Find a GPU driver on hosts without /run/opengl-driver

[donk-shopify]

Builds on your nix-flake branch (tobi#26 on tobi/disktree), so it's aimed at your branch rather than main — merge it and tobi#26 picks it up.

The symptom

On Arch (Omarchy/Hyprland, Radeon 890M, Wayland) nix run github:pcasaretto/disktree/nix-flake never draws a window:

thread 'main' panicked at crates/disktree-app/src/main.rs:140:18:
open the disktree window: Failed to create surface: Failed to create surface for any enabled backend: {}

The empty brace set is the tell: wgpu isn't reporting a backend that failed, it's reporting that it had no backend to try.

Why

The patchelf --add-rpath in postFixup is doing its job — dlopen("libvulkan.so.1") resolves to the closure's vulkan-loader. The loader then has to find a driver, which it discovers through an ICD manifest, and a Nix-built loader looks for those under /run/opengl-driver/share/vulkan/icd.d. Only NixOS creates that path. With VK_LOADER_DEBUG=error:

[Vulkan Loader] ERROR | DRIVER: vkCreateInstance: Found no drivers!

The host's own Mesa can't be borrowed to fix it. /usr/share/vulkan/icd.d/radeon_icd.json names its library relatively (libvulkan_radeon.so) and Nix's glibc doesn't read /etc/ld.so.cache, so it isn't found; rewriting the manifest with an absolute path then fails on libLLVM.so.22.1 for the same reason; and LD_LIBRARY_PATH=/usr/lib shadows glibc itself and kills the process outright (/usr/lib/libc.so.6: undefined symbol: __pointer_chk_guard, version GLIBC_PRIVATE). The driver has to come from the closure.

The change

postFixup gains two lines' worth of work:

• addDriverRunpath $out/bin/disktree — appends /run/opengl-driver/lib, so NixOS and proprietary-NVIDIA hosts keep working exactly as before.
• a wrapProgram offering this closure's Mesa as an additional driver source: VK_ADD_DRIVER_FILES for Vulkan, __EGL_VENDOR_LIBRARY_DIRS and LIBGL_DRIVERS_PATH for GL.

Everything is --suffix, and VK_ADD_DRIVER_FILES (rather than VK_DRIVER_FILES) is searched after the ICDs the loader discovers for itself — so a working system driver still wins and NixOS behaviour is unchanged. addDriverRunpath and makeWrapper join nativeBuildInputs under the existing isLinux guard.

A machine driven by the proprietary NVIDIA stack is still out of scope — Mesa cannot drive that card, so those hosts need nixGL. The comment above postFixup says so.

Verified

nix build .#disktree on the pinned nixpkgs, then both $out/bin/disktree and nix run .#disktree open the window on Arch with no environment variables set. readelf -d on .disktree-wrapped shows /run/opengl-driver/lib first in the RUNPATH, and the desktop entry's Exec still resolves to the wrapper rather than the inner binary.

Two smaller things I left alone

• fontconfig, freetype, libx11, libxcursor, libxi and libxrandr appear in neither NEEDED nor the RUNPATH of the built binary (which wants only libxcb, libxkbcommon, libxkbcommon-x11, libgcc_s, libm, libc), and the only dlopened names in the binary are libvulkan.so.1, libEGL.so.1 and libwayland-client.so.0. They may well be build-time pkg-config needs, but if they aren't, dropping them would stop the input list implying an X11 story the RUNPATH doesn't support.
• $out/bin/disktree --help exits 0 without a display, so an installCheckPhase one-liner would catch a broken wrapper in future.