scheduler: fix cancellations

artiq/management/scheduler.py

@@ -10,7 +10,7 @@ class Scheduler:
         self.worker = Worker(worker_handlers)
         self.next_rid = 0
         self.queue = Notifier([])
-        self.queue_count = asyncio.Semaphore(0)
+        self.queue_modified = asyncio.Event()
         self.periodic = Notifier(dict())
         self.periodic_modified = asyncio.Event()
 
@@ -39,12 +39,16 @@ class Scheduler:
     def run_once(self, run_params, timeout):
         rid = self.new_rid()
         self.queue.append((rid, run_params, timeout))
-        self.queue_count.release()
+        self.queue_modified.set()
         return rid
 
     def cancel_once(self, rid):
-        idx = next(idx for idx, (qrid, _, _) in enumerate(self.queue)
+        idx = next(idx for idx, (qrid, _, _)
+                   in enumerate(self.queue.backing_struct)
                    if qrid == rid)
+        if idx == 0:
+            # Cannot cancel when already running
+            raise NotImplementedError
         del self.queue[idx]
 
     def run_periodic(self, run_params, timeout, period):
@@ -97,18 +101,17 @@ class Scheduler:
     def _schedule(self):
         while True:
             next_periodic = yield from self._run_periodic()
-            ev_queue = asyncio.Task(self.queue_count.acquire())
-            ev_periodic = asyncio.Task(self.periodic_modified.wait())
-            done, pend = yield from asyncio.wait(
-                [ev_queue, ev_periodic],
-                timeout=next_periodic,
-                return_when=asyncio.FIRST_COMPLETED)
-            self.periodic_modified.clear()
-            for t in pend:
-                t.cancel()
-
-            yield from self._run_periodic()
-            if ev_queue in done:
+            if self.queue.backing_struct:
                 rid, run_params, timeout = self.queue.backing_struct[0]
                 yield from self._run(rid, run_params, timeout)
                 del self.queue[0]
+            else:
+                self.queue_modified.clear()
+                self.periodic_modified.clear()
+                done, pend = yield from asyncio.wait(
+                    [
+                        self.queue_modified.wait(),
+                        self.periodic_modified.wait()
+                    ],
+                    timeout=next_periodic,
+                    return_when=asyncio.FIRST_COMPLETED)