[xcb] Unknown sequence number while processing queue - XInitThreads

[yamadapc]

Hey there, I've been working on Haskell bindings as well as extensions to this library on https://github.com/beijaflor-io/haskell-libui

Upon running a FRP click-to-resize multi-threaded example, I get the following exception on Linux:

[xcb] Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not been called
[xcb] Aborting, sorry about that.
libui-reactive-banana-click-to-resize: xcb_io.c:274: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.

Indeed XInitThreads does't seem to be called in libui/unix/main.c in uiInit. Adding it fixed the problem for me.

Here's a patch to do it: https://gist.github.com/c6f4f0bc5fa5aec33bb83ac2d4c53f3d

I'm unfamiliar with the build system here and had to re-instruct Haskell's build system to work anyway, so I don't need any other changes but this for the bindings to work, but I'm open to submitting a PR with a fix to discuss, understand and fix the issue.

[yamadapc]

Note

Just to be clear, even though there're multiple threads in Haskell-land, that code is using uiQueueMain on every UI update, and even adding a lock so uiQueueMain was only called once at a time (which seemed closer to the C++ example in this repository, though I can't really tell) didn't fix the issue.

EDIT - So I don't spam 164 people again

Actually, there're more details... Maybe it's exclusive to the Haskell bindings. If you never get this bug I should just close this issue. I forgot my version of uiMain was ticking the UI loop from Haskell, so it could be interrupted by other threads.

Sorry for not writing everything in one comment.

[andlabs]

There is no way to pin uiMain() to a thread in Haskell? If there is, you could hack around it using uiQueueMain().