init: nixgl module

This module enables wrapping programs which require access to libGL with
nixGL on non-NixOS systems.

modules/misc/nixgl.nix

@@ -0,0 +1,58 @@
+{ config, lib, pkgs, ... }:
+
+let cfg = config.nixGL;
+in {
+  meta.maintainers = [ lib.maintainers.smona ];
+
+  options.nixGL.prefix = lib.mkOption {
+    type = lib.types.str;
+    default = "";
+    example = lib.literalExpression
+      ''"''${inputs.nixGL.packages.x86_64-linux.nixGLIntel}/bin/nixGLIntel"'';
+    description = ''
+      The nixGL command that `lib.nixGL.wrap` should wrap packages with.
+      This can be used to provide libGL access to applications on non-NixOS systems.
+
+      Some packages are wrapped by default (e.g. kitty, firefox), but you can wrap other packages
+      as well, with `(config.lib.nixGL.wrap <package>)`. If this option is empty (the default),
+      then `lib.nixGL.wrap` is a no-op.
+    '';
+  };
+
+  config = {
+    lib.nixGL.wrap = # Wrap a single package with the configured nixGL wrapper
+      pkg:
+
+      if cfg.prefix == "" then
+        pkg
+      else
+      # Wrap the package's binaries with nixGL, while preserving the rest of
+      # the outputs and derivation attributes.
+        (pkg.overrideAttrs (old: {
+          name = "nixGL-${pkg.name}";
+          buildCommand = ''
+            set -eo pipefail
+
+            ${
+            # Heavily inspired by https://stackoverflow.com/a/68523368/6259505
+            pkgs.lib.concatStringsSep "\n" (map (outputName: ''
+              echo "Copying output ${outputName}"
+              set -x
+              cp -rs --no-preserve=mode "${
+                pkg.${outputName}
+              }" "''$${outputName}"
+              set +x
+            '') (old.outputs or [ "out" ]))}
+
+            rm -rf $out/bin/*
+            shopt -s nullglob # Prevent loop from running if no files
+            for file in ${pkg.out}/bin/*; do
+              echo "#!${pkgs.bash}/bin/bash" > "$out/bin/$(basename $file)"
+              echo "exec -a \"\$0\" ${cfg.prefix} $file \"\$@\"" >> "$out/bin/$(basename $file)"
+              chmod +x "$out/bin/$(basename $file)"
+            done
+            shopt -u nullglob # Revert nullglob back to its normal default state
+          '';
+        }));
+  };
+}

modules/modules.nix

@@ -31,6 +31,7 @@ let
     ./misc/gtk.nix
     ./misc/lib.nix
     ./misc/news.nix
+    ./misc/nixgl.nix
     ./misc/numlock.nix
     ./misc/pam.nix
     ./misc/qt.nix