From 7d8202a63c00f343bef09fd9572a9cdc9f7f95ee Mon Sep 17 00:00:00 2001
From: Tyler Goodlet <jgbt@protonmail.com>
Date: Thu, 30 Sep 2021 07:33:43 -0400
Subject: [PATCH] Fix shm index update race

There was a lingering issue where the fsp daemon would sync its shm
array with the source data and we'd set the start/end indices to the
same value. Under some races a reader would then read an empty `.array`
which it wasn't expecting. This fixes that as well as tidies up the
`ShmArray.push()` logic and adds a temporary check in `.array` for zero
length if the array hasn't been written yet.

We can now start removing read array length checks in consumer code
and hopefully no more races will show up.
---
 piker/data/_sharedmem.py |  2 +-
 piker/ui/_chart.py       | 33 ++++++++++++++++-----------------
 2 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/piker/data/_sharedmem.py b/piker/data/_sharedmem.py
index 2169e262..8afa3214 100644
--- a/piker/data/_sharedmem.py
+++ b/piker/data/_sharedmem.py
@@ -220,7 +220,7 @@ class ShmArray:
         self,
         length: int = 1,
     ) -> np.ndarray:
-        return self.array[-length:]
+        return self.array[-length]
 
     def push(
         self,
diff --git a/piker/ui/_chart.py b/piker/ui/_chart.py
index 04f24d50..dc9387ea 100644
--- a/piker/ui/_chart.py
+++ b/piker/ui/_chart.py
@@ -936,11 +936,12 @@ class ChartPlotWidget(pg.PlotWidget):
         **kwargs,
 
     ) -> pg.GraphicsObject:
-        """Update the named internal graphics from ``array``.
-
-        """
+        '''Update the named internal graphics from ``array``.
 
+        '''
+        assert len(array)
         data_key = array_key or graphics_name
+
         if graphics_name not in self._overlays:
             self._arrays['ohlc'] = array
         else:
@@ -948,21 +949,19 @@ class ChartPlotWidget(pg.PlotWidget):
 
         curve = self._graphics[graphics_name]
 
-        if len(array):
-            # TODO: we should instead implement a diff based
-            # "only update with new items" on the pg.PlotCurveItem
-            # one place to dig around this might be the `QBackingStore`
-            # https://doc.qt.io/qt-5/qbackingstore.html
+        # NOTE: back when we weren't implementing the curve graphics
+        # ourselves you'd have updates using this method:
+        # curve.setData(y=array[graphics_name], x=array['index'], **kwargs)
 
-            # NOTE: back when we weren't implementing the curve graphics
-            # ourselves you'd have updates using this method:
-            # curve.setData(y=array[graphics_name], x=array['index'], **kwargs)
-
-            curve.update_from_array(
-                x=array['index'],
-                y=array[data_key],
-                **kwargs
-            )
+        # NOTE: graphics **must** implement a diff based update
+        # operation where an internal ``FastUpdateCurve._xrange`` is
+        # used to determine if the underlying path needs to be
+        # pre/ap-pended.
+        curve.update_from_array(
+            x=array['index'],
+            y=array[data_key],
+            **kwargs
+        )
 
         return curve