Cannot run GUI applcations when started under systemd except when run under strace -f too

[LordMZTE]

• Version: 1.7.8+wayland1
• OS: NixOS
• config.rasi (note: this is run through a template engine first): https://git.mzte.de/LordMZTE/dotfiles/src/commit/2ed52f6c52dc258cb91444ac394b89f05cba81b5/.config/rofi/config.rasi.cgt

I'm having an incredibly strange issue: when I run rofi under systemd using
systemd-run --user -- rofi -show combi, I cannot start GUI applications. The log output from rofi
does never include anything from the launched application but also doesn't show any error starting
it, here I've set G_MESSAGES_DEBUG=all and attempted to run nheko, but any (both GTK and Qt) GUI I
tried exhibited similar behavior:

<snip>
Feb 19 12:17:01 mzte-fw rofi[4209]: Theme entry: #element-text alternate.normal property background-image unset.
Feb 19 12:17:01 mzte-fw rofi[4209]: Theme entry: #element-text alternate.normal property text-outline unset.
Feb 19 12:17:01 mzte-fw rofi[4209]: Theme entry: #window  property border-color unset.
Feb 19 12:17:01 mzte-fw rofi[4209]: 2.602955 (0.006422): ../source/wayland/view.c:wayland_rofi_view_update:356 widgets
Feb 19 12:17:01 mzte-fw rofi[4209]: Found property: 'delay' on 'timeout', but type Integer does not match expected type Double.
Feb 19 12:17:01 mzte-fw rofi[4209]: Parsed command: |nheko %u| into |nheko |.
Feb 19 12:17:01 mzte-fw rofi[4209]: Teardown
Feb 19 12:17:01 mzte-fw rofi[4209]: Cleanup.
Feb 19 12:17:01 mzte-fw rofi[4209]: Entry filename output: '/home/lordmzte/.local/cache/rofi-entry-history.txt'
Feb 19 12:17:01 mzte-fw rofi[4209]: 2.773887 (0.000000): Stopped

Now, I decided to debug this using strace to find out what's happening using
systemd-run --user -EG_MESSAGES_DEBUG=all -- strace -o ~/Downloads/trace.log rofi -show combi.
Same behavior, and also no (interesting) calls to execve in the trace, so I figured rofi must be
spawning the child on a different thread and included the -f option for strace, too. For some
reason, though, now rofi started working! The program will run if and only if rofi is run with
strace -f!

One more thought I had is that this would somehow cause the environment variables to be different,
but by running a command to save the output of env in both instances of rofi to a file and then
diffing it, I found out that, apart from some variables containing unique IDs that systemd sets,
everything is identical.

I also doubt anything with the systemd environment is fundamentally wrong, as rofi's GUI shows just fine and any other GUI applications I have tried to start also work when run directly with systemd-run.

[DaveDavenport]

I think its because rofi binary exits, then systemd-run cleans up that session, including the spawned program.

if you keep the service around (-r) it works, but this then will stay around.

I don't use systemd-run, so not sure what the solution is? maybe to make rofi use systemd-run to launch the client?

[LordMZTE]

That's it! I managed to find a pretty good solution by also passing -pType=forking to systemd-run. This has the added bonus of the service only being considered as started once something was launched.