feat(unit): add dropin capability for unit files

This allows to create drop-ins for unit files as described in
https://www.freedesktop.org/software/systemd/man/systemd.unit.html.

Either create a full unit or create a dropin for an existing unit to
alter some details.

Caveat: no top-level drop-ins are supported

pillar.example

@@ -69,6 +69,16 @@ systemd:
       Install:
         WantedBy: multi-user.target
 
+    systemd-journald:
+      enabled: true
+      status: start
+      dropin: true
+      Unit:
+        Description: Fancy new description
+      Service:
+        WatchdocSec: 2min
+        RestartSec: 2
+
   path:
     trigger-service-on-changes:
       # this parameter is passed to systemctl to enable/disable the unit

systemd/units/init.sls

@@ -7,10 +7,16 @@ include:
   {% if unittype in unittypes.get('Valid') %}
     {% for unit, unitconfig in units.items() %}
       {% set unit_status = 'disable' if unitconfig.enabled is defined and unitconfig.enabled == false else 'enable' %}
+      {% set dropin = unitconfig.dropin | default(false) %}
 
 systemd_systemd_units_file_{{ unit }}_{{ unittype }}:
   file.managed:
+      {%- if dropin %}
+    - name: /etc/systemd/system/{{ unit }}.{{ unittype }}.d/salt-override.conf
+    - makedirs: True
+      {%- else %}
     - name: /etc/systemd/system/{{ unit }}.{{ unittype }}
+      {%- endif %}
     - template: jinja
     - source: salt://systemd/units/unit.jinja
     - context:

systemd/units/unit.jinja

@@ -2,7 +2,7 @@
 # This file managed by Salt, do not edit
 #
 {% for section, sectioncontent in config.items() %}
-  {%- if section not in ['enabled', 'status'] %}
+  {%- if section not in ['enabled', 'status', 'dropin'] %}
 [{{ section }}]
     {%- for key, value in sectioncontent.items() %}
       {%- if value is list %}

test/integration/default/controls/unit_spec.rb

@@ -22,6 +22,7 @@
     its('group') { should eq 'root' }
     its('content') { should_not include '[status]' }
     its('content') { should_not include '[enabled]' }
+    its('content') { should_not include '[dropin]' }
     its('content') { should include 'Documentation=man:rsync(1) man:rsyncd.conf(5)' }
     its('content') { should include 'ConditionPathExists=/etc/rsyncd.conf' }
     its('content') { should include 'ConditionPathExists=/etc/passwd' }
@@ -34,4 +35,20 @@
     it { should be_enabled }
     it { should be_running }
   end
+
+  describe file('/etc/systemd/system/systemd-journald.service.d/salt-override.conf') do
+    its('type') { should eq :file }
+    its('mode') { should cmp '0644' }
+    its('owner') { should eq 'root' }
+    its('group') { should eq 'root' }
+    its('content') { should_not include '[status]' }
+    its('content') { should_not include '[enabled]' }
+    its('content') { should_not include '[dropin]' }
+    its('content') { should include '[Unit]' }
+    its('content') { should include 'Description=Fancy new description' }
+    its('content') { should include '[Service]' }
+    its('content') { should include 'WatchdocSec=2min' }
+    its('content') { should include 'RestartSec=2' }
+    its('content') { should_not include '[Install]' }
+  end
 end