targets.genericLinux: add module

PR #797

modules/misc/news.nix

@@ -1443,6 +1443,22 @@ in
           'services.picom' as soon as possible.
         '';
       }
+
+      {
+        time = "2020-04-08T09:33:05+00:00";
+        condition = hostPlatform.isLinux;
+        message = ''
+          A new module is available: 'targets.genericLinux'.
+
+          When enabled, this module will configure various settings
+          and environment variables to make Home Manager and programs
+          installed through Nix work better on GNU/Linux distributions
+          other than NixOS.
+
+          It should not be enabled if your Home Manager configuration
+          is deployed on a NixOS host.
+        '';
+      }
     ];
   };
 }

modules/modules.nix

@@ -168,6 +168,7 @@ let
     (loadModule ./services/xscreensaver.nix { })
     (loadModule ./services/xsuspender.nix { condition = hostPlatform.isLinux; })
     (loadModule ./systemd.nix { })
+    (loadModule ./targets/generic-linux.nix { condition = hostPlatform.isLinux; })
     (loadModule ./xcursor.nix { })
     (loadModule ./xresources.nix { })
     (loadModule ./xsession.nix { })

modules/targets/generic-linux.nix

@@ -0,0 +1,39 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  profileDirectory = config.home.profileDirectory;
+
+in {
+  options.targets.genericLinux.enable = mkEnableOption "" // {
+    description = ''
+      Whether to enable settings that make Home Manager work better on
+      GNU/Linux distributions other than NixOS.
+    '';
+  };
+
+  config = mkIf config.targets.genericLinux.enable {
+    home.sessionVariables = let
+      profiles = [ "/nix/var/nix/profiles/default" profileDirectory ];
+      dataDirs =
+        concatStringsSep ":" (map (profile: "${profile}/share") profiles);
+    in { XDG_DATA_DIRS = "${dataDirs}\${XDG_DATA_DIRS:+:}$XDG_DATA_DIRS"; };
+
+    home.sessionVariablesExtra = ''
+      . "${pkgs.nix}/etc/profile.d/nix.sh"
+    '';
+
+    # We need to source both nix.sh and hm-session-vars.sh as noted in
+    # https://github.com/rycee/home-manager/pull/797#issuecomment-544783247
+    programs.bash.initExtra = ''
+      . "${pkgs.nix}/etc/profile.d/nix.sh"
+      . "${profileDirectory}/etc/profile.d/hm-session-vars.sh"
+    '';
+
+    systemd.user.sessionVariables = {
+      NIX_PATH = "$HOME/.nix-defexpr/channels\${NIX_PATH:+:}$NIX_PATH";
+    };
+  };
+}

tests/default.nix

@@ -57,5 +57,6 @@ import nmt {
     ./modules/services/sxhkd
     ./modules/services/window-managers/i3
     ./modules/systemd
+    ./modules/targets
   ]);
 }

tests/modules/targets/default.nix

@@ -0,0 +1 @@
+{ targets-generic-linux = ./generic-linux.nix; }

tests/modules/targets/generic-linux-session-vars-expected.txt

@@ -0,0 +1,6 @@
+# Only source this once.
+if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
+export __HM_SESS_VARS_SOURCED=1
+
+export XDG_DATA_DIRS="/nix/var/nix/profiles/default/share:/homeless-shelter/.nix-profile/share${XDG_DATA_DIRS:+:}$XDG_DATA_DIRS"
+. "@nix@/etc/profile.d/nix.sh"

tests/modules/targets/generic-linux.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    home.homeDirectory = "/homeless-shelter";
+
+    targets.genericLinux.enable = true;
+
+    nmt.script = ''
+      assertFileExists home-path/etc/profile.d/hm-session-vars.sh
+      assertFileContent \
+        home-path/etc/profile.d/hm-session-vars.sh \
+        ${
+          pkgs.substituteAll {
+            src = ./generic-linux-session-vars-expected.txt;
+            nix = pkgs.nix;
+          }
+        }
+    '';
+  };
+}