Enable tracing back insert backfills

Namely insertion writes which over-fill the shm buffer past the latest
tsdb sample via `.tsp._history.shm_push_in_between()`.

Deats,
- check earliest `to_push` timestamp and enter pause point if it's
  earlier then the tsdb's `backfill_until_dt` stamp.
- requires actually passing the `backfill_until_dt: datetime` thru,
  * `get_null_segs()`
  * `maybe_fill_null_segments()`
  * `shm_push_in_between()` (obvi XD)

piker/tsp/_history.py

@@ -75,7 +75,6 @@ from piker.brokers._util import (
 )
 from piker.storage import TimeseriesNotFound
 from ._anal import (
-
     dedupe,
     get_null_segs,
     iter_null_segs,
@@ -128,7 +127,8 @@ def diff_history(
     # no diffing with tsdb dt index possible..
     if (
         prepend_until_dt is None
-        and append_until_dt is None
+        and
+        append_until_dt is None
     ):
         return array
 
@@ -140,15 +140,26 @@ def diff_history(
         return array[times >= prepend_until_dt.timestamp()]
 
 
-# TODO: can't we just make this a sync func now?
 async def shm_push_in_between(
     shm: ShmArray,
     to_push: np.ndarray,
     prepend_index: int,
+    backfill_until_dt: datetime,
 
     update_start_on_prepend: bool = False,
 
 ) -> int:
+
+    # XXX, try to catch bad inserts by peeking at the first/last
+    # times and ensure we don't violate order.
+    f_times: np.ndarray = to_push['time']
+    f_start: float = f_times[0]
+    f_start_dt = from_timestamp(f_start)
+    if (
+       f_start_dt < backfill_until_dt
+    ):
+        await tractor.pause()
+
     # XXX: extremely important, there can be no checkpoints
     # in the body of this func to avoid entering new ``frames``
     # values while we're pipelining the current ones to
@@ -181,6 +192,7 @@ async def maybe_fill_null_segments(
     get_hist: Callable,
     sampler_stream: tractor.MsgStream,
     mkt: MktPair,
+    backfill_until_dt: datetime,
 
     task_status: TaskStatus[trio.Event] = trio.TASK_STATUS_IGNORED,
 
@@ -191,6 +203,10 @@ async def maybe_fill_null_segments(
 
     frame: Frame = shm.array
 
+    # TODO, put in parent task/daemon root!
+    import greenback
+    await greenback.ensure_portal()
+
     null_segs: tuple|None = get_null_segs(
         frame,
         period=timeframe,
@@ -237,6 +253,7 @@ async def maybe_fill_null_segments(
             shm,
             to_push,
             prepend_index=absi_end,
+            backfill_until_dt=backfill_until_dt,
             update_start_on_prepend=False,
         )
         # TODO: UI side needs IPC event to update..
@@ -352,15 +369,12 @@ async def start_backfill(
     mkt: MktPair,
     shm: ShmArray,
     timeframe: float,
-
     backfill_from_shm_index: int,
     backfill_from_dt: datetime,
-
     sampler_stream: tractor.MsgStream,
 
     backfill_until_dt: datetime|None = None,
     storage: StorageClient|None = None,
-
     write_tsdb: bool = True,
 
     task_status: TaskStatus[tuple] = trio.TASK_STATUS_IGNORED,
@@ -495,7 +509,14 @@ async def start_backfill(
 
             assert time[-1] == next_end_dt.timestamp()
 
-            expected_dur: Interval = last_start_dt - next_start_dt
+            expected_dur: Interval = (
+                last_start_dt.subtract(
+                    seconds=timeframe
+                    # ^XXX, always "up to" the bar *before*
+                )
+                -
+                next_start_dt
+            )
 
             # frame's worth of sample-period-steps, in seconds
             frame_size_s: float = len(array) * timeframe
@@ -556,6 +577,7 @@ async def start_backfill(
                     shm,
                     to_push,
                     prepend_index=next_prepend_index,
+                    backfill_until_dt=backfill_until_dt,
                     update_start_on_prepend=update_start_on_prepend,
                 )
                 await sampler_stream.send({
@@ -585,6 +607,7 @@ async def start_backfill(
                     shm,
                     to_push,
                     prepend_index=next_prepend_index,
+                    backfill_until_dt=backfill_until_dt,
                     update_start_on_prepend=update_start_on_prepend,
                 )
                 await sampler_stream.send({
@@ -1056,7 +1079,7 @@ async def tsdb_backfill(
                 trio.open_nursery() as tn,
             ):
 
-                bf_done = await tn.start(
+                bf_done: trio.Event = await tn.start(
                     partial(
                         start_backfill,
                         get_hist=get_hist,
@@ -1077,7 +1100,9 @@ async def tsdb_backfill(
                     )
                 )
                 nulls_detected: trio.Event|None = None
+
                 if last_tsdb_dt is not None:
+
                     # calc the index from which the tsdb data should be
                     # prepended, presuming there is a gap between the
                     # latest frame (loaded/read above) and the latest
@@ -1148,7 +1173,7 @@ async def tsdb_backfill(
                     # TODO: ideally these can never exist!
                     # -[ ] somehow it seems sometimes we're writing zero-ed
                     #    segments to tsdbs during teardown?
-                    # -[ ] can we ensure that the backcfiller tasks do this
+                    # -[ ] can we ensure that the backfiller tasks do this
                     #    work PREVENTAVELY instead?
                     # -[ ] fill in non-zero epoch time values ALWAYS!
                     # await maybe_fill_null_segments(
@@ -1160,6 +1185,7 @@ async def tsdb_backfill(
                         get_hist=get_hist,
                         sampler_stream=sampler_stream,
                         mkt=mkt,
+                        backfill_until_dt=last_tsdb_dt,
                     ))
 
             # 2nd nursery END