db: Give the Idle Queue a cancel_task() function

It's sometimes desireable to cancel a pending task and re-add it with
new parameters to cut out some unnecessary work.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

emmental/db/idle.py

@@ -52,6 +52,11 @@ class Queue(GObject.GObject):
         self.running = False
         self._idle_id = None
 
+    def cancel_task(self, func: typing.Callable) -> None:
+        """Remove all instances of a specific task from the Idle Queue."""
+        self._tasks = [t for t in self._tasks if t[0] != func]
+        self.__update_counters()
+
     def complete(self) -> None:
         """Complete all pending tasks."""
         if self.running:

tests/db/test_idle.py

@@ -51,6 +51,26 @@ class TestIdleQueue(unittest.TestCase):
         self.assertEqual(self.queue.total, 0)
         self.assertEqual(self.queue.progress, 0.0)
 
+    def test_cancel_task(self, mock_idle_add: unittest.mock.Mock,
+                         mock_source_removed: unittest.mock.Mock):
+        """Test canceling a specific task."""
+        self.queue.push(1)
+        self.queue.push(2)
+        self.queue.push(1)
+
+        self.queue.cancel_task(1)
+        self.assertListEqual(self.queue._tasks, [(2,)])
+        self.assertEqual(self.queue.total, 3)
+        self.assertAlmostEqual(self.queue.progress, 2 / 3)
+        mock_source_removed.assert_not_called()
+
+        self.queue.cancel_task(2)
+        self.assertListEqual(self.queue._tasks, [])
+        self.assertIsNone(self.queue._idle_id)
+        self.assertEqual(self.queue.total, 0)
+        self.assertEqual(self.queue.progress, 0.0)
+        mock_source_removed.assert_called_with(42)
+
     def test_complete(self, mock_idle_add: unittest.mock.Mock,
                       mock_source_removed: unittest.mock.Mock):
         """Test completing queued tasks."""