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

@@ -63,12 +63,14 @@ 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,
     )
 
+    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(
@@ -78,6 +80,11 @@ def xy_downsample(
 
         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(
     # NOTE: need to construct this manually for readonly

piker/ui/_render.py

@@ -211,11 +211,13 @@ class Renderer(msgspec.Struct):
 
             elif should_ds and uppx > 1:
 
-                x_1d, y_1d, ymn, ymx = xy_downsample(
+                ds_out = xy_downsample(
                     x_1d,
                     y_1d,
                     uppx,
                 )
+                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}')