Improvements to task processing

* Moves clean up to separate function for organization
* moves the `self.dequeued.append(item)` immediately after `self.queue.get()` so that we don't wait for it to show up

(cherry picked from commit bc9372e88f0c8855694431f811702d7fb899a97e)

cps/services/worker.py

@@ -82,16 +82,8 @@ class WorkerThread(threading.Thread):
             tasks = self.queue.to_list() + self.dequeued
             return sorted(tasks, key=lambda x: x.num)
 
-
-    # Main thread loop starting the different tasks
-    def run(self):
-        main_thread = _get_main_thread()
-        while main_thread.is_alive():
-            item = self.queue.get()
-
+    def cleanup_tasks(self):
         with self.doLock:
-                # once we hit our trigger, start cleaning up dead tasks
-                if len(self.dequeued) > TASK_CLEANUP_TRIGGER:
             dead = []
             alive = []
             for x in self.dequeued:
@@ -106,9 +98,21 @@ class WorkerThread(threading.Thread):
                 ret = sorted(dead, key=lambda x: x.task.end_time)[-TASK_CLEANUP_TRIGGER:] + alive
 
             self.dequeued = sorted(ret, key=lambda x: x.num)
+
+    # Main thread loop starting the different tasks
+    def run(self):
+        main_thread = _get_main_thread()
+        while main_thread.is_alive():
+            # this blocks until something is available
+            item = self.queue.get()
+            with self.doLock:
                 # add to list so that in-progress tasks show up
                 self.dequeued.append(item)
 
+            # once we hit our trigger, start cleaning up dead tasks
+            if len(self.dequeued) > TASK_CLEANUP_TRIGGER:
+                self.cleanup_tasks()
+
             # sometimes tasks (like Upload) don't actually have work to do and are created as already finished
             if item.task.stat is STAT_WAITING:
                 # CalibreTask.start() should wrap all exceptions in it's own error handling