WxRenderWidget cannot be used on its own (segfault)

[tlambert03]

running the following example causes a segfault (rendercanvas v2.1.2, wxpython 4.2.3, macos, python 3.12)

import rendercanvas.wx
import wx

app = wx.App()
widget = rendercanvas.wx.WxRenderWidget()

while WxRenderCanvas() works just fine...

the fault seems to be happening in a size-related method:

➜ lldb python -- x.py
(lldb) target create "python"
Current executable set to '/Users/talley/dev/self/ndv/.venv/bin/python' (arm64).
(lldb) settings set -- target.run-args  "x.py"
(lldb) run
Process 8261 launched: '/Users/talley/dev/self/ndv/.venv/bin/python' (arm64)
Process 8261 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x00000001032dd158 libwx_osx_cocoau_core-3.2.0.4.0.dylib`wxWindow::DoGetPosition(int*, int*) const + 52
libwx_osx_cocoau_core-3.2.0.4.0.dylib`wxWindow::DoGetPosition:
->  0x1032dd158 <+52>: ldr    x8, [x0]
    0x1032dd15c <+56>: ldr    x8, [x8, #0x98]
    0x1032dd160 <+60>: add    x1, sp, #0xc
    0x1032dd164 <+64>: add    x2, sp, #0x8
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
  * frame #0: 0x00000001032dd158 libwx_osx_cocoau_core-3.2.0.4.0.dylib`wxWindow::DoGetPosition(int*, int*) const + 52
    frame #1: 0x0000000102ae55a8 _core.cpython-312-darwin.so`sipwxWindow::DoGetPosition(int*, int*) const + 156
    frame #2: 0x00000001032de498 libwx_osx_cocoau_core-3.2.0.4.0.dylib`wxWindow::DoSetSize(int, int, int, int, int) + 72
    frame #3: 0x0000000102ae5390 _core.cpython-312-darwin.so`sipwxWindow::DoSetSize(int, int, int, int, int) + 184
    frame #4: 0x0000000102af8828 _core.cpython-312-darwin.so`meth_wxWindow_SetSize(_object*, _object*, _object*) + 660
    frame #5: 0x0000000101631154 libpython3.12.dylib`cfunction_call + 72
    frame #6: 0x00000001014693b0 libpython3.12.dylib`_PyEval_EvalFrameDefault + 164064
    frame #7: 0x000000010165eb70 libpython3.12.dylib`method_vectorcall.llvm.13786586648554541543 + 172
    frame #8: 0x000000010146cd70 libpython3.12.dylib`_PyEval_EvalFrameDefault + 178848

and I tracked it down to this line in _rc_set_logical_size (which is triggered by _final_canvas_init)

rendercanvas/rendercanvas/wx.py

Line 359 in 557cb3f

self.SetSize(width, height)

[tlambert03]

a follow up question (not sure if it's a bug or my misunderstanding of how WxRenderWidget() should be used)... but if I comment out that SetSize line above, I run into another issue, when trying to create a pygfx renderer targeting the widget:

import pygfx.renderers
import rendercanvas.wx
import wx

app = wx.App()
canvas = rendercanvas.wx.WxRenderWidget()
renderer = pygfx.renderers.WgpuRenderer(canvas)  # this line blocks and prevents further setup 

it gets caught in a loop here, which was triggered by WxRenderWidget._rc_get_present_methods:

rendercanvas/rendercanvas/wx.py

Lines 188 to 189 in 557cb3f

	while event_loop.Pending():
	event_loop.Dispatch()

[almarklein]

From what I understand, a toplevel widget should inherit from wx.Frame, and subwidgets from wx.Window. So the WxRenderWidget is intended to be embedded in a larger GUI, while the rendercanvas.wx.RenderCanvas is intended as a toplevel widget.

[tlambert03]

Yes, I was going to be placing it within the context of a larger widget. But it still doesn’t work. Even if I pass in a parent frame it doesn’t work

Can you show me a simple example of embedding that doesn’t segfault or freeze?

[almarklein]

Trying your original sample on Linux, I get not a segfault but it indeed errors at SetSize:

  File "/home/almar/dev/py/rendercanvas/rendercanvas/base.py", line 523, in set_logical_size
    self._rc_set_logical_size(width, height)
  File "/home/almar/dev/py/rendercanvas/rendercanvas/wx.py", line 359, in _rc_set_logical_size
    self.SetSize(width, height)
wx._core.wxAssertionError: C++ assertion ""m_widget"" failed at ./src/gtk/window.cpp(4057) in DoSetSize(): invalid window

When trying to come up with a minimal example, I indeed run into errors and segfaults rather easily. The order of operations matters.

Simplest form:

import rendercanvas.wx
import wx

app = wx.App()

frame = wx.Frame(None)

canvas = rendercanvas.wx.WxRenderWidget(parent=frame)

frame.SetSize(640, 480)
frame.Show()

app.MainLoop()

Slightly more advanced:

import rendercanvas.wx
import wx
import numpy as np


app = wx.App()

frame = wx.Frame(None)

canvas = rendercanvas.wx.WxRenderWidget(parent=frame)

frame.SetSize(640, 480)
frame.Show()

context = canvas.get_context("bitmap")
bitmap= np.full((100, 100, 4), 255, np.uint8)
bitmap[:,:,0] = 0
context.set_bitmap(bitmap)

app.MainLoop()

With an animation:

import rendercanvas.wx
import wx
import numpy as np


app = wx.App()

frame = wx.Frame(None)

canvas = rendercanvas.wx.WxRenderWidget(parent=frame, update_mode='continuous')

frame.SetSize(640, 480)
frame.Show()

context = canvas.get_context("bitmap")

@canvas.request_draw
def animate():
    w, h = canvas.get_logical_size()
    shape = int(h) // 4, int(w) // 4

    bitmap = np.random.uniform(0, 255, shape).astype(np.uint8)
    context.set_bitmap(bitmap)

app.MainLoop()

And then there is https://github.com/pygfx/rendercanvas/blob/main/examples/wx_app.py

[tlambert03]

ok, fair enough! sorry I missed that example. was trying something too simple I guess.

do you think it would be worth guarding

rendercanvas/rendercanvas/wx.py

Line 359 in 557cb3f

self.SetSize(width, height)

behind if parent is not None?

[almarklein]

See #92

I also found that if you use wx.Frame() instead of wx.Frame(None) in the snippets above, it segfaults 🤷 but we cannot detect that case.

[tlambert03]

I commented at #92. The warning is fine, but I think it slightly misses the point that this is a little bit of a bug/limitation in rendercanvas. There shouldn’t be anything conceptually wrong with creating a widget without a parent and then later putting it into another widget before the event loop starts. It’s still not using it as a top level widget and I would have expected it to be a supported pattern