https://bugs.kde.org/show_bug.cgi?id=523545
Bug ID: 523545
Summary: Switching users from a KDE Plasma session does not
work when using GDM as the display manager.
Classification: Plasma
Product: plasmashell
Version First 6.7.3
Reported In:
Platform: Arch Linux
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: Session Management
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Target Milestone: 1.0
Created attachment 194636
--> https://bugs.kde.org/attachment.cgi?id=194636&action=edit
Diagnostic outputs for GDM user switching issue.
# User switching does not work in Plasma Wayland session when using GDM 50.1
## Summary
Switching users from a KDE Plasma session does not work when using GDM as the
display manager.
When attempting to switch users, Plasma logs:
```text
Cannot switch user: XDG_SEAT_PATH not set
```
The session itself appears to be correctly registered with systemd-logind, but
Plasma's current implementation appears to depend on a display manager D-Bus
interface that is not provided by modern GDM.
## Environment
* Distribution: Arch Linux x86_64
* KDE Plasma: 6.7.3
* GDM: 50.1
* systemd: 261 (261.1-1-arch)
* Session type: Wayland
* Window manager: KWin (Wayland)
## Steps to reproduce
1. Use GDM as the display manager.
2. Log into a KDE Plasma Wayland session.
3. Open the Plasma session menu.
4. Select "Switch User".
## Expected behavior
The system should switch to the login greeter and allow another user to log in.
## Actual behavior
Nothing happens.
The following warning is logged:
```text
Cannot switch user: XDG_SEAT_PATH not set
```
## Investigation
The warning originates from `libkworkspace` in
`SessionManagement::switchUser()`:
```cpp
if (!qEnvironmentVariableIsSet("XDG_SEAT_PATH")) {
qCWarning(LIBKWORKSPACE_DEBUG) << "Cannot switch user: XDG_SEAT_PATH not
set";
return;
}
```
The function then uses `XDG_SEAT_PATH` to call:
```text
org.freedesktop.DisplayManager.Seat.SwitchToGreeter
```
However, on this system:
### systemd-logind correctly knows the session seat
`loginctl` reports the session seat as:
```text
seat0
```
so the session appears to be correctly registered with logind.
### The expected DisplayManager D-Bus service is not available
Running:
```sh
busctl tree org.freedesktop.DisplayManager
```
returns:
```text
Failed to introspect object / of service org.freedesktop.DisplayManager: The
name is not activatable
No objects discovered.
```
### GDM exposes a different D-Bus service
Running:
```sh
busctl --system list | grep -i gdm
```
shows:
```text
org.gnome.DisplayManager
```
The exported object tree is:
```text
/org/gnome/DisplayManager
├── Displays
├── LocalDisplayFactory
├── Manager
└── RemoteDisplayFactory
```
The manager interface exposes:
```text
org.gnome.DisplayManager.Manager
OpenSession
OpenReauthenticationChannel
RegisterDisplay
RegisterSession
```
but there does not appear to be a `SwitchToGreeter` method or a seat object
equivalent to:
```text
org.freedesktop.DisplayManager.Seat
```
## Question
Is Plasma's current user switching implementation relying on an obsolete
display manager interface?
Should user switching be implemented through systemd-logind or GDM's current
D-Bus API instead of relying on `XDG_SEAT_PATH` and
`org.freedesktop.DisplayManager.Seat.SwitchToGreeter`?
I am happy to provide additional logs or test patches if need
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