NixOS works: document the full FHS recipe, drop the nix-ld dead end

First successful Linux field test (NixOS 26.05, Plasma 6, KiCad 10).
Getting there peeled three layers the error messages misattribute:
the FHS env itself (libgthread), then libs the xcb-cursor hint hides
(the xcb-util family, and libzstd for libQt6Core - in zstd.out, not
zstd's default output), and finally Plasma's QT_PLUGIN_PATH feeding
the system Qt's platform plugins to the wheel's bundled Qt, a
private-ABI mismatch that fails exactly like a missing library.

New docs/NIXOS.md carries the verified buildFHSEnv wrapper, the
steam-run quick test (now with env -u QT_PLUGIN_PATH), and a guide
for chasing future missing sonames. README upgrades Linux from
untested to works and links the doc. The in-plugin error no longer
suggests programs.nix-ld: the venv python is a Nix binary, so the
nix-ld shim never engages for it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -31,13 +31,13 @@ SWIG API. Requires KiCad **10.0.1+**.
 ## Setup (one-time)
 
 The plugin is developed and tested on **Windows**; **macOS works**
-(field-tested on KiCad 10 after a round of mac-specific fixes).
+(field-tested on KiCad 10 after a round of mac-specific fixes), and
-**Linux is expected to work but is untested so far** — the code and
+**Linux works** (field-tested on NixOS — the hardest Linux to run pip
-the dependency stack have been audited (KiCad builds the plugin a
+wheels on; mainstream FHS distributions should be no harder, reports
-private Python venv from `requirements.txt` on every platform, from
+welcome). KiCad builds the plugin a private Python venv from
-pre-built wheels only, no compiler needed), but nobody has run the
+`requirements.txt` on every platform, from pre-built wheels only, no
-plugin there yet. Reports welcome! Steps 1–4 are the same everywhere;
+compiler needed. Steps 1–4 are the same everywhere; OS specifics are
-OS specifics are spelled out per step and in *Platform notes* below.
+spelled out per step and in *Platform notes* below.
 
 1. **Enable the API server**: KiCad → Preferences → Plugins → check
    *Enable KiCad API*.
@@ -87,11 +87,23 @@ OS specifics are spelled out per step and in *Platform notes* below.
   Plot and dialog windows may open **behind** the KiCad window (they
   are raised best-effort) — check the Dock if nothing seems to appear
   after a solve.
-- **Linux** — **untested** (audited only, same caveat). The venv uses
+- **Linux** — **works** (field-tested on NixOS, KiCad 10; mainstream
+  distributions are audited but not yet field-tested). The venv uses
   the system Python (3.9+), so the stack matches your distribution.
   On **ARM64 (aarch64)** there are no pyamg wheels —
   `requirements.txt` skips pyamg there and the solver falls back to
   Jacobi-CG: same results, noticeably slower on large grids.
+  **NixOS**: **works** (field-tested on NixOS 26.05, Plasma 6). pip's
+  Linux wheels link against standard FHS library paths, which NixOS
+  does not provide — PySide6 fails with `libgthread-2.0.so.0: cannot
+  open shared object file`. The plugin cannot fix this from inside
+  its venv (KiCad installs wheels only); run KiCad inside an FHS
+  environment built with `buildFHSEnv`, and — on KDE Plasma — unset
+  `QT_PLUGIN_PATH`, which otherwise poisons the wheel's bundled Qt
+  with the system's Qt plugins. The tested wrapper (exact package
+  list incl. the non-obvious `zstd.out` and xcb-util family), a
+  `steam-run` quick test, and a debugging guide are in
+  [docs/NIXOS.md](docs/NIXOS.md).
 
 ## Usage
 

docs/NIXOS.md

@@ -0,0 +1,128 @@
+# Running Fill Resistance on NixOS
+
+KiCad builds the plugin a private venv from pip wheels
+(`requirements.txt`). Those Linux wheels — PySide6 in particular —
+`dlopen` system libraries at standard FHS paths (`/usr/lib`), which
+NixOS does not provide. Nothing inside the venv can fix that; KiCad
+must be **launched in an environment that supplies the libraries**.
+This page gives a verified, declarative setup (field-tested on
+NixOS 26.05, KiCad 10, Plasma 6 on Wayland) and maps each failure
+mode to its cause, because the error messages are misleading.
+
+## The three failure layers
+
+You will hit these in order; each fix below removes one.
+
+1. **`libgthread-2.0.so.0: cannot open shared object file`**
+   (shown in the plugin's own error figure) — no FHS environment at
+   all. PySide6's bundled Qt cannot load glib & friends.
+2. **`qt.qpa.plugin: From 6.5.0, xcb-cursor0 or libxcb-cursor0 is
+   needed…` / `no Qt platform plugin could be initialized`** —
+   FHS present but incomplete. Beyond the obvious `xcb-util-cursor`,
+   the bundled Qt needs the whole **xcb-util family** (`icccm`,
+   `image`, `keysyms`, `render-util`, `util`) and **`libzstd`**
+   (a hard dependency of `libQt6Core`; note that `pkgs.zstd`'s
+   default output ships only the CLI — the library lives in
+   `zstd.out`). Qt prints the xcb-cursor hint for *any* missing
+   dependency of the platform plugin, so don't trust it literally.
+3. **`Could not load the Qt platform plugin "wayland"/"xcb" in ""
+   even though it was found`** with all libraries present — the
+   desktop session (KDE Plasma does this) exports **`QT_PLUGIN_PATH`**
+   pointing at the system's Qt plugin directories. That variable takes
+   precedence over the wheel's bundled plugins, so the venv's PySide6
+   (its own Qt, e.g. 6.11.1) tries to load platform plugins built
+   against the system Qt (e.g. 6.11.0) — a private-ABI mismatch that
+   fails exactly like a missing library. The fix is to **unset
+   `QT_PLUGIN_PATH`** for KiCad and everything it spawns. KiCad
+   itself is wxWidgets/GTK and does not use the variable.
+
+## Recommended setup: an FHS wrapper
+
+Wrap KiCad in `buildFHSEnv` so a plain `kicad` launch carries
+everything. Home-manager example (`home.nix`); for a system-wide
+install put the same package in `environment.systemPackages` in
+`configuration.nix` instead:
+
+```nix
+{ pkgs, ... }:
+
+let
+  kicad-fhs = pkgs.buildFHSEnv {
+    name = "kicad";
+    targetPkgs = p: with p; [
+      kicad glib fontconfig freetype dbus libGL libxkbcommon
+      xcb-util-cursor wayland zlib zstd.out
+      xorg.libX11 xorg.libxcb xorg.libXext xorg.libXrender
+      xorg.libSM xorg.libICE xorg.libXrandr xorg.libXi
+      xorg.libXcursor xorg.libXfixes
+      # xcb-util family needed by PySide6's bundled xcb platform plugin
+      xorg.xcbutil xorg.xcbutilwm xorg.xcbutilimage
+      xorg.xcbutilkeysyms xorg.xcbutilrenderutil
+    ];
+    # Plasma exports QT_PLUGIN_PATH (system Qt plugin dirs); the venv's
+    # bundled Qt chokes on those mismatched plugins. KiCad is wx/GTK,
+    # so dropping the variable is safe.
+    profile = "unset QT_PLUGIN_PATH";
+    runScript = "kicad";
+  };
+in
+{
+  home.packages = [ kicad-fhs /* replaces bare pkgs.kicad */ ];
+
+  # buildFHSEnv ships no .desktop file; restore the menu launcher.
+  xdg.desktopEntries.kicad = {
+    name = "KiCad";
+    exec = "kicad %F";
+    icon = "kicad";
+    categories = [ "Development" "Electronics" ];
+    mimeType = [ "application/x-kicad-project" ];
+  };
+}
+```
+
+Rebuild, **fully quit any running KiCad** (a running instance keeps
+its old environment), relaunch, click the Ω button. A one-line
+`Could not load the Qt platform plugin "wayland"` warning may remain
+if the wayland client stack is unhappy — it is harmless; Qt falls
+back to xcb (XWayland) and the dialog appears.
+
+## Quick test without a rebuild
+
+`steam-run` provides a broad FHS that is one library short
+(`libxcb-cursor`, required by Qt ≥ 6.5) and, on Plasma, still leaks
+`QT_PLUGIN_PATH`:
+
+```sh
+XCBCUR=$(nix build --no-link --print-out-paths nixpkgs#xcb-util-cursor)
+env -u QT_PLUGIN_PATH QT_QPA_PLATFORM=xcb \
+    LD_LIBRARY_PATH=$XCBCUR/lib steam-run kicad
+```
+
+(Fish: `set XCBCUR (nix build --no-link --print-out-paths
+nixpkgs#xcb-util-cursor)`, then the same `env …` line.)
+
+## Debugging further failures
+
+If a new wheel version needs another library, reproduce the plugin's
+Qt startup *outside* KiCad against the wrapper's library set — the
+venv survives at
+`~/.cache/kicad/10.0/python-environments/th.co.b4l.fill-resistance`:
+
+```sh
+ROOTFS=$(nix-store -qR "$(readlink -f "$(command -v kicad)")" \
+         | grep fhsenv-rootfs | head -1)
+env -i HOME=$HOME DISPLAY=$DISPLAY XAUTHORITY=$XAUTHORITY \
+    LD_LIBRARY_PATH=$ROOTFS/usr/lib64 QT_DEBUG_PLUGINS=1 \
+    ~/.cache/kicad/10.0/python-environments/th.co.b4l.fill-resistance/bin/python3 \
+    -c 'from PySide6.QtWidgets import QApplication; \
+print(QApplication([]).platformName())'
+```
+
+A missing library shows up as a plain `ImportError: libfoo.so.N:
+cannot open shared object file` — map the soname to its nixpkgs
+attribute (`nix-locate libfoo.so.N`, from `nix-index`) and add it to
+`targetPkgs`. If instead every library loads and only the platform
+plugin fails, compare the environment of the *running* KiCad
+(`tr '\0' '\n' < /proc/$(pgrep -x kicad)/environ`) for Qt variables
+leaking in from the session — `QT_PLUGIN_PATH` above was found
+exactly this way.

fill_resistance/main.py

@@ -39,9 +39,24 @@ def _fail(message: str, outdir) -> None:
 def main() -> None:
     outdir = None
     try:
-        from kipy.errors import ApiError
+        try:
+            from kipy.errors import ApiError
 
-        from . import board_io, dialog
+            from . import board_io, dialog
+        except ImportError as e:
+            if "cannot open shared object file" not in str(e):
+                raise
+            # pip's Linux wheels link against FHS system libraries;
+            # on NixOS those paths don't exist and PySide6/pynng die
+            # exactly like this. Nothing inside the venv can fix it.
+            raise UserFacingError(
+                f"A compiled dependency cannot load its system "
+                f"libraries: {e}\nThe plugin venv is built from pip "
+                f"wheels, which expect standard (FHS) library paths. "
+                f"On NixOS, run KiCad inside an FHS environment "
+                f"(buildFHSEnv wrapper, or steam-run for a quick "
+                f"test) - see docs/NIXOS.md in the plugin repo."
+            )
         try:
             kicad, board = board_io.connect()
             stackup = board_io.get_stackup_info(board)

fill_resistance/plots.py

@@ -24,10 +24,16 @@ def _pick_backend():
     dependency and the selection dialog / progress window put a Qt event
     loop in this process, after which matplotlib refuses TkAgg
     ("Cannot load backend 'TkAgg' ... as 'qt' is currently running") -
-    exactly what happened on macOS, whose bundled Python ships tkinter."""
+    exactly what happened on macOS, whose bundled Python ships tkinter.
+
+    The probe must import the binding's native core, not just the
+    package: on NixOS `import PySide6` succeeds (pure __init__) while
+    QtCore's .so cannot find the system libraries pip wheels expect
+    ("libgthread-2.0.so.0: cannot open shared object file") - promising
+    QtAgg then kills even the error figure at switch_backend time."""
     for qt in ("PySide6", "PyQt6", "PyQt5", "PySide2"):
         try:
-            __import__(qt)
+            __import__(qt + ".QtCore")
             return "QtAgg" if qt in ("PySide6", "PyQt6") else "Qt5Agg"
         except Exception:
             continue

tests/test_platform_fallbacks.py

@@ -20,6 +20,25 @@ def test_backend_prefers_qt_over_tk():
     assert plots._pick_backend() in ("QtAgg", "Qt5Agg")
 
 
+def test_backend_probe_requires_working_qtcore(monkeypatch):
+    # NixOS: `import PySide6` succeeds (a pure-Python __init__) while
+    # QtCore's .so cannot load the FHS system libraries pip wheels
+    # expect. The probe must import the native core and fall through -
+    # promising QtAgg kills even the error figure at switch_backend
+    # time, and the failure report with it.
+    import builtins
+    real_import = builtins.__import__
+
+    def broken_qt(name, *args, **kwargs):
+        if name.split(".")[0] in ("PySide6", "PyQt6", "PyQt5", "PySide2"):
+            raise ImportError("libgthread-2.0.so.0: cannot open shared "
+                              "object file: No such file or directory")
+        return real_import(name, *args, **kwargs)
+
+    monkeypatch.setattr(builtins, "__import__", broken_qt)
+    assert plots._pick_backend() in ("TkAgg", None)
+
+
 def test_output_dir_falls_back_when_board_dir_unwritable(
         tmp_path, monkeypatch, capsys):
     board_dir = tmp_path / "board"