drawArc doesn't work consistently

[williamngan]

Describe the bug
canvas.drawArc() fails silently in some cases

To Reproduce

Code to reproduce as follows. Run this multiple times to see some arcs may fail to draw.

import skia
import numpy as np
import matplotlib.pyplot as plt

width, height = 512, 512
array = np.zeros((height, width, 4), dtype=np.uint8)
rects = np.random.rand(10, 2, 2) * 512

with skia.Surface(array) as canvas:
    for r in rects:
        rect = skia.Rect.MakeLTRB(r[0][0], r[0][1], r[1][0], r[1][1])
        # fill oval in red
        paint = skia.Paint(
            AntiAlias=True,
            Color=0x33ff0000,
            Style=skia.Paint.kFill_Style,
        )
        canvas.drawOval(rect, paint)
        # draw arc in black line
        paint = skia.Paint(
            AntiAlias=True,
            Color=0xFF000000,
            StrokeWidth=5,
            Style=skia.Paint.kStroke_Style,
        )
        canvas.drawArc(rect, 30, 180, False, paint)

plt.imshow(array)
plt.show()

Result I got. Notice that some ovals don't have stroked arcs.

Expected behavior
drawArc() should work consistently as drawOval() above.

Desktop (please complete the following information):

• OS: Ubuntu 20.04.5 LTS
• Python: 3.11.0
• skia-python version: 87.5

Additional context
Add any other context about the problem here.

[HinTak]

I am wondering if it is some kind of "optimization" where some of the drawing operations are re-ordered and dropped. So it might be interesting to re-write your code with a opengl backend and draw interactively (pause, and in response to a event to draw the next bit), and only convert to CPU array at the end (if you want a graphic file).

I have a suspicion that "earlier" drawarc's are dropped.

[HinTak]

So doing a deterministic seed for the random number, and actually writing the order out, which one is missing, would be interesting.