r/promtail: Optionally run with DAC_READ_SEARCH

The *promtail* service runs as an unprivileged user by default, which is
fine in most cases (i.e. when scraping only the Journal), but may not
always be sufficient to read logs from other files.  Rather than run
Promtail as root in these cases, we can assign it the
CAP_DAC_READ_SEARCH capability, which will allow it to read any file,
but does not grant it any of root's other privileges.

To enable this functionality, the `promtail_dac_read_search` Ansible
variable can be set to `true` for a host or group.  This will create a
systemd unit configuration extension that configures the service to have
the CAP_DAC_READ_SEARCH capability in its ambient set.

roles/promtail/defaults/main.yml

@@ -1,3 +1,5 @@
+promtail_dac_read_search: false
+
 promtail_positions_file: /tmp/positions.yaml
 
 promtail_clients:

roles/promtail/handlers/main.yml

@@ -1,4 +1,8 @@
-- name: reload promtail
+- name: reload systemd
+  systemd:
+    daemon_reload: true
+
+- name: restart promtail
   service:
     name: promtail
     state: restarted

roles/promtail/tasks/deploy.yml

@@ -18,7 +18,7 @@
     owner: root
     group: root
   notify:
-  - reload promtail
+  - restart promtail
   tags:
   - config
 
@@ -31,11 +31,33 @@
     group: root
     mode: u=rw,go=r
   notify:
-  - reload promtail
+  - restart promtail
   tags:
   - config
   - cert
 
+- name: ensure promtail systemd unit extension directory exists
+  file:
+    path: /etc/systemd/system/promtail.service.d
+    owner: root
+    group: root
+    mode: u=rwx,go=rx
+    state: directory
+  tags:
+  - systemd
+- name: ensure promtail service capabilities are configured
+  template:
+    src: capabilities.conf.j2
+    dest: /etc/systemd/system/promtail.service.d/capabilities.conf
+    owner: root
+    group: root
+    mode: u=rw,go=r
+  notify:
+  - reload systemd
+  - restart promtail
+  tags:
+  - systemd
+
 - name: ensure promtail service starts at boot
   service:
     name: promtail
@@ -43,6 +65,8 @@
   tags:
   - service
 
+- meta: flush_handlers
+
 - name: ensure promtail is running
   service:
     name: promtail

roles/promtail/templates/capabilities.conf.j2

@@ -0,0 +1,4 @@
+[Service]
+{% if promtail_dac_read_search %}
+AmbientCapabilities=CAP_DAC_READ_SEARCH
+{% endif %}