ckb: currently broken due to recent Qt5 changes

[kierdavis]

Issue description

ckb currently fails on startup with the following message:

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

Reinstalling the application may fix this problem.

I believe it's looking for libqxcb.so, which is currently located at /nix/store/kgvmhibzszc84p2ddzbhhsw54pdb8awh-qtbase-5.9.2-bin/lib/qt-5.9/plugins/platforms/libqxcb.so.

git bisect revealed that this problem has been present since commit 6fa4178.

I've not been keeping up with the recent changes to Qt5 - what's the most canonical way to fix this and allow ckb to find the xcb plugin again?

Steps to reproduce

cd <nixpkgs>
nix-build -A ckb
result/bin/ckb

Technical details

• System: 17.03.1862.ea06690f1f (Gorilla)
• Nix version: 1.11.14
• Nixpkgs version: master as of the time of writing (b304695)
• Sandboxing enabled: no

[NickHu]

This seems to be the same issue as #30713

[kierdavis]

cc Qt5.9 maintainers: @qknight @ttuegel @periklis

[lukateras]

Can't reproduce:

• System: 18.03pre118631.66b63d2f5a (Impala)
• Nix version: 1.11.15
• Nixpkgs version: e2edcca
• Sandboxing enabled: yes

[GallagherCommaJack]

Reproduced with slightly different error:

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

Available platform plugins are: minimal, offscreen, vnc, xcb, wayland-org.kde.kwin.qpa.

Reinstalling the application may fix this problem.

• System: 17.09.2075.ac35504065 (Hummingbird)
• Nix version: 1.11.15
• Sandboxing enabled: no

(I would include nixpkgs version, but I don't actually know how to find that out)

[lukateras]

@GallagherCommaJack Nixpkgs version is important in this case. Could you please run:

nix-shell -p nix-info --run 'nix-info -m'

[GallagherCommaJack]

Uh, this is the output from that command, both with and without sudo:

 - system: `"x86_64-linux"`
 - host os: `Linux 4.9.61, NixOS, 17.09.2075.ac35504065 (Hummingbird)`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 1.11.15`
 - channels(jack): `"unstable-18.03pre120398.aeff3080d0"`
 - channels(root): `"nixos-17.09.2075.ac35504065"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs`

EDIT:
In the nixpkgs directory there's a file called svn-revision. Contents:

.2075.ac35504065

[GallagherCommaJack]

Distressingly enough, this wasn't fixed by rolling back to a previous configuration built on Nov 1 when I wasn't having this problem.

[kierdavis]

After finally getting round to upgrading to 17.09, I can't reproduce anymore.

NixOS version: 17.09.2075.ac35504065 (Hummingbird)
Nix version: 1.11.15
nixpkgs version: ac35504

Output of nix-info -m:

- system: `"x86_64-linux"`
- host os: `Linux 4.9.61, NixOS, 17.09.2075.ac35504065 (Hummingbird)`
- multi-user?: `yes`
- sandbox: `no`
- version: `nix-env (Nix) 1.11.15`
- channels(root): `"nixos-17.09.2075.ac35504065"`
- channels(kier): `""`
- nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs`

[kierdavis]

I think I may have a lead.

I currently have (at least) two instances of ckb in my store: one built from nixpkgs ac35504 as part of my NixOS build, and one built from nixpkgs master (963204e) just now. The former works fine, while the latter exhibits this issue. Comparing the contents of the store paths shows that the working one contains a file named /nix/store/.../nix-support/propagated-native-build-inputs, which contains a reference to /nix/store/i1mgik5x2f8akjwhjcd0r62259s89592-qtbase-5.9.1-bin. This is the package that contains libqxcb.so. The version of ckb that exhibits the missing plugin problem lacks this file, so I think it's very likely that this issue is caused by qtbase-bin not being recorded in the nix-support/propagated-native-build-inputs file.

The contents of <nixpkgs>/pkgs/tools/misc/ckb/default.nix are unchanged between these two versions of nixpkgs, so it seems this issue is due to a change elsewhere in nixpkgs. Now running a git bisect...

[orivej]

nix-build -A ckb
result/bin/ckb

This may work, but currently it is not supported for Qt applications. Also the unfortunate best practice on NixOS is not to install Qt into the user profile at all, so do not try nix-env -iA ckb, or use this workaround: #30551 (comment)

[GallagherCommaJack]

Weirdly enough, I've got it working if I try to launch it from sudo su.
If I run sudo su, then nvim-qt (another command having the same issue), it opens.
If I run the same binary from my user account, it can't find xcb.

Removing user environment from the path also worked. So I guess that's the issue.

Is there a way to figure out what package is causing the issue?
EDIT: by trial and error, turns out I had installed Calibre as a user, which had pulled in QT and messed everything up.
Is there any reasonable way to install a QT application as a user and have it use root's QT? I guess I can just sudo nix-env -iA, but that feels dirty.

[orivej]

Likely there is a way to make ~/.nix-profile/lib/qt-5.9 disappear even when Qt is installed into the user env, but this may and likely will break Qt applications installed into the user env (unless system and user versions of Qt are exactly the same). Let's continue in #30551.