napari: Bug: Segfaults appearing with "QObject::setParent: Cannot set parent, new parent is in a different thread"

🐛 Bug

In the last couple days, I’ve seen this segfault a couple times on main. I’m not quite sure what’s causing it yet, but it tends to happen with mouse movements, perhaps on the canvas, maybe on sliders, not sure… but it crashes napari out to the console

WARNING: QObject::setParent: Cannot set parent, new parent is in a different thread
10:19:54 WARNING QObject::setParent: Cannot set parent, new parent is in a different thread
WARNING: QObject::setParent: Cannot set parent, new parent is in a different thread
10:19:54 WARNING QObject::setParent: Cannot set parent, new parent is in a different thread
WARNING: QObject::setParent: Cannot set parent, new parent is in a different thread
10:19:54 WARNING QObject::setParent: Cannot set parent, new parent is in a different thread
[1]    19897 segmentation fault  napari

About this issue

Original URL
State: closed
Created 2 years ago
Comments: 20 (15 by maintainers)

Most upvoted comments

Hey, I’m facing this issue now and was wondering if there has been any update already. I see #5195 was working on it. I’m also trying to update a layer outside the main thread.

I don’t think exact issue ever made into an official release as it should have been fixed between the release of v0.4.16 and v0.4.17 (first in https://github.com/napari/napari/pull/4983 then in https://github.com/napari/napari/pull/4985).

The goal of https://github.com/napari/napari/pull/5195 was to add some tooling to better detect when this happens, but it’s only available on main (i.e. not on v0.4.17). If you’re developing against main, then you can set NAPARI_ENSURE_PLUGIN_MAIN_THREAD=1 to get more info about where this is happening in your code.

Generally, napari is not thread safe, so I would only update a layer on the main thread unless some napari documentation says otherwise. If you can directly depend on superqt (the full napari viewer does), then you can use the ensure_main_thread decorator to ensure that a particular function/callback executes on the qt main thread.

If you still have problems, you might want to create a separate issue with the details.

andy-sweet on Dec 13, 2022

yeah, that seems like a good plan to me. “update status bar” is a strictly gui related thing, so that should probably have been on the _qt side of things all along

tlambert03 on Aug 25, 2022