Kept small on purpose so it's easy to review. I have a few more small
fixes like this from going back over recently merged PRs. Would you
rather get them as separate PRs like this one, or combined into one?
#1361 made `cancel_download` write under the queue lock, which stops a
cancel from overwriting a download that finished first. The other
direction is still open. The worker checks `cancel_flag.is_set()` and
then calls `update_status(COMPLETE)` a few lines later in
`_process_single_download`. If a cancel lands between those two lines
the task goes CANCELLED, then the worker writes COMPLETE over it.
`_apply_status_locked` allows terminal to terminal moves, so the
terminal hook also fires a second time and records two events for one
task.
This makes the first terminal status win: `_apply_status_locked` ignores
a different terminal status once the task already has one. Requeueing
from a terminal status still works because QUEUED isn't terminal.
I replaced the existing race test. It patched `queue.update_status`,
which `cancel_download` no longer calls, so it never exercised the path
it names, and its last assertion only looked at the final hook event.
The new tests call the queue directly: complete then cancel is refused,
cancel then complete stays cancelled with one hook event, error then
complete stays error, and a terminal task can still be requeued. The two
late-write tests fail on current `main`.
Ruff and basedpyright are clean. `tests/core` and `tests/download` pass
apart from 4 `seleniumbase` import failures in my environment that also
fail on `main`.
The one red check is the existing clock-dependent test
`test_a_queued_torrent_asks_for_a_grace_once`, which fails on any runner
that booted recently. It is fixed in #1426 and is not caused by this
change.
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