scheduler: consider the pipeline flushed if everything has a lower priority than us. Closes #640

RELEASE_NOTES.rst

@@ -43,6 +43,9 @@ Release notes
   those logs to the master. See the example device databases to see how to
   instantiate this controller. Using ``artiq_session`` ensures that a controller
   manager is running simultaneously with the master.
+* Experiments scheduled with the "flush pipeline" option now proceed when there
+  are lower-priority experiments in the pipeline. Only experiments at the current
+  (or higher) priority level are flushed.
 
 
 2.3

artiq/dashboard/experiments.py

@@ -294,7 +294,8 @@ class _ExperimentDock(QtWidgets.QMdiSubWindow):
         priority.valueChanged.connect(update_priority)
 
         flush = QtWidgets.QCheckBox("Flush")
-        flush.setToolTip("Flush the pipeline before starting the experiment")
+        flush.setToolTip("Flush the pipeline (of current- and higher-priority "
+                         "experiments) before starting the experiment")
         self.layout.addWidget(flush, 2, 2, 1, 2)
 
         flush.setChecked(scheduling["flush"])

artiq/master/scheduler.py

@@ -204,6 +204,7 @@ class PrepareStage(TaskObject):
                     run.status = RunStatus.flushing
                     while not all(r.status in (RunStatus.pending,
                                                RunStatus.deleting)
+                                  or r.priority < run.priority
                                   or r is run
                                   for r in self.pool.runs.values()):
                         ev = [self.pool.state_changed.wait(),