Sync history recalcs to diff checks via a "task tracker"

piker/fsp/_engine.py

@@ -18,6 +18,7 @@
 core task logic for processing chains
 
 '''
+from dataclasses import dataclass
 from functools import partial
 from typing import AsyncIterator, Callable, Optional
 
@@ -44,6 +45,12 @@ _fsp_builtins = {
 }
 
 
+@dataclass
+class TaskTracker:
+    complete: trio.Event
+    cs: trio.CancelScope
+
+
 async def filter_quotes_by_sym(
 
     sym: str,
@@ -134,7 +141,8 @@ async def fsp_compute(
 
     # setup a respawn handle
     with trio.CancelScope() as cs:
-        task_status.started((cs, index))
+        tracker = TaskTracker(trio.Event(), cs)
+        task_status.started((tracker, index))
         profiler(f'{func_name} yield last index')
 
         # import time
@@ -232,7 +240,7 @@ async def cascade(
                 func=func
             )
 
-            cs, index = await n.start(fsp_target)
+            tracker, index = await n.start(fsp_target)
 
             if zero_on_step:
                 last = dst.array[-1:]
@@ -263,17 +271,20 @@ async def cascade(
                         new_len > last_len + 1 or
                         abs(diff) > 1
                     ):
-                        log.warning(f're-syncing fsp {func_name} to source')
-                        cs.cancel()
-                        cs, index = await n.start(fsp_target)
-
                         # TODO: adopt an incremental update engine/approach
                         # where possible here eventually!
+                        log.warning(f're-syncing fsp {func_name} to source')
+                        tracker.cs.cancel()
+                        await tracker.complete.wait()
+                        tracker, index = await n.start(fsp_target)
+
+                        # skip adding a new bar since we should be fully aligned.
+                        continue
 
                     # read out last shm row, copy and write new row
                     array = dst.array
 
-                    # some metrics, like vlm should be reset
+                    # some metrics like vlm should be reset
                     # to zero every step.
                     if zero_on_step:
                         last = zeroed