Allow upstream systemd units to be extended

If you define a unit, and either systemd or a package in
systemd.packages already provides that unit, then we now generate a
file /etc/systemd/system/<unit>.d/overrides.conf. This makes it
possible to use upstream units, while allowing them to be customised
from the NixOS configuration. For instance, the module nix-daemon.nix
now uses the units provided by the Nix package. And all unit
definitions that duplicated upstream systemd units are finally gone.

This makes the baseUnit option unnecessary, so I've removed it.

nixos/modules/system/boot/systemd.nix

@@ -31,7 +31,6 @@ let
       "sockets.target"
       "graphical.target"
       "multi-user.target"
-      "getty.target"
       "network.target"
       "network-online.target"
       "nss-lookup.target"
@@ -50,10 +49,17 @@ let
       # Udev.
       "systemd-udevd-control.socket"
       "systemd-udevd-kernel.socket"
-      #"systemd-udevd.service"
+      "systemd-udevd.service"
       "systemd-udev-settle.service"
       "systemd-udev-trigger.service"
 
+      # Consoles.
+      "getty.target"
+      "getty@.service"
+      "serial-getty@.service"
+      "container-getty@.service"
+      "systemd-vconsole-setup.service"
+
       # Hardware (started by udev when a relevant device is plugged in).
       "sound.target"
       "bluetooth.target"
@@ -80,7 +86,8 @@ let
       "systemd-initctl.service"
 
       # Kernel module loading.
-      #"systemd-modules-load.service"
+      "systemd-modules-load.service"
+      "kmod-static-nodes.service"
 
       # Filesystems.
       "systemd-fsck@.service"
@@ -146,6 +153,9 @@ let
       "systemd-tmpfiles-clean.timer"
       "systemd-tmpfiles-setup.service"
       "systemd-tmpfiles-setup-dev.service"
+
+      # Misc.
+      "systemd-sysctl.service"
     ]
 
     ++ optionals cfg.enableEmergencyMode [
@@ -198,7 +208,7 @@ let
 
   serviceConfig = { name, config, ... }: {
     config = mkMerge
-      [ (mkIf (config.baseUnit == null) { # Default path for systemd services.  Should be quite minimal.
+      [ { # Default path for systemd services.  Should be quite minimal.
           path =
             [ pkgs.coreutils
               pkgs.findutils
@@ -207,7 +217,7 @@ let
               systemd
             ];
           environment.PATH = config.path;
-        })
+        }
         (mkIf (config.preStart != "")
           { serviceConfig.ExecStartPre = makeJobScript "${name}-pre-start" ''
               #! ${pkgs.stdenv.shell} -e
@@ -275,10 +285,7 @@ let
         (if isList value then value else [value]))
         as));
 
-  commonUnitText = def:
-    optionalString (def.baseUnit != null) ''
-      .include ${def.baseUnit}
-    '' + ''
+  commonUnitText = def: ''
       [Unit]
       ${attrsToSection def.unitConfig}
     '';
@@ -358,6 +365,8 @@ let
   units = pkgs.runCommand "units" { preferLocalBuild = true; }
     ''
       mkdir -p $out
+
+      # Copy the upstream systemd units we're interested in.
       for i in ${toString upstreamUnits}; do
         fn=${systemd}/example/systemd/system/$i
         if ! [ -e $fn ]; then echo "missing $fn"; false; fi
@@ -368,6 +377,8 @@ let
         fi
       done
 
+      # Copy .wants links, but only those that point to units that
+      # we're interested in.
       for i in ${toString upstreamWants}; do
         fn=${systemd}/example/systemd/system/$i
         if ! [ -e $fn ]; then echo "missing $fn"; false; fi
@@ -376,18 +387,35 @@ let
         for i in $fn/*; do
           y=$x/$(basename $i)
           cp -pd $i $y
-          if ! [ -e $y ]; then rm -v $y; fi
+          if ! [ -e $y ]; then rm $y; fi
         done
       done
 
-      for i in ${toString (mapAttrsToList (n: v: v.unit) cfg.units)}; do
-        ln -fs $i/* $out/
-      done
-
+      # Symlink all units provided listed in systemd.packages.
       for i in ${toString cfg.packages}; do
-        ln -s $i/etc/systemd/system/* $out/
+        ln -s $i/etc/systemd/system/* $i/lib/systemd/system/* $out/
       done
 
+      # Symlink all units defined by systemd.units. If these are also
+      # provided by systemd or systemd.packages, then add them as
+      # <unit-name>.d/overrides.conf, which makes them extend the
+      # upstream unit.
+      for i in ${toString (mapAttrsToList (n: v: v.unit) cfg.units)}; do
+        fn=$(basename $i/*)
+        if [ -e $out/$fn ]; then
+          if [ "$(readlink -f $i/$fn)" = /dev/null ]; then
+            ln -sfn /dev/null $out/$fn
+          else
+            mkdir $out/$fn.d
+            ln -s $i/$fn $out/$fn.d/overrides.conf
+          fi
+       else
+          ln -fs $i/$fn $out/
+        fi
+      done
+
+      # Created .wants and .requires symlinks from the wantedBy and
+      # requiredBy options.
       ${concatStrings (mapAttrsToList (name: unit:
           concatMapStrings (name2: ''
             mkdir -p $out/'${name2}.wants'
@@ -400,6 +428,7 @@ let
             ln -sfn '../${name}' $out/'${name2}.requires'/
           '') unit.requiredBy) cfg.units)}
 
+      # Stupid misc. symlinks.
       ln -s ${cfg.defaultUnit} $out/default.target
 
       ln -s rescue.target $out/kbrequest.target
@@ -411,7 +440,7 @@ let
             ../nss-user-lookup.target ../swap.target $out/multi-user.target.wants/
 
       ${ optionalString config.services.journald.enableHttpGateway ''
-      ln -s ../systemd-journal-gatewayd.service $out/multi-user-target.wants/
+        ln -s ../systemd-journal-gatewayd.service $out/multi-user-target.wants/
       ''}
     ''; # */