fish: isolated sourcePaths configuration

nix/homeModules/common/fish/sourcePaths.nix

@@ -0,0 +1,31 @@
+{ config, lib, ... }:
+
+{
+
+  options.programs.fish.sourcePaths = lib.mkOption {
+    type = with lib.types; nonEmptyListOf str;
+    description = ''
+      Paths to be sourced idempotently at the start of a login-shell.
+    '';
+  };
+
+  # We need to handle path idempotently, because fish in home-manager is
+  # unable to depend on nixos/nix-darwin configurations to figure out the profile.d/nix.sh
+  # to source.
+  #
+  # The hack is to make terminal emulators  run fish as a login shell
+  # whenever fish should handle the path.
+  # tmux should NOT call fish as a login shell, because it would inherit the
+  # environment variables from its parent shell, which is a login shell.
+  config.programs.fish.loginShellInit = ''
+    begin
+        set ps ${builtins.concatStringsSep " " config.programs.fish.sourcePaths}
+
+        set -e fish_user_paths
+        for p in $ps
+            test -d $p && set --append fish_user_paths $p
+        end
+    end
+  '';
+
+}