Wayland: Node Graph Panel is transparent and causes rendering artifacts/flickering in auto_nodes example

[mara004]

Steps to reproduce:

• Log in to a Wayland session
• run example_auto_nodes.py
  -> The Node Graph Panel is transparent; moving the window causes rendering artifacts and flickering
  

Workaround: set QT_QPA_PLATFORM=xcb to run as an XWayland instance.
The problems described were not reproducable with example.py, only with auto_nodes

Note: This is something I have been experiencing with many applications under Wayland. I initially filed an issue with kde, but the devs say those are definitely problems that need to be addressed in the client apps, and that it would be a common problem they can help fixing.

[davidedmundson]

Please attach output of WAYLAND_DEBUG=1 example.py.

[mara004]

@davidedmundson You mean example_auto_nodes.py, right?
nodegraphqt_wayland.txt

Thank you for looking into this.

[davidedmundson]

[3451683,422] -> wl_subcompositor@15.get_subsurface(new id wl_subsurface@125, wl_surface@122, wl_surface@121)

Thought so.

We've seen this quite a few times.

What typically happens is you have a QWidget.
You promote this widget to an indepdenent toplevel window (by errronously someWidget->wID() or something)
But you never call some someWidget->show() so it never appears as a separate window it's in a half bodged state - which happens to have no effect on X11.

If you can run this in gdb and breaking on QPlatformWindow::QPlatformWindow then tracking back to where in your client code you're doing this. You should only have this once per actual window, if you get more, trace it back and find where we have the bug in the client / used library.

[mara004]

If you can run this in gdb and breaking on QPlatformWindow::QPlatformWindow then tracking back to where in your client code you're doing this. You should only have this once per actual window, if you get more, trace it back and find where we have the bug in the client / used library.

The approach will need to be a bit different since this is Python and not C++ code, but maybe we can use pdb or something similar to find the problematic calls.

[davidedmundson]

We certainly need the breakpoint on the C++ side so we can break QPlatformWindow does something.

https://askubuntu.com/questions/175412/how-can-i-get-python-stack-trace-information-using-gdb
implies it's possible to get the python backtrace from within GDB.

If you do find something, please paste here and I can try and help out

[mara004]

Hmm, interesting. I'd not known that. I tried to run the script in gdb with gdb --args python3 example_auto_nodes.py but py-bt seems not to work, output only says unable to locate python frame, although I followed the instructions from the askubuntu answers

[mara004]

Ah, it suddenly worked. I can now get python backtraces that look like this:

Thread 1 "python3" hit Breakpoint 1, 0x00007fffd2f13a50 in QPlatformWindow::QPlatformWindow(QWindow*) ()
   from /usr/local/lib/python3.8/dist-packages/PySide2/Qt/lib/libQt5Gui.so.5
(gdb) py-bt
Traceback (most recent call first):
  <built-in method setWidget of NodeLineEdit object at remote 0x7fffc0785a80>
  File "/home/manuel/Downloads/builds/NodeGraphQt/NodeGraphQt/widgets/node_widgets.py", line 188, in set_custom_widget
    self.setWidget(group)
  File "/home/manuel/Downloads/builds/NodeGraphQt/NodeGraphQt/widgets/node_widgets.py", line 303, in __init__
    self.set_custom_widget(ledit)
  File "/home/manuel/Downloads/builds/NodeGraphQt/NodeGraphQt/base/node.py", line 962, in add_text_input
    Delete output port.
  File "/home/manuel/Downloads/builds/NodeGraphQt/example_auto_nodes/viewer_nodes.py", line 11, in __init__
    self.add_text_input('data', 'Data Viewer', multi_line=True)
  File "/home/manuel/Downloads/builds/NodeGraphQt/NodeGraphQt/base/graph.py", line 1279, in _deserialize
    node = NodeCls()
  File "/home/manuel/Downloads/builds/NodeGraphQt/example_auto_nodes/subgraph_nodes.py", line 126, in create_from_file
    children = self.graph._deserialize(children_data, set_parent=False)
  File "/home/manuel/Downloads/builds/NodeGraphQt/example_auto_nodes/subgraph_nodes.py", line 96, in set_graph
    self.create_from_file()
  File "/home/manuel/Downloads/builds/NodeGraphQt/NodeGraphQt/base/graph.py", line 1209, in add_node
    root_node = self.root_node()
  File "/home/manuel/Downloads/builds/NodeGraphQt/NodeGraphQt/base/graph.py", line 1292, in _deserialize
    self.add_node(node, n_data.get('pos'), unique_name=set_parent)
  File "/home/manuel/Downloads/builds/NodeGraphQt/NodeGraphQt/base/graph.py", line 1442, in import_session
    self._deserialize(layout_data)
  File "/home/manuel/Downloads/builds/NodeGraphQt/NodeGraphQt/base/graph.py", line 1418, in load_session
    self.import_session(file_path)
  File "example_auto_nodes.py", line 82, in <module>
    graph.load_session(r'example_auto_nodes/networks/example_SubGraph.json')

[mara004]

@davidedmundson Could you please explain more detailed how I should run the program and navigate in gdb? I'd barely used it so far.

[davidedmundson]

Sure, some of these backtraces will show something legit happening, we're looking for the one that isn't, so we'll need to collect them all.

You've clearly run and got one trace.

Next steps are to type "continue" (the program will freeze again) and then again do "py-bit"
Then we keep doing that until we've reproduced the bug.

Can you include "bt" as well, for the c++ backtrace

One trick is you can run "set logging on" in gdb before run and it will save everything to a single file, which will save trying to scroll through a terminal.

[davidedmundson]

This backtrace shows that void QGraphicsProxyWidget::setWidget(QWidget *widget) ended up creating a complete new window, that is unexpected.

Hopefully the C++ side of the trace will tell us something

[mara004]

Next steps are to type "continue" (the program will freeze again) and then again do "py-bit"
Then we keep doing that until we've reproduced the bug.

Okay, that's basically what I started to do, but this will be very tedious. There are about 200 such breakpoints. Could we automate repeatedly running continue and py-bt somehow? If it was for X I'd probably use something like AutoKeyQt, but that doesn't work for Wayland I think.

[davidedmundson]

This should really only happen once when you open the window, and then again if you open a menu or something.

Lets get the C++ side of this one trace that we do have and try to sort that out - hopefully the rest will be the same root cause.