Kate fails to start unless QT_QPA_PLATFORM_PLUGIN_PATH is set

[clacke]

Issue description

Kate fails with:

This application failed to start because it could not find or load the Qt platform plugin "xcb" in "".

Steps to reproduce

$ nix-shell --pure -p kate --run "XAUTHORITY=$XAUTHORITY DISPLAY=$DISPLAY kate"
This application failed to start because it could not find or load the Qt platform plugin "xcb"
in "".

This works (kate starts, opens its main window, runs fine):

$ nix-shell --pure -p kate --run "XAUTHORITY=$XAUTHORITY DISPLAY=$DISPLAY QT_QPA_PLATFORM_PLUGIN_PATH=$(nix-build '<nixpkgs>' -A qt5.qtbase)/lib/qt-5.10/plugins/platforms kate"

qtbase should probably set that variable, and kate wrap it?

Technical details

• system: "x86_64-linux"
• host os: Linux 4.9.86, NixOS, 17.09.3254.967e40787c5 (Hummingbird)
• multi-user?: yes
• sandbox: no
• version: nix-env (Nix) 2.0.2
• channels(root): "nixpkgs-18.09pre140004.425ff431aba, nixos-17.09.3254.967e40787c5"
• channels(clacke): "nixpkgs-18.09pre140004.425ff431aba"
• nixpkgs: /home/clacke/.nix-defexpr/channels/nixpkgs

[clacke]

Alternatively, QLibraryInfo could be made to return the right path to platforms.

http://doc.qt.io/archives/qt-4.8/qlibraryinfo.html

[peterhoeg]

Works here without:

peter@dolores:~ $ echo $QT_QPA_PLATFORM_PLUGIN_PATH

peter@dolores:~ $ file $(which kate)
/run/current-system/sw/bin/kate: symbolic link to /nix/store/pm35jrp2nagsj0cw88svj8k9pn94jmi4-kate-18.04.1/bin/kate
peter@dolores:~ $ file /nix/store/pm35jrp2nagsj0cw88svj8k9pn94jmi4-kate-18.04.1/bin/kate                                                                                                                    ~
/nix/store/pm35jrp2nagsj0cw88svj8k9pn94jmi4-kate-18.04.1/bin/kate: ELF 64-bit LSB executable, x86-64, version 1 (GNU/Linux), dynamically linked, interpreter /nix/store/27x7pinqdsl9f3rpbm8bsszd9fhwq266-glibc-2.27/lib/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, not stripped

And kate then launches as normal.

[clacke]

Huh.

Did you run nix-shell with --pure to check that there isn't some other variable that makes it work? How is e.g. your LD_LIBRARY_PATH?

[clacke]

Same kate hash:

$ nix-shell -p kate --run 'file $(which kate)'
/nix/store/pm35jrp2nagsj0cw88svj8k9pn94jmi4-kate-18.04.1/bin/kate: ELF 64-bit [ . . . ]

[peterhoeg]

The issue is that the qt/kde applications don't work well (or at all) unless installed using environment.systemPackages. Sorry, I should have noticed that from the beginning.

[clacke]

What is it that makes them work as system packages? Must be some environment variable somewhere, right?

[peterhoeg]

No, it's not quite that simple unfortunately: #24256 (comment)

Your two options are:

1. Install in environment.systemPackages
2. Set the variable manually

[clacke]

I believe setting that variable in the wrapper will break the application under KDE.

Ok, that settles it then. Thanks for the clear information.

[clacke]

For any passers-by who happen to read this:

It's not necessary to install KDE and it's not necessary to install anything system-wide. Simply installing with nix-env -iA nixpkgs.kate makes kate work without issues.

It finds the appropriate things via your profile, it's only the environment method via nix-shell that doesn't work.

[peterhoeg]

While that is technically true, having Qt applications in both the system environment as well as user environments has its own problems in case they happen to be updated at different times. If you have application A in the system environment that depends on Qt 5.9.2 and application B that depends on Qt 5.9.3 just because they were installed at different times, depending on the load order one of them will stop working.

[clacke]

Right, that would be an issue. My comment is meant only from the perspective of "what do I need to do to make my program start?". To reframe it to be clear:

If you have a Qt program that won't start from platform plugin load failure when you nix-shell it, your three options are:

1. Install in environment.systemPackages.
2. Set QT_QPA_PLATFORM_PLUGIN_PATH.
3. Install in your profile with nix-env.

[clacke]

The my Qt vs system Qt issue sounds like a tough nut to crack.