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Three "database is locked" failures on the docker WROLPi, one cause each:
* `INSERT INTO download` from POST /api/download -- a user creating a
video download while a refresh ran (2026-08-06 14:06:46);
* six `INSERT INTO file_group` from `upsert_file`, uploads landing in
the same three-minute window;
* `UPDATE download SET status` from `renew_recurring_downloads`.
All three are the deferred read-then-write trap: a transaction that
begins as a reader and upgrades at its first write is refused
*instantly* when another connection wrote in between, because
busy_timeout is skipped for lock upgrades. `get_immediate_db_session`
existed to avoid it, but an opt-in helper only works if every author
remembers, and read-then-write is the ordinary shape of a write. So it
is gone: `get_db_session(commit=True)` -- the existing declaration of
write intent -- now begins IMMEDIATE. Rails 7.1 and Django 5.1 made the
same call, defaulting SQLite transactions to IMMEDIATE rather than
asking application code to opt in.
That default is only safe if write transactions stay short, and one did
not. `video_cleanup` ran its whole Channel-claiming join --
`UPDATE video ... FROM channel ... LEFT JOIN file_group`, O(channels x
file_groups) -- inside one write transaction: 110 to 130 seconds each of
the eight times it appears in the log, on a library with ~1500 channels.
While it ran, *every* writer on the box failed, including ones that
began IMMEDIATE, because the hold far exceeds the 30s busy_timeout. The
14:04-14:06 failures above sit inside its 130-second window. The join
is now a read -- WAL readers never block writers -- and only the
resulting UPDATEs take the lock, 500 rows per transaction.
`claim_videos_for_channels` had the same join and the same fix; channel
import calls it with every channel in channels.yaml.
Also here:
* `renew_recurring_downloads` was a read session with an inner commit;
it is a write session now. Its computed `next_download` values now
persist even when nothing was renewed -- that is the point of
computing them.
* POST /api/download no longer writes through `request.ctx.session`.
That session is committed by the response middleware, so a writer
would hold the lock across the whole request; a write session
commits before the handler returns, ahead of the background dispatch
it fires.
* `_immediate_txn` is a ContextVar, not a threading.local. The
writers that set it include coroutines, and one asyncio thread runs
many of them interleaved.
* `write_lock_held_briefly` moved from the move tests into conftest,
since two suites now need it.
Both regression tests were verified by mutation: reverting either site
alone fails its own test, and the API one fails with the production
error verbatim.
Sources: fractaledmind.com/2023/12/11/sqlite-on-rails-improving-concurrency,
tenthousandmeters.com/blog/sqlite-concurrent-writes-and-database-is-locked-errors,
berthub.eu/articles/posts/a-brief-post-on-sqlite3-database-locked-despite-timeout
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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| __init__.py | ||