initrd: Opt-in bare bones systemd-based initrd

nixos/lib/systemd-lib.nix

@@ -120,18 +120,23 @@ in rec {
         (if isList value then value else [value]))
         as));
 
-  generateUnits = generateUnits' true;
-
-  generateUnits' = allowCollisions: type: units: upstreamUnits: upstreamWants:
-    pkgs.runCommand "${type}-units"
+  generateUnits = { allowCollisions ? true, type, units, upstreamUnits, upstreamWants, packages ? cfg.packages, package ? cfg.package }:
+    let
+      typeDir = ({
+        system = "system";
+        initrd = "system";
+        user = "user";
+        nspawn = "nspawn";
+      }).${type};
+    in pkgs.runCommand "${type}-units"
       { preferLocalBuild = true;
         allowSubstitutes = false;
       } ''
       mkdir -p $out
 
       # Copy the upstream systemd units we're interested in.
       for i in ${toString upstreamUnits}; do
-        fn=${cfg.package}/example/systemd/${type}/$i
+        fn=${package}/example/systemd/${typeDir}/$i
         if ! [ -e $fn ]; then echo "missing $fn"; false; fi
         if [ -L $fn ]; then
           target="$(readlink "$fn")"
@@ -148,7 +153,7 @@ in rec {
       # Copy .wants links, but only those that point to units that
       # we're interested in.
       for i in ${toString upstreamWants}; do
-        fn=${cfg.package}/example/systemd/${type}/$i
+        fn=${package}/example/systemd/${typeDir}/$i
         if ! [ -e $fn ]; then echo "missing $fn"; false; fi
         x=$out/$(basename $fn)
         mkdir $x
@@ -160,14 +165,14 @@ in rec {
       done
 
       # Symlink all units provided listed in systemd.packages.
-      packages="${toString cfg.packages}"
+      packages="${toString packages}"
 
       # Filter duplicate directories
       declare -A unique_packages
       for k in $packages ; do unique_packages[$k]=1 ; done
 
       for i in ''${!unique_packages[@]}; do
-        for fn in $i/etc/systemd/${type}/* $i/lib/systemd/${type}/*; do
+        for fn in $i/etc/systemd/${typeDir}/* $i/lib/systemd/${typeDir}/*; do
           if ! [[ "$fn" =~ .wants$ ]]; then
             if [[ -d "$fn" ]]; then
               targetDir="$out/$(basename "$fn")"
@@ -286,17 +291,11 @@ in rec {
     };
   };
 
-  serviceConfig = { name, config, ... }: {
+  mkServiceConfig = path: { name, config, ... }: {
     config = mkMerge
       [ { # Default path for systemd services.  Should be quite minimal.
-          path = mkAfter
-            [ pkgs.coreutils
-              pkgs.findutils
-              pkgs.gnugrep
-              pkgs.gnused
-              systemd
-            ];
-          environment.PATH = "${makeBinPath config.path}:${makeSearchPathOutput "bin" "sbin" config.path}";
+          path = mkAfter path;
+          environment.PATH = mkIf (config.path != []) "${makeBinPath config.path}:${makeSearchPathOutput "bin" "sbin" config.path}";
         }
         (mkIf (config.preStart != "")
           { serviceConfig.ExecStartPre =
@@ -325,6 +324,16 @@ in rec {
       ];
   };
 
+  serviceConfig = mkServiceConfig [
+    pkgs.coreutils
+    pkgs.findutils
+    pkgs.gnugrep
+    pkgs.gnused
+    systemd
+  ];
+
+  initrdServiceConfig = mkServiceConfig [];
+
   mountConfig = { config, ... }: {
     config = {
       mountConfig =
@@ -382,6 +391,22 @@ in rec {
         '';
     };
 
+  initrdServiceToUnit = name: def:
+    { inherit (def) aliases wantedBy requiredBy enable;
+      text = commonUnitText def +
+        ''
+          [Service]
+          ${let env = def.environment;
+            in concatMapStrings (n:
+              let s = optionalString (env.${n} != null)
+                "Environment=${builtins.toJSON "${n}=${env.${n}}"}\n";
+              # systemd max line length is now 1MiB
+              # https://github.com/systemd/systemd/commit/e6dde451a51dc5aaa7f4d98d39b8fe735f73d2af
+              in if stringLength s >= 1048576 then throw "The value of the environment variable ‘${n}’ in systemd service ‘${name}.service’ is too long." else s) (attrNames env)}
+          ${attrsToSection def.serviceConfig}
+        '';
+    };
+
   socketToUnit = name: def:
     { inherit (def) aliases wantedBy requiredBy enable;
       text = commonUnitText def +

nixos/lib/systemd-types.nix

@@ -0,0 +1,30 @@
+{ lib, systemdUtils }:
+
+with systemdUtils.lib;
+with systemdUtils.unitOptions;
+with lib;
+
+rec {
+  units = with types;
+    attrsOf (submodule ({ name, config, ... }: {
+      options = concreteUnitOptions;
+      config = { unit = mkDefault (systemdUtils.lib.makeUnit name config); };
+    }));
+
+  services = with types; attrsOf (submodule [ { options = serviceOptions; } unitConfig serviceConfig ]);
+  initrdServices = with types; attrsOf (submodule [ { options = serviceOptions; } unitConfig initrdServiceConfig ]);
+
+  targets = with types; attrsOf (submodule [ { options = targetOptions; } unitConfig ]);
+
+  sockets = with types; attrsOf (submodule [ { options = socketOptions; } unitConfig ]);
+
+  timers = with types; attrsOf (submodule [ { options = timerOptions; } unitConfig ]);
+
+  paths = with types; attrsOf (submodule [ { options = pathOptions; } unitConfig ]);
+
+  slices = with types; attrsOf (submodule [ { options = sliceOptions; } unitConfig ]);
+
+  mounts = with types; listOf (submodule [ { options = mountOptions; } unitConfig mountConfig ]);
+
+  automounts = with types; listOf (submodule [ { options = automountOptions; } unitConfig automountConfig ]);
+}

nixos/lib/utils.nix

@@ -197,5 +197,6 @@ rec {
   systemdUtils = {
     lib = import ./systemd-lib.nix { inherit lib config pkgs; };
     unitOptions = import ./systemd-unit-options.nix { inherit lib systemdUtils; };
+    types = import ./systemd-types.nix { inherit lib systemdUtils; };
   };
 }

nixos/modules/module-list.nix

@@ -1172,6 +1172,7 @@
   ./system/boot/systemd/nspawn.nix
   ./system/boot/systemd/tmpfiles.nix
   ./system/boot/systemd/user.nix
+  ./system/boot/systemd/initrd.nix
   ./system/boot/timesyncd.nix
   ./system/boot/tmp.nix
   ./system/etc/etc-activation.nix

nixos/modules/system/boot/stage-1.nix

@@ -706,8 +706,12 @@ in
       }
     ];
 
-    system.build =
-      { inherit bootStage1 initialRamdisk initialRamdiskSecretAppender extraUtils; };
+    system.build = mkMerge [
+      { inherit bootStage1 initialRamdiskSecretAppender extraUtils; }
+
+      # generated in nixos/modules/system/boot/systemd/initrd.nix
+      (mkIf (!config.boot.initrd.systemd.enable) { inherit initialRamdisk; })
+    ];
 
     system.requiredKernelConfig = with config.lib.kernelConfig; [
       (isYes "TMPFS")

nixos/modules/system/boot/systemd.nix

@@ -11,14 +11,7 @@ let
   systemd = cfg.package;
 
   inherit (systemdUtils.lib)
-    makeUnit
     generateUnits
-    makeJobScript
-    unitConfig
-    serviceConfig
-    mountConfig
-    automountConfig
-    commonUnitText
     targetToUnit
     serviceToUnit
     socketToUnit
@@ -185,13 +178,7 @@ in
     systemd.units = mkOption {
       description = "Definition of systemd units.";
       default = {};
-      type = with types; attrsOf (submodule (
-        { name, config, ... }:
-        { options = concreteUnitOptions;
-          config = {
-            unit = mkDefault (makeUnit name config);
-          };
-        }));
+      type = systemdUtils.types.units;
     };
 
     systemd.packages = mkOption {
@@ -203,37 +190,37 @@ in
 
     systemd.targets = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = targetOptions; } unitConfig] );
+      type = systemdUtils.types.targets;
       description = "Definition of systemd target units.";
     };
 
     systemd.services = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = serviceOptions; } unitConfig serviceConfig ]);
+      type = systemdUtils.types.services;
       description = "Definition of systemd service units.";
     };
 
     systemd.sockets = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = socketOptions; } unitConfig ]);
+      type = systemdUtils.types.sockets;
       description = "Definition of systemd socket units.";
     };
 
     systemd.timers = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = timerOptions; } unitConfig ]);
+      type = systemdUtils.types.timers;
       description = "Definition of systemd timer units.";
     };
 
     systemd.paths = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = pathOptions; } unitConfig ]);
+      type = systemdUtils.types.paths;
       description = "Definition of systemd path units.";
     };
 
     systemd.mounts = mkOption {
       default = [];
-      type = with types; listOf (submodule [ { options = mountOptions; } unitConfig mountConfig ]);
+      type = systemdUtils.types.mounts;
       description = ''
         Definition of systemd mount units.
         This is a list instead of an attrSet, because systemd mandates the names to be derived from
@@ -243,7 +230,7 @@ in
 
     systemd.automounts = mkOption {
       default = [];
-      type = with types; listOf (submodule [ { options = automountOptions; } unitConfig automountConfig ]);
+      type = systemdUtils.types.automounts;
       description = ''
         Definition of systemd automount units.
         This is a list instead of an attrSet, because systemd mandates the names to be derived from
@@ -253,7 +240,7 @@ in
 
     systemd.slices = mkOption {
       default = {};
-      type = with types; attrsOf (submodule [ { options = sliceOptions; } unitConfig] );
+      type = systemdUtils.types.slices;
       description = "Definition of slice configurations.";
     };
 
@@ -352,10 +339,11 @@ in
       type = types.listOf types.str;
       example = [ "systemd-backlight@.service" ];
       description = ''
-        A list of units to suppress when generating system systemd configuration directory. This has
+        A list of units to skip when generating system systemd configuration directory. This has
         priority over upstream units, <option>systemd.units</option>, and
         <option>systemd.additionalUpstreamSystemUnits</option>. The main purpose of this is to
-        suppress a upstream systemd unit with any modifications made to it by other NixOS modules.
+        prevent a upstream systemd unit from being added to the initrd with any modifications made to it
+        by other NixOS modules.
       '';
     };
 
@@ -471,7 +459,12 @@ in
       enabledUpstreamSystemUnits = filter (n: ! elem n cfg.suppressedSystemUnits) upstreamSystemUnits;
       enabledUnits = filterAttrs (n: v: ! elem n cfg.suppressedSystemUnits) cfg.units;
     in ({
-      "systemd/system".source = generateUnits "system" enabledUnits enabledUpstreamSystemUnits upstreamSystemWants;
+      "systemd/system".source = generateUnits {
+        type = "system";
+        units = enabledUnits;
+        upstreamUnits = enabledUpstreamSystemUnits;
+        upstreamWants = upstreamSystemWants;
+      };
 
       "systemd/system.conf".text = ''
         [Manager]