Move sync diffing helpers out of index loop

piker/fsp/_engine.py

@@ -44,6 +44,11 @@ _fsp_builtins = {
     'vwap': _tina_vwap,
 }
 
+# TODO: things to figure the heck out:
+# - how to handle non-plottable values (pyqtgraph has facility for this
+#   now in `arrayToQPath()`)
+# - composition of fsps / implicit chaining syntax (we need an issue)
+
 
 @dataclass
 class TaskTracker:
@@ -54,10 +59,11 @@ class TaskTracker:
 async def filter_quotes_by_sym(
 
     sym: str,
-    quote_stream,
+    quote_stream: tractor.MsgStream,
 
 ) -> AsyncIterator[dict]:
-    '''Filter quote stream by target symbol.
+    '''
+    Filter quote stream by target symbol.
 
     '''
     # TODO: make this the actual first quote from feed
@@ -187,8 +193,9 @@ async def cascade(
     loglevel: Optional[str] = None,
 
 ) -> None:
-    '''Chain streaming signal processors and deliver output to
+    '''
-    destination mem buf.
+    Chain streaming signal processors and deliver output to
+    destination shm array buffer.
 
     '''
     profiler = pg.debug.Profiler(delayed=False, disabled=False)
@@ -260,6 +267,46 @@ async def cascade(
                 await tracker.complete.wait()
                 return await n.start(fsp_target)
 
+            def is_synced(
+                src: ShmArray,
+                dst: ShmArray
+            ) -> tuple[bool, int, int]:
+                '''Predicate to dertmine if a destination FSP
+                output array is aligned to its source array.
+
+                '''
+                step_diff = src.index - dst.index
+                len_diff = abs(len(src.array) - len(dst.array))
+                return not (
+                    # the source is likely backfilling and we must
+                    # sync history calculations
+                    len_diff > 2 or
+
+                    # we aren't step synced to the source and may be
+                    # leading/lagging by a step
+                    step_diff > 1 or
+                    step_diff < 0
+                ), step_diff, len_diff
+
+            async def poll_and_sync_to_step(
+
+                tracker: TaskTracker,
+                src: ShmArray,
+                dst: ShmArray,
+
+            ) -> tuple[TaskTracker, int]:
+
+                synced, step_diff, _ = is_synced(src, dst)
+                while not synced:
+                    tracker, index = await resync(tracker)
+                    synced, step_diff, _ = is_synced(src, dst)
+
+                return tracker, step_diff
+
+            s, step, ld = is_synced(src, dst)
+            if step or ld:
+                await tractor.breakpoint()
+
             # Increment the underlying shared memory buffer on every
             # "increment" msg received from the underlying data feed.
             async with feed.index_stream() as stream:
@@ -269,41 +316,20 @@ async def cascade(
 
                 async for msg in stream:
 
-                    def desynced(src: ShmArray, dst: ShmArray) -> bool:
-                        diff = src.index - dst.index
-                        len_diff = abs(len(src.array) - len(dst.array))
-                        return (
-                            # the source is likely backfilling and we must
-                            # sync history calculations
-                            len_diff > 2 or
-
-                            # we aren't step synced to the source and may be
-                            # leading/lagging by a step
-                            diff > 1 or
-                            diff < 0
-                        )
-
-                    async def poll_and_sync_to_step(tracker):
-                        while desynced(src, dst):
-                            tracker, index = await resync(tracker)
-                            # log.info(
-                            #     '\n'.join((
-                            #         f'history index after sync: {index}',
-                            #         f'diff after sync: {diff}',
-                            #     ))
-                            # )
-
-                        return tracker, diff
-
                     # respawn the compute task if the source
                     # array has been updated such that we compute
                     # new history from the (prepended) source.
-                    if desynced(src, dst):
+                    synced, step_diff, _ = is_synced(src, dst)
-                        tracker, diff = await poll_and_sync_to_step(tracker)
+                    if not synced:
+                        tracker, step_diff = await poll_and_sync_to_step(
+                            tracker,
+                            src,
+                            dst,
+                        )
 
-                        # skip adding a last bar since we should be
+                        # skip adding a last bar since we should already
-                        # source alinged
+                        # be step alinged
-                        if diff == 0:
+                        if step_diff == 0:
                             continue
 
                     # read out last shm row, copy and write new row