Detect and request sample period in fsp engine

piker/fsp/_engine.py

@@ -123,7 +123,6 @@ async def fsp_compute(
     # TODO: push using a[['f0', 'f1', .., 'fn']] = .. syntax no?
     # if the output array is multi-field then push
     # each respective field.
-    # await tractor.breakpoint()
     fields = getattr(dst.array.dtype, 'fields', None).copy()
     fields.pop('index')
     # TODO: nptyping here!
@@ -381,14 +380,19 @@ async def cascade(
 
             s, step, ld = is_synced(src, dst)
 
+            # detect sample period step for subscription to increment
+            # signal
+            times = src.array['time']
+            delay_s = times[-1] - times[times != times[-1]][-1]
+
             # Increment the underlying shared memory buffer on every
             # "increment" msg received from the underlying data feed.
-            async with feed.index_stream() as stream:
+            async with feed.index_stream(int(delay_s)) as istream:
 
                 profiler(f'{func_name}: sample stream up')
                 profiler.finish()
 
-                async for msg in stream:
+                async for _ in istream:
 
                     # respawn the compute task if the source
                     # array has been updated such that we compute