Skip platform-irrelevant dotfiles packages during stow

iterm2 was being stowed on Linux and herbstluftwm/xorg on Darwin.
Add a built-in Darwin/Linux exclude default plus an excludePackages
override option in modules/home/dotfiles.nix, superseding the earlier
"accept harmless unused symlinks" decision.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BBSd4VBfoKHjE1HR3P8kmn

5 files changed, 90 insertions, 0 deletions

diff --git a/openspec/changes/filter-dotfiles-by-platform/.openspec.yaml b/openspec/changes/filter-dotfiles-by-platform/.openspec.yaml
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+schema: spec-driven
+created: 2026-09-18
diff --git a/openspec/changes/filter-dotfiles-by-platform/design.md b/openspec/changes/filter-dotfiles-by-platform/design.md
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+## Context
+
+See proposal.md - Why. `modules/home/dotfiles.nix` currently loops over every non-hidden top-level directory in the cloned external dotfiles repo and stows all of them unconditionally (lines 22-28). The only existing platform-branching precedent in this codebase is the simple `if pkgs.stdenv.isDarwin then ... else ...` in `users/tom.nix:6`. Only two real `homeConfigurations` exist today (`tom@linux`, `tom@darwin` in `flake.nix:44-53`); no NixOS hosts are defined yet (`hosts/` has only `template`). The known current package inventory in the external repo includes `iterm2` (Darwin-only), and `herbstluftwm` + `xorg` (Linux-only).
+
+## Goals / Non-Goals
+
+**Goals:**
+- Skip stowing packages that are known to be irrelevant to the current platform, using a minimal, easily-extended mechanism.
+- Preserve an escape hatch so a host can exclude additional packages without further code changes if the external repo's contents change.
+- Keep all changes scoped to `modules/home/dotfiles.nix`; no changes to the external dotfiles repo, `flake.nix`, or host definitions.
+
+**Non-Goals:**
+- Building a full per-host "which programs/WMs does this host use" model. The current problem is fully explained by OS (Darwin vs. Linux), so no new host-level option or metadata is introduced.
+- Handling a future NixOS host that runs a desktop environment (e.g. GNOME, per `add-gnome-desktop`) but would still be Linux and thus still match the Linux exclude branch for `herbstluftwm`/`xorg` as "relevant." No such host exists yet; if it becomes a real problem later, the `excludePackages` escape hatch this change adds already covers it per-host.
+
+## Decisions
+
+**Decision: a small built-in platform-keyed exclude list, plus an `excludePackages` override option, computed with `pkgs.stdenv.isDarwin`/`isLinux`.**
+
+Inside `modules/home/dotfiles.nix`, add:
+```nix
+options.home.dotfiles.excludePackages = lib.mkOption {
+  type = lib.types.listOf lib.types.str;
+  default = [ ];
+  description = "Names of dotfiles stow packages to always skip, in addition to built-in platform defaults.";
+};
+```
+and a `let`-bound list combining a small built-in default (`iterm2` when not Darwin; `herbstluftwm`, `xorg` when not Linux) with `cfg.excludePackages`. Pass the combined list into the activation script via `lib.escapeShellArgs` as a bash array, and check membership in the existing `for pkg in ...` loop before calling `stow`, skipping with a warning message (mirroring the existing stow-failure warning style at line 27) instead of silently stowing.
+
+Alternatives considered:
+- **Marker-file convention in the external dotfiles repo** (e.g. each stow package declares its own target platform via a `.nix-platform` file that `dotfiles.nix` reads). This would keep nix-config fully generic and free of package-name knowledge, matching the original design philosophy most closely. Rejected because the dotfiles repo lives outside this git repository and is not something this change can edit or verify; it would also require the mechanism to handle a missing/malformed marker file gracefully, adding complexity for no immediate benefit over a 3-name list.
+- **Fully generic attrs-based platform-mapping option** (e.g. `home.dotfiles.packagePlatforms = { darwin = [...]; linux = [...]; }`), configured per-host. Rejected as overengineering relative to the current need (only two platforms, three known mismatched packages); the simpler list-plus-override captures the same override capability with far less surface area.
+- **Do nothing (status quo)**: the previously accepted "harmless unused symlinks" trade-off. Rejected per the user's explicit request; this change supersedes that decision.
+
+## Risks / Trade-offs
+
+- [Reintroducing external-repo package-name knowledge into nix-config, which the original design explicitly avoided] → Mitigation: kept minimal (3 names in one `let` binding) and overridable via `excludePackages`, so it degrades gracefully if the external repo's package list changes rather than silently breaking.
+- [A future Linux host that both is Linux and doesn't want `herbstluftwm`/`xorg` (e.g. a GNOME NixOS host) would still get them stowed, since the built-in default only branches on Darwin vs. Linux, not on WM in use] → Mitigation: no such host exists yet; the `excludePackages` option already provides a per-host override once one does.
diff --git a/openspec/changes/filter-dotfiles-by-platform/proposal.md b/openspec/changes/filter-dotfiles-by-platform/proposal.md
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+## Why
+
+The dotfiles activation in `modules/home/dotfiles.nix` stows every top-level package directory from the external personal dotfiles repo into `$HOME` unconditionally, with no platform filtering. In practice this means macOS-only config (`iterm2`, for iTerm2) gets stowed on the Linux home-manager target, and Linux X11-only config (`herbstluftwm`, `xorg`) gets stowed on the Darwin machine — producing dead, irrelevant symlinks. This was previously an explicitly accepted trade-off (see the archived `add-dotfiles-stow-bootstrap` change's design non-goals, reaffirmed by `add-gnome-desktop`'s out-of-scope note), but the user now wants platform-irrelevant packages skipped, so this change supersedes that earlier "accept the risk" decision.
+
+## What Changes
+
+- `modules/home/dotfiles.nix` gains a new `home.dotfiles.excludePackages` option (list of package names to skip regardless of platform), as a manual per-host escape hatch.
+- The module computes a small built-in default exclude set from the current known platform mismatches, using the same `pkgs.stdenv.isDarwin`/`isLinux` pattern already used in `users/tom.nix`: `iterm2` is skipped when not on Darwin; `herbstluftwm` and `xorg` are skipped when not on Linux.
+- The stow activation loop skips any package in the combined exclude set (built-in + `excludePackages`), printing a visible "skipped — not relevant on this platform" message instead of silently stowing it. Packages not in the exclude set continue to stow exactly as before.
+
+## Capabilities
+
+### Modified Capabilities
+- `dotfiles-management`: adds a requirement that platform-irrelevant dotfiles packages (built-in defaults or explicitly configured via `excludePackages`) are skipped during stow, rather than always stowing every top-level package directory.
+
+## Impact
+
+- **Code**: `modules/home/dotfiles.nix` (new option, exclude-set computation, activation script loop).
+- **Specs**: `openspec/specs/dotfiles-management/spec.md` gains a new requirement; existing requirements (opt-in via repo URL, no re-clone, package-to-symlink mapping, per-package fault isolation) are unaffected.
+- **Prior decisions superseded**: the "no filtering, accept harmless unused symlinks" non-goal recorded in the archived `add-dotfiles-stow-bootstrap` design and reaffirmed in `add-gnome-desktop` no longer holds; this change intentionally reverses it.
+- No changes to `flake.nix`, host definitions, or the external dotfiles repo itself.
diff --git a/openspec/changes/filter-dotfiles-by-platform/specs/dotfiles-management/spec.md b/openspec/changes/filter-dotfiles-by-platform/specs/dotfiles-management/spec.md
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+## ADDED Requirements
+
+### Requirement: Platform-irrelevant packages are skipped
+The system SHALL skip stowing a dotfiles package when that package is platform-irrelevant, either because it matches a built-in platform default (based on the current OS) or because it has been explicitly listed for exclusion, rather than always stowing every top-level package directory.
+
+#### Scenario: A package is excluded by built-in platform default
+- **WHEN** the dotfiles repository contains a top-level package directory that is known to be relevant only to a different platform than the one currently running (for example, a macOS-only package on a Linux system, or a Linux-only package on a macOS system)
+- **THEN** that package is not symlinked, and a visible message indicates it was skipped as not relevant on this platform
+
+#### Scenario: A package is excluded via explicit configuration
+- **WHEN** a package name has been explicitly configured for exclusion, regardless of the built-in platform defaults
+- **THEN** that package is not symlinked, and a visible message indicates it was skipped
+
+#### Scenario: A non-excluded package is still stowed
+- **WHEN** a top-level package directory is neither a built-in platform-mismatched package nor explicitly excluded
+- **THEN** that package is symlinked into the home directory as described by the existing package-symlinking requirement
diff --git a/openspec/changes/filter-dotfiles-by-platform/tasks.md b/openspec/changes/filter-dotfiles-by-platform/tasks.md
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+## 1. Exclude-list option and platform defaults
+
+- [x] 1.1 In `modules/home/dotfiles.nix`, add the `home.dotfiles.excludePackages` option (`lib.types.listOf lib.types.str`, default `[ ]`) and verify `nix flake check` still evaluates the module without errors
+- [x] 1.2 Add a `let`-bound built-in exclude list computed from `pkgs.stdenv.isDarwin`/`isLinux` (`iterm2` excluded when not Darwin; `herbstluftwm` and `xorg` excluded when not Linux), combined with `cfg.excludePackages`, and verify by inspecting the evaluated module (e.g. `nix eval` or a scratch `home-manager build`) that the combined list contains the expected names for each platform
+
+## 2. Skip excluded packages during stow
+
+- [x] 2.1 Pass the combined exclude list into the `home.activation.dotfiles` script via `lib.escapeShellArgs` and extend the `for pkg in ...` loop (currently `modules/home/dotfiles.nix:22-28`) to skip any package name present in that list, printing a message that it was skipped as not relevant on this platform, and verify by rendering the activation script (`home-manager build --flake .#tom@darwin` or `.#tom@linux`, or `nix eval .#homeConfigurations.\"tom@darwin\".activationPackage` and inspecting the generated script) that the skip branch is present
+- [x] 2.2 Verify non-excluded packages are unaffected: confirm the rendered activation script still calls `stow` for packages outside the exclude list (e.g. `git`, `neovim`, `tmux`)
+
+## 3. Validation
+
+- [x] 3.1 Run `openspec validate filter-dotfiles-by-platform --strict` and confirm it passes