Allow building/testing individual systemd units

nixos/doc/manual/development.xml

@@ -652,6 +652,37 @@ $ qemu-system-x86_64 -kernel ./kernel/bzImage -initrd ./initrd/initrd -hda /dev/
     </listitem>
   </varlistentry>
 
+  <varlistentry>
+    <term><varname>systemd.units.<replaceable>unit-name</replaceable>.unit</varname></term>
+    <listitem>
+      <para>This builds the unit with the specified name.  Note that
+      since unit names contain dots
+      (e.g. <literal>httpd.service</literal>), you need to put them
+      between quotes, like this:
+
+<screen>
+$ nix-build -A 'config.systemd.units."httpd.service".unit'
+</screen>
+
+      You can also test individual units, without rebuilding the whole
+      system, by putting them in
+      <filename>/run/systemd/system</filename>:
+
+<screen>
+$ cp $(nix-build -A 'config.systemd.units."httpd.service".unit')/httpd.service \
+    /run/systemd/system/tmp-httpd.service
+$ systemctl daemon-reload
+$ systemctl start tmp-httpd.service
+</screen>
+
+      Note that the unit must not have the same name as any unit in
+      <filename>/etc/systemd/system</filename> since those take
+      precedence over <filename>/run/systemd/system</filename>.
+      That’s why the unit is installed as
+      <filename>tmp-httpd.service</filename> here.</para>
+    </listitem>
+  </varlistentry>
+
 </variablelist>
 
 </para>

nixos/modules/system/boot/systemd.nix

@@ -311,8 +311,6 @@ let
         '';
     };
 
-  nixosUnits = mapAttrsToList makeUnit cfg.units;
-
   units = pkgs.runCommand "units" { preferLocalBuild = true; }
     ''
       mkdir -p $out
@@ -338,7 +336,7 @@ let
         done
       done
 
-      for i in ${toString nixosUnits}; do
+      for i in ${toString (mapAttrsToList (n: v: v.unit) cfg.units)}; do
         ln -s $i/* $out/
       done
 
@@ -387,32 +385,41 @@ in
       description = "Definition of systemd units.";
       default = {};
       type = types.attrsOf types.optionSet;
-      options = {
+      options = { name, config, ... }:
-        text = mkOption {
+        { options = {
-          type = types.str;
+            text = mkOption {
-          description = "Text of this systemd unit.";
+              type = types.str;
+              description = "Text of this systemd unit.";
+            };
+            enable = mkOption {
+              default = true;
+              type = types.bool;
+              description = ''
+                If set to false, this unit will be a symlink to
+                /dev/null. This is primarily useful to prevent specific
+                template instances (e.g. <literal>serial-getty@ttyS0</literal>)
+                from being started.
+              '';
+            };
+            requiredBy = mkOption {
+              default = [];
+              type = types.listOf types.string;
+              description = "Units that require (i.e. depend on and need to go down with) this unit.";
+            };
+            wantedBy = mkOption {
+              default = [];
+              type = types.listOf types.string;
+              description = "Units that want (i.e. depend on) this unit.";
+            };
+            unit = mkOption {
+              internal = true;
+              description = "The generated unit.";
+            };
+          };
+          config = {
+            unit = makeUnit name config;
+          };
         };
-        enable = mkOption {
-          default = true;
-          type = types.bool;
-          description = ''
-            If set to false, this unit will be a symlink to
-            /dev/null. This is primarily useful to prevent specific
-            template instances (e.g. <literal>serial-getty@ttyS0</literal>)
-            from being started.
-          '';
-        };
-        requiredBy = mkOption {
-          default = [];
-          type = types.listOf types.string;
-          description = "Units that require (i.e. depend on and need to go down with) this unit.";
-        };
-        wantedBy = mkOption {
-          default = [];
-          type = types.listOf types.string;
-          description = "Units that want (i.e. depend on) this unit.";
-        };
-      };
     };
 
     systemd.packages = mkOption {