wrap-qt-apps-hooks leads to very slow wrapper scripts

[matklad]

Describe the bug

Wrapper scripts for plasma apps such as krunner or plasma-emojier are slow. For example, executing just the wrapper for plasma-emojier takes 100ms on my fast machine.

The reason for slowness is that the wrapper itself repeatedly adds various paths:

$ rg -F "QT_PLUGIN_PATH='" $(which plasma-emojier) | wc -l
44

This leads to quadratic behavior. Profiling with perf shows that the bulk of the time is spend in string manipulation routines.

The relevant logic is here:

nixpkgs/pkgs/development/libraries/qt-6/hooks/wrap-qt-apps-hook.sh

Line 43 in 07ab967

makeWrapper "$original" "$wrapper" "${qtWrapperArgs[@]}" "$@"

Essentially, we are calling

wrapProgram program \
  --prefix QT_PLUGIN_PATH ...
  --prefix QT_PLUGIN_PATH ...
  --prefix QT_PLUGIN_PATH ...

and makeWrapper then faithfully translates each --prefix, instead of pre-computing thee concatenation.

Steps To Reproduce

# Strip the `exec` line
$ <$(which plasma-emojier) head -n -1 > wrapper.sh && chmod a+x ./wrapper.sh

# Time the execution of the wrapper itself
$ time ./wrapper.sh
real	0m0.117s  😱
user	0m0.115s
sys	0m0.002s

Expected behavior

Wrapper itself shouldn't take more than a handful of milliseconds. It seems achievable if I manually replace incremental quadratic addition to environmental variables with setting them once.

Not sure what's the best fix here, some options I see are:

• fix wrap-qt-apps-hook to pre-concatenate variables and pass just a single --prefix argument per variable to wrapProgram
• make wrapProgram itself smarter and teach it to collate identical prefixes
• fix wrap-qt-apps-hook to use --set rather then --prefix

As I don't know bash, I don't plan on working on the fix myself

Notify maintainers

cc @ttuegel

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[matklad@Ishmael:~/tmp]$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 6.2.10, NixOS, 23.05 (Stoat), 23.05.20230411.3fb8eed`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.13.3`
 - channels(matklad): `""`
 - nixpkgs: `/nix/store/k9vs66va04886gl91m6lpcwaymqiihqx-9lkm36ac7hfl7f9h2z9dj00mdh30mjhi-source`

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/plasma-emojier-is-very-slow-makeqtwrapper-is-accidentally-quadratic/27160/3