Rustdesk not working when connected screen is turned off

[hatl]

Bug Description

When connecting to a Linux PC with which has a monitor attached that's powered-off, I get: "Remote Error" "No Display"

How to Reproduce

Start rustdesk.
Remove power cable from monitor.
Try to connect --> error

Expected Behavior

Connection is working

Operating system(s) on local side and remote side

Ubuntu 23.10 -> Ubuntu 23.10

RustDesk Version(s) on local side and remote side

1.2.3 -> 1.2.3

Screenshots

Error message:

Working connection via x11vnc:

Additional Context

The corresponding X11 screen exists and should be used by rustdesk.

xrandr output:

Screen 0: minimum 8 x 8, current 3840 x 2160, maximum 32767 x 32767
DVI-D-0 disconnected (normal left inverted right x axis y axis)
HDMI-0 disconnected primary (normal left inverted right x axis y axis)
DP-0 disconnected (normal left inverted right x axis y axis)

---

The same use case works, when using x11vnc - while using the existing screen :0 (see screenshot)
You can even see that rustdesk is running when connecting via vnc

[rustdesk]

This is not vnc

[rustdesk]

#7032

[hatl]

I know that this is not VNC - the hint was just for reference.

This is not a duplicate of #4296
The main difference is, that in 4296 the screen won't be updated.

This is also not a headless system as described in #7032
There is a monitor connected initially and is powered-off afterwards.
So the whole desktop environment + X11 screen are running.

In my case, the screen is updated because the X11 screen still exists and can be interacted with.
For some reason rustdesk doesn't find/use the existing screen.

[hatl]

I did some analysis:
seems like the screen should be part of the iterator:

rustdesk/libs/scrap/src/x11/iter.rs

Line 20 in c97cc15

let mut outer = xcb_setup_roots_iterator(server.setup());

I've created a minimum example to show that the screen is actually available:

#include <stdio.h>
#include <stdlib.h>
#include <xcb/xcb.h>

int main(void) {
    /* Establish a connection to the X server */
    xcb_connection_t *connection = xcb_connect(NULL, NULL);
    if (xcb_connection_has_error(connection)) {
        fprintf(stderr, "Error: Cannot open display\n");
        exit(1);
    }

    /* Retrieve the setup data */
    const xcb_setup_t *setup = xcb_get_setup(connection);
    if (!setup) {
        fprintf(stderr, "Error: Cannot retrieve setup information\n");
        xcb_disconnect(connection);
        exit(1);
    }

    /* Use xcb_setup_roots_iterator to iterate through screens */
    xcb_screen_iterator_t iter = xcb_setup_roots_iterator(setup);
    int screen_num = 0;

    for (; iter.rem; xcb_screen_next(&iter), screen_num++) {
        xcb_screen_t *screen = iter.data;
        printf("Screen %d: %dx%d\n", screen_num, screen->width_in_pixels, screen->height_in_pixels);
    }

    /* Clean up and close the connection */
    xcb_disconnect(connection);
    return 0;
}

compile using:

gcc -o list_screens list_screens.c $(pkg-config --cflags --libs xcb)

this prints:
Screen 0: 3840x2160

---

Maybe the issue is somewhere here?

rustdesk/libs/scrap/src/common/x11.rs

Lines 89 to 93 in c97cc15

	let mut displays = x11::Server::displays(server);
	let mut best = displays.next();
	if best.as_ref().map(|x| x.is_default()) == Some(false) {
	best = displays.find(|x| x.is_default()).or(best);
	}

[aki42]

Thanks a lot for your bug report - otherwise I never would have thought that the (sort of) solution to this problem really can be to turn on the monitor of the machine I want to control remotely. Of course this is no real solution because that monitor now consumes power without any use than to avoid the "Remote Error"/"No Display" errors. But the connection works at least. Really astonishing this bug got closed though you even provided demo code and a hint regarding where the code looking for displays might fail. 🤷

[hatl]

I was also quite surprised.
I still think this should be easily fixable (without turning the monitor on).

[cubicYYY]

#6682