation details. Maybe push this off into pytools._sqlite?

[matthiasdiener]

Moving these functions into a separate module makes sense I think, but I thought these functions are pretty generic? Which implementation details do they encode?

[inducer]

How cpython and threads and sqlite interact. The details of how this works at the moment is kind of insane ATM, and I try to avoid insanity in publicly exposed interfaces. Hence my preference to keep this out of public view.

[matthiasdiener]

How cpython and threads and sqlite interact. The details of how this works at the moment is kind of insane ATM

I see - is this in reference to the translation between thread support levels in 07412b2#diff-2d67e0c2f525bd7f05e4909f1248231d4b1f3b92d2f1be94487d5b2d4cb46b78R2992-R2996, or are there other problems in interacting between threads/sqlite? If so, could those be the issue for the failures I mentioned in #234 (comment)?

[matthiasdiener]

Another idea: in case the sqlite library is not built with thread safety, can we just use a mutex every time we use the connection (e.g., in _exec_sql_fn), and also set check_same_thread=False?

[matthiasdiener]

It seems it is crashing in the multithreaded case where the connection is shared among threads, a situation which should not crash even without the mutex :-(

  File "/home/runner/work/pytools/pytools/pytools/persistent_dict.py", line 778, in fetch
    stored_key, value = pickle.loads(row[0])
                        ^^^^^^^^^^^^^^^^^^^^
TypeError: a bytes-like object is required, not 'NoneType'

[matthiasdiener]

Well, that turned out to be more ... interesting... than expected. The final error only happened with Python 3.12, which made debugging this more difficult.

This is ready for testing/review.

@vincefn: Could you test this PR against the failure you had mentioned in #233?

It does avoid the warning I had seen in the boxtree test in #233.

[vincefn]

Sorry it took a while - I could only reproduce that inside a gitlab CI test... But the tests which failed are now passing with this fix :-)