fix(queue): a late progress update no longer reopens a finished task (#1538)

#1429 made the first terminal status win, but the guard only refused
terminal to terminal changes. A worker's poll that reports DOWNLOADING
after a cancel still overwrote CANCELLED (the poll loop calls the status
callback before it waits on the cancel flag), and the COMPLETE or ERROR
that followed was then accepted as a first terminal status, so the
terminal hook ran a second time.

This refuses any status change away from a terminal status. `add()` and
`enqueue_existing()` reset the status themselves, so retries are
unaffected.

The new test cancels a downloading task, sends DOWNLOADING and then
COMPLETE, and expects CANCELLED and one hook call. It fails on main and
passes here. Full non-e2e suite passes, ruff and basedpyright are clean.

Found in a Claude pass over 1.5.0, same as my other small fixes from it.

shelfmark/core/queue.py

@@ -168,14 +168,13 @@ class BookQueue:
         hook: Callable[[str, QueueStatus, DownloadTask], None] | None = None
         hook_task: DownloadTask | None = None
         previous_status = self._status.get(book_id)
-        if (
-            previous_status in TERMINAL_QUEUE_STATUSES
-            and status in TERMINAL_QUEUE_STATUSES
-            and previous_status != status
-        ):
+        if previous_status in TERMINAL_QUEUE_STATUSES and previous_status != status:
             # The first terminal status wins. A worker that checked its cancel flag just before
             # a cancel landed would otherwise overwrite CANCELLED with COMPLETE and fire the
-            # terminal hook a second time.
+            # terminal hook a second time. A late progress update (DOWNLOADING) is refused for
+            # the same reason: it would reopen the task and let the next terminal status
+            # through. A retry goes through add() or enqueue_existing(), which reset the
+            # status themselves.
             return None, None
         self._update_status(book_id, status)
 

tests/core/test_queue.py

@@ -88,6 +88,23 @@ def test_a_late_complete_does_not_overwrite_a_cancel():
     assert terminal_events == [QueueStatus.CANCELLED]
 
 
+def test_a_late_progress_update_does_not_reopen_a_cancelled_task():
+    """A poll that reports DOWNLOADING after the cancel must not reopen the task.
+
+    Without this the next COMPLETE or ERROR is accepted as a first terminal status and
+    the terminal hook fires a second time.
+    """
+    queue = BookQueue()
+    terminal_events = _downloading_task(queue, "race-task")
+
+    assert queue.cancel_download("race-task") is True
+    queue.update_status("race-task", QueueStatus.DOWNLOADING)
+    queue.update_status("race-task", QueueStatus.COMPLETE)
+
+    assert queue.get_task_status("race-task") == QueueStatus.CANCELLED
+    assert terminal_events == [QueueStatus.CANCELLED]
+
+
 def test_a_task_that_ended_in_error_is_not_completed_afterwards():
     queue = BookQueue()
     terminal_events = _downloading_task(queue, "race-task")