rta: Disallow invalid composition of delayed tasks

This example:

    wl = RTA(te.target, 'mywl', calibration=calib)
    wl.conf('profile',
            params = {'t': Periodic(delay_s=3, duration_s=1) +
                    Periodic(delay_s=2, duration_s=1)).get()})

Should semantically result in

1. 3s delay
2. 1s of periodic behaviour
3. 2s delay
4. 1s of periodic behaviour.

But part 3 (the second delay) will be ignored due to the way we use
the "delay" field.

We could probably fix this using a "sleep" event, but to keep things
simple let's just disallow this type of composition, unless we later
find that we need that feature.

1 files changed, 15 insertions, 0 deletions

diff --git a/tests/lisa/test_wlgen_rtapp.py b/tests/lisa/test_wlgen_rtapp.py
index 8513af4..3984d31 100644
--- a/tests/lisa/test_wlgen_rtapp.py
+++ b/tests/lisa/test_wlgen_rtapp.py
@@ -216,6 +216,21 @@ class TestRTAComposition(RTABase):
         self.assert_output_file_exists('rt-app-task_ramp-0.log')
         self.assert_can_read_logfile(exp_tasks=['task_ramp'])
 
+    def test_invalid_composition(self):
+        """Test that you can't compose tasks with a delay in the second task"""
+        t1 = Periodic()
+        t2 = Periodic(delay_s=1)
+
+        # Should work fine if delayed task is the first one
+        try:
+            t3 = t2 + t1
+        except Exception as e:
+            raise AssertionError("Couldn't compose tasks: {}".format(e))
+
+        # But not the other way around
+        with self.assertRaises(ValueError):
+            t3 = t1 + t2
+
 
 class TestRTACustom(RTABase):
     def _test_custom_smoke(self, calibration):