Breakpoint bad (-ve or too large) x-ranges to m4

This should never really happen but when it does it appears to be a race
with writing startup pre-graphics-formatter array data where we get
`x_end` epoch value subtracting some really small offset value (like
`-/+0.5`) or the opposite where the `x_start` is epoch and `x_end` is
small.

This adds a warning msg and `breakpoint()` as well as guards around the
entire code downsampling code path so that when resumed the downsampling
cycle should just be skipped and avoid a crash.

piker/data/_m4.py

@@ -91,6 +91,14 @@ def ds_m4(
         x_end = x[-1]  # x end value/highest in domain
         xrange = (x_end - x_start)
 
+    if xrange < 0:
+        log.error(f'-VE M4 X-RANGE: {x_start} -> {x_end}')
+        # XXX: broken x-range calc-case, likely the x-end points
+        # are wrong and have some default value set (such as
+        # x_end -> <some epoch float> while x_start -> 0.5).
+        breakpoint()
+        return None
+
     # XXX: always round up on the input pixels
     # lnx = len(x)
     # uppx *= max(4 / (1 + math.log(uppx, 2)), 1)

piker/data/_pathops.py

@@ -63,20 +63,27 @@ def xy_downsample(
     # downsample whenever more then 1 pixels per datum can be shown.
     # always refresh data bounds until we get diffing
     # working properly, see above..
-    bins, x, y, ymn, ymx = ds_m4(
+    m4_out = ds_m4(
         x,
         y,
         uppx,
     )
 
-    # flatten output to 1d arrays suitable for path-graphics generation.
-    x = np.broadcast_to(x[:, None], y.shape)
-    x = (x + np.array(
-        [-x_spacer, 0, 0, x_spacer]
-    )).flatten()
-    y = y.flatten()
-
-    return x, y, ymn, ymx
+    if m4_out is not None:
+        bins, x, y, ymn, ymx = m4_out
+        # flatten output to 1d arrays suitable for path-graphics generation.
+        x = np.broadcast_to(x[:, None], y.shape)
+        x = (x + np.array(
+            [-x_spacer, 0, 0, x_spacer]
+        )).flatten()
+        y = y.flatten()
+
+        return x, y, ymn, ymx
+
+    # XXX: we accept a None output for the case where the input range
+    # to ``ds_m4()`` is bad (-ve) and we want to catch and debug
+    # that (seemingly super rare) circumstance..
+    return None
 
 
 @njit(

piker/ui/_render.py

@@ -211,17 +211,19 @@ def render(
 
             elif should_ds and uppx > 1:
 
-                x_1d, y_1d, ymn, ymx = xy_downsample(
+                ds_out = xy_downsample(
                     x_1d,
                     y_1d,
                     uppx,
                 )
-                self.viz.yrange = ymn, ymx
-                # print(f'{self.viz.name} post ds: ymn, ymx: {ymn},{ymx}')
-
-                reset = True
-                profiler(f'FULL PATH downsample redraw={should_ds}')
-                self._in_ds = True
+                if ds_out is not None:
+                    x_1d, y_1d, ymn, ymx = ds_out
+                    self.viz.yrange = ymn, ymx
+                    # print(f'{self.viz.name} post ds: ymn, ymx: {ymn},{ymx}')
+
+                    reset = True
+                    profiler(f'FULL PATH downsample redraw={should_ds}')
+                    self._in_ds = True
 
             path = self.draw_path(
                 x=x_1d,