[Bug] [Flatpak] Stale KDSingleApplication lock file prevents launch after logout/login

[Aqua1ung]

Component: Nextcloud Desktop (Flatpak)

Version: 33.0.0daily (Flatpak)

OS: blendOS (Arch Linux), GNOME/Wayland

Description

After a logout/login cycle (not a reboot), the Nextcloud Desktop Flatpak client fails to start. The app immediately exits with status 255, logging "Already running, exiting...".

The cause is a stale lock file at:

~/.var/app/com.nextcloud.desktopclient.nextcloud/cache/tmp/kdsingleapp-dad-nextcloud.lock

KDSingleApplication creates this lock to enforce single-instance behavior, but it is not cleaned up when the session ends via logout. On the next login, the new instance finds the lock file, assumes another instance is running, and exits.

Steps to reproduce

1. Launch Nextcloud Desktop (Flatpak) — it runs normally
2. Log out of the GNOME session
3. Log back in
4. Nextcloud Desktop fails to start (autostart or manual launch)

Journal output

nextcloud.gui.application: Already running, exiting...
dbus-:1.2-com.nextcloudgmbh.Nextcloud@0.service: Main process exited, code=exited, status=255/EXCEPTION

Workaround

rm ~/.var/app/com.nextcloud.desktopclient.nextcloud/cache/tmp/kdsingleapp-dad-nextcloud.lock

Expected behavior

KDSingleApplication should detect stale locks — for example by using flock() (which auto-releases when the process dies) or by recording the PID in the lock file and checking if it's still alive. Alternatively, Nextcloud could register a cleanup handler for SIGTERM/session-end signals.

Additional context

This likely happens because the Flatpak sandbox receives SIGKILL during logout before the app has time to clean up, or because SIGTERM isn't properly forwarded through the Flatpak/systemd scope. The issue does not occur after a full reboot (since /run/user/ or $XDG_CACHE_HOME/tmp gets wiped).

A secondary issue is that config migration from data/Nextcloud to config/Nextcloud also fails repeatedly for sync log files that exist in both locations, producing log noise on every launch.

File against

• Primary: nextcloud/desktop
• Upstream (if triaged as library issue): KDAB/KDSingleApplication

[jubiman]

I have the same issue but on MacOS. I believe I have found the folder where it should be (/var/folders/st/.../T/com.nextcloud.desktopclient/), but I am unable to ls or rm -r it, even when super user (Operation not permitted). This means that the nextcloud app is entirely unable to launch.
For me it happened due to a crash

[i2h3]

@jubiman Are you referring to #9694? I opened KDAB/KDSingleApplication#115 which is a different root cause, though (too long path for UNIX socket).

[jubiman]

That is very possible. When I launch Nextcloud through the terminal I get the following output:

/Applications/Nextcloud.app/Contents/MacOS/Nextcloud 
KDSingleApplication: unable to make the primary instance listen on kdsingleapp-501-Nextcloud: QLocalServer::listen: Unknown error 1
nextcloud.sync.configfile: Default update channel is "stable"
nextcloud.gui.application: Already running, exiting...
kdsingleapplication.localsocket: Connection timed out

Though, it is running macOS 13.7.8, meaning it is entirely possible that the crash did not cause this issue, but that I am having trouble with #9694.

[RobinBouvier]

Hi, I have the same issue on flatpak version 33.0.2 using Bazzite with GNOME/Wayland.
The only difference is that it doesn't launch even after a fresh start of my computer, when I launch it via CLI, I get this :

me@bazzite:~$ flatpak run com.nextcloud.desktopclient.nextcloud
nextcloud.gui.application: Migrating old config from "/home/me/.var/app/com.nextcloud.desktopclient.nextcloud/data/Nextcloud" to "/home/me/.var/app/com.nextcloud.desktopclient.nextcloud/config/Nextcloud"
nextcloud.gui.application: Failed to move the old config directory to its new location ( "/home/me/.var/app/com.nextcloud.desktopclient.nextcloud/data/Nextcloud" to "/home/me/.var/app/com.nextcloud.desktopclient.nextcloud/config/Nextcloud" )
nextcloud.gui.application: Will move the individual files QList("Nextcloud_sync.log")
nextcloud.gui.application: Fallback move of  "Nextcloud_sync.log" also failed
nextcloud.gui.application: Already running, exiting...
kdsingleapplication.localsocket: Connection timed out

Also I can't exec
rm ~/.var/app/com.nextcloud.desktopclient.nextcloud/cache/tmp/kdsingleapp-dad-nextcloud.lock because the file does not exist :

rm ~/.var/app/com.nextcloud.desktopclient.nextcloud/cache/tmp/kdsingleapp-dad-nextcloud.lock
rm: impossible de supprimer '/home/me/.var/app/com.nextcloud.desktopclient.nextcloud/cache/tmp/kdsingleapp-dad-nextcloud.lock': Aucun fichier ou dossier de ce nom

It means I can't open the client at all

(After some research it appears my problems is mixed with #6201)

[Aqua1ung]

Hi, I have the same issue on flatpak version 33.0.2 using Bazzite with GNOME/Wayland. The only difference is that it doesn't launch even after a fresh start of my computer, when I launch it via CLI, I get this

You need to remove that file. Replace dad with your user name.

[RobinBouvier]

Hi, I have the same issue on flatpak version 33.0.2 using Bazzite with GNOME/Wayland. The only difference is that it doesn't launch even after a fresh start of my computer, when I launch it via CLI, I get this

You need to remove that file. Replace dad with your user name.

Yeah it worked ! Thank you
I'm sorry, I should look better at answers...