swww-daemon crashes after a few image switching with error `RustixIoErrno(Os { code: 9, kind: Uncategorized, message: "Bad file descriptor" })`

[Aeldit]

BEFORE ANYTHING, PLEASE ENSURE YOU ARE TESTING IN THE LATEST GIT VERSION.
I tested using the latest git and in debug mode

Describe the bug
The swww-daemon crashes when changing the background a few times in a row when (I suspect) a monitor is detected and prepared to render to, but when the rendering starts it is no longer detected (the monitor).

To Reproduce
Run the following script multiple times in a row in Hyprland with the command cargo run -- path/to/image monitor ~/.config/hypr/hyprland.conf (where monitor is the name of your monitor, which for me is either eDP-1 for the integrated screen or HDMI-1-A for the external screen):

The hyprland.conf file is read and written to, but nothing is changed

The script uses swww-git, this is just the compiled file that you put in you /bin/ so it doesn't overwrite the swww you are already using

// File: src/main.rs
use std::{env, fs, io};

use hyprland::{
    dispatch::{Dispatch, DispatchType},
    keyword::Keyword,
};

#[allow(unused_must_use)]
fn main() -> io::Result<()> {
    let bg_path = env::args().nth(1).unwrap_or_default();
    let monitor = env::args().nth(2).unwrap_or_default();
    let hyprland_conf_path = env::args().nth(3).unwrap_or_default();

    let hyprland_contents = fs::read_to_string(&hyprland_conf_path)?;

    fs::write(&hyprland_conf_path, hyprland_contents)?;
    Keyword::set("monitor", format!("{monitor},disable"));
    Dispatch::call(DispatchType::Exec(&format!("swww-git img {bg_path}")));

    Ok(())
}

# File: Cargo.toml
[package]
name = "repr-bug"
version = "1.0.0"
edition = "2021"

[dependencies]
hyprland = "0.4.0-beta.2"

In your hyprland config, monitor must be enabled like so monitor = eDP-1, 1920x1080@60, auto, 1 (for me it is eDP-1 but for you it is the same as the command used to run the repoduce script)

Screenshots
Not really applicable here, it just crashes

Desktop (please complete the following information):

• Desktop Environment: Hyprland
• Monitor setup: Laptop monitor + external monitor connected via HDMI (the laptop monitor is disabled by a script when the external one is connected)

Additional context
I have a script that changes my theme from dark to light and the other way, and it happens to write to my hyprland config files. In these files, my laptop's integrated monitor is set to enabled, but I have another script that turns it off when an external monitor is connected. When the file is written to, the laptop's monitor is enabled and almost instantly disabled by my other script.
I suspect that when this happens, swww-daemon detects it and prepares it for image rendering, but then when it is disabled it is no longer present and swww-daemon crashes with the error code RustixIoErrno(Os { code: 9, kind: Uncategorized, message: "Bad file descriptor" })

Log when run in debug mode from latest git source:

         0ms [DEBUG] Created socket in "/run/user/1000/wayland-1-swww-daemon..sock"
         0ms [DEBUG] Finished setting up signal handlers
         0ms [DEBUG] available shm format: Xrbg
         0ms [DEBUG] available shm format: Xbgr
         0ms [INFO]  selected pixel format: Xbgr
         0ms [DEBUG] New output: 60
         0ms [DEBUG] Output 60 name: HDMI-A-1
         0ms [DEBUG] Output 60 description: HP Inc. HP V24e 1CR12816B3 (HDMI-A-1)
error: unexpected argument '--namespace' found

  tip: to pass '--namespace' as a value, use '-- --namespace'

Usage: swww img --outputs <OUTPUTS> --filter <FILTER> <IMAGE>

For more information, try '--help'.
     12448ms [DEBUG] output Some("HDMI-A-1") - drawing: image: /home/aeldit/Dev/Rust/Reproduce/bg.jpg
     12448ms [INFO]  BumpPool with: 1 buffers. Size: 8100Kb
     12481ms [INFO]  BumpPool with: 2 buffers. Size: 16200Kb
     12494ms [DEBUG] Output 9: Surface Enter
     15548ms [INFO]  BumpPool with: 3 buffers. Size: 24300Kb
     15582ms [INFO]  BumpPool with: 4 buffers. Size: 32400Kb
     15615ms [INFO]  BumpPool with: 5 buffers. Size: 40500Kb
     15648ms [INFO]  BumpPool with: 6 buffers. Size: 48600Kb
     15681ms [INFO]  BumpPool with: 7 buffers. Size: 56700Kb
     15715ms [INFO]  BumpPool with: 8 buffers. Size: 64800Kb
     15748ms [INFO]  BumpPool with: 9 buffers. Size: 72900Kb
     15781ms [INFO]  BumpPool with: 10 buffers. Size: 81000Kb
     15815ms [INFO]  BumpPool with: 11 buffers. Size: 89100Kb
     15848ms [INFO]  BumpPool with: 12 buffers. Size: 97200Kb
     15881ms [INFO]  BumpPool with: 13 buffers. Size: 105300Kb
     15915ms [INFO]  BumpPool with: 14 buffers. Size: 113400Kb
     15931ms [DEBUG] New output: 106
     15948ms [INFO]  BumpPool with: 15 buffers. Size: 121500Kb
     15982ms [INFO]  BumpPool with: 16 buffers. Size: 129600Kb
     16015ms [INFO]  BumpPool with: 17 buffers. Size: 137700Kb
     16048ms [INFO]  BumpPool with: 18 buffers. Size: 145800Kb
     16081ms [INFO]  BumpPool with: 19 buffers. Size: 153900Kb
     16115ms [INFO]  BumpPool with: 20 buffers. Size: 162000Kb
     16148ms [INFO]  BumpPool with: 21 buffers. Size: 170100Kb
     16182ms [INFO]  BumpPool with: 22 buffers. Size: 178200Kb
     16215ms [INFO]  BumpPool with: 23 buffers. Size: 186300Kb
     16248ms [INFO]  BumpPool with: 24 buffers. Size: 194400Kb
     16281ms [INFO]  BumpPool with: 25 buffers. Size: 202500Kb
     16296ms [DEBUG] Destroyed output ? - ?
     16697ms [ERROR] We failed to find wayland buffer with id: 11. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 18. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 17. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 20. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 22. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 24. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 26. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 28. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 30. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 32. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 34. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 36. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 38. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 40. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 50. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 52. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 54. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 56. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 58. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 60. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 62. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 64. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 66. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 68. This should be impossible.
     16697ms [ERROR] We failed to find wayland buffer with id: 70. This should be impossible.
     21358ms [DEBUG] New output: 107
     21701ms [DEBUG] Destroyed output ? - ?
     25193ms [DEBUG] New output: 108
     25552ms [DEBUG] Destroyed output ? - ?
     29940ms [DEBUG] New output: 109
     30303ms [DEBUG] Destroyed output ? - ?
     32255ms [DEBUG] New output: 110
     32618ms [DEBUG] Destroyed output ? - ?
     34248ms [DEBUG] New output: 111
     34248ms [DEBUG] Destroyed output ? - ?
     34248ms [DEBUG] Destroyed output HDMI-A-1 - HP Inc. HP V24e 1CR12816B3 (HDMI-A-1)
     34248ms [INFO]  Removed socket at "/run/user/1000/wayland-1-swww-daemon..sock"
Error: RustixIoErrno(Os { code: 9, kind: Uncategorized, message: "Bad file descriptor" })

[Aeldit]

From what I can see the only place that could return this error is the

daemon.backend.flush()?;

in daemon/src/main.rs at line 750

When we follow the calls, it leads to

net::sendmsg(wayland_fd, &[iov], &mut control, net::SendFlags::NOSIGNAL)
            .map_err(Error::RustixIoErrno)?;

in wire.rs at line 482 (from the waybackend crate)

EDIT: I have confirmed that the crash happens precisely at this line (simply put a println!() before and after and only the first prints when a crash happens).
I believe the solution would be to check for available outputs just before flushing, to prevent removed ones from being rendered to (causing the crash)

[LGFae]

Thanks for finding where the error is! Unfortunately, I don't think the outputs can be the problem. The EBADF error (bad file descriptor) is returned when the socket is not a valid descriptor. This would mean the wayland file descriptor is wrong. Which is bizarre, because it would imply the wayland file descriptor just... arbitrarily changed out of nowhere. I need to investigate it further.