modules/users: prototype s6 integration: ~/.config/s6/{sources,compiled}

modules/users/default.nix

@@ -241,6 +241,7 @@ let
 in
 {
   imports = [
+    ./s6-rc.nix
     ./systemd.nix
   ];
 

modules/users/s6-rc.nix

@@ -0,0 +1,129 @@
+{ lib, pkgs, ... }:
+let
+  normalizeName = name: lib.removeSuffix ".service" (lib.removeSuffix ".target" name);
+
+  # infers the service type from the arguments and dispatches appropriately
+  genService = { name, run, depends }: if run != null then
+    genService' (normalizeName name) "longrun" depends [
+      (pkgs.writeTextFile {
+        name = "s6-${name}-run";
+        destination = "/${normalizeName name}/run";
+        text = run;
+      })
+    ]
+  else
+    # TODO: a bundle can totally have dependencies. i can't just map them *all* to contents.
+    # genService' (normalizeName name) "bundle" [] (
+    #   (builtins.map
+    #     (d: pkgs.writeTextFile {
+    #       name = "s6-${name}-contains-${d}";
+    #       destination = "/${normalizeName name}/contents.d/${normalizeName d}";
+    #       text = "";
+    #     })
+    #     depends
+    #   ) ++ [
+    #     # in case the bundle has no contents, ensure `contents.d` still gets made
+    #     (pkgs.runCommandLocal "s6-${name}-contains.d" {} ''
+    #       mkdir -p $out/"${normalizeName name}"/contents.d
+    #     '')
+    #   ]
+    # )
+    genService' (normalizeName name) "bundle" [] [
+      (pkgs.writeTextFile {
+        name = "s6-${name}-contents";
+        destination = "/${normalizeName name}/contents";
+        text = lib.concatStringsSep "\n" (builtins.map normalizeName depends);
+      })
+    ]
+  ;
+  genService' = name: type: depends: others: pkgs.symlinkJoin {
+    name = "s6-${name}";
+    paths = others ++ [
+      (pkgs.writeTextFile {
+        name = "s6-${name}-type";
+        destination = "/${name}/type";
+        text = type;
+      })
+    ] ++ builtins.map
+      (d: pkgs.writeTextFile {
+        name = "s6-${name}-depends-${d}";
+        destination = "/${name}/dependencies.d/${normalizeName d}";
+        text = "";
+      })
+      depends;
+  };
+
+  # create a directory, containing N subdirectories:
+  # - svc-a/
+  #   - type
+  #   - run
+  # - svc-b/
+  #   - type
+  #   - run
+  # - ...
+  genServices = svcs: pkgs.symlinkJoin {
+    name = "s6-user-services";
+    paths = builtins.map genService svcs;
+  };
+
+  # output is a directory containing:
+  # - db
+  # - lock
+  # - n
+  # - resolve.cdb
+  # - servicedirs/
+  #   - svc-a/
+  #   - svc-b/
+  #   - ...
+  #
+  # this can then be used by s6-rc-init, like:
+  # s6-svscan scandir &
+  # s6-rc-init -c $compiled -l $PWD/live -d $PWD/scandir
+  # s6-rc -l $PWD/live start svc-a
+  compileServices = sources: with pkgs; stdenv.mkDerivation {
+    name = "s6-user-services";
+    src = sources;
+    nativeBuildInputs = [ s6-rc ];
+    buildPhase = ''
+      s6-rc-compile $out $src
+    '';
+  };
+
+  # transform the `user.services` attrset into a s6 services list.
+  s6SvcsFromConfigServices = services: lib.mapAttrsToList
+    (name: service: {
+      inherit name;
+      run = service.serviceConfig.ExecStart;
+      depends = service.wants ++ builtins.attrNames (
+        lib.filterAttrs (_: cfg: lib.elem name cfg.wantedBy) services
+      );
+    })
+    services
+  ;
+
+  # in the systemd service management, these targets are implicitly defined and used
+  # to accomplish something like run-levels, or service groups.
+  # map them onto s6 "bundles". their contents are determined via reverse dependency mapping (`wantedBy` of every other service).
+  implicitServices = {
+    "default.target" = {
+      serviceConfig.ExecStart = null;
+      wants = [];
+      wantedBy = [];
+    };
+    "graphical-session.target" = {
+      serviceConfig.ExecStart = null;
+      wants = [];
+      wantedBy = [];
+    };
+  };
+in
+{
+  options.sane.users = with lib; mkOption {
+    type = types.attrsOf (types.submodule ({ config, ...}: let
+      sources = genServices (s6SvcsFromConfigServices (implicitServices // config.services));
+    in {
+      fs.".config/s6/sources".symlink.target = sources;
+      fs.".config/s6/compiled".symlink.target = compileServices sources;
+    }));
+  };
+}