Merge pull request #773 from joseivanlopez/service_actions

Add actions to SystemService

library/systemd/README.md

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+# Systemd actions
+
+The following table describes the actions to perform over a systemd service and its associated socked (if any) when we try to start, stop, restart or reload the service.
+
+
+| Status | Start | Stop | Restart | Reload |
+|---|---|---|---|---|
+| Socket running | nothing | stop socket | stop socket and start service/socket (depending on start mode) | nothing |
+| Service running | nothing  | stop service  | stop service and start service/socket (depending on start mode) | reload service (if it support, otherwise restart) |
+| Socket & Service running | nothing | stop service and socket | stop service and start service/socket (depending on start mode) | reload service (if it support, otherwise restart) |
+| nothing runs | start service/socket (depending on start mode) | nothing | start service/socket (depending on start mode) | start service/socket (depending on start mode) |
+
+
+## Actions over a `SystemService`
+
+In systemd, it might happen that a service is compose by a set of units (services, sockets, paths and so on). This class `SystemService` is able to group those units and handle them together.
+
+### `SystemService` status
+
+* [1] A service is stopped when neither the systemd socket nor service are running
+* [2] A service is running when the systemd socket and/or service are running
+
+### Start
+
+* Target
+  * when the service is stopped (see [1]), the service is started again (see [2])
+
+* Actions
+  * when neither systemd socket nor service are running
+    * and start mode is `on demand`
+      * starts systemd socket
+    * and start mode is `on boot` or `manually`
+      * starts systemd service
+
+### Stop
+
+* Target
+  * service is stopped (see [1])
+
+* Actions
+  * stops systemd service if it is running
+  * stops systemd socket if it is running
+
+### Restart
+
+* Target
+  * service is stopped (see [1]) and started again (see [2])
+
+* Actions
+  * calls to "stop service action" (see stop section) if the service is running (see [2])
+  * calls to "start service action" (see start section)
+
+### Reload
+
+* Target
+  * service is reloaded or restarted or started
+
+* Actions
+  * when the service supports reload
+    * and start mode is `on demand`
+      * and systemd socket is running
+        * and systemd service is running
+          * reloads systemd service
+      * and systemd socket is not running
+        * and systemd service is running
+          * reloads systemd service
+        * and systemd service is not running
+          * calls to "start service action" (see start section)
+    * and start mode is `on boot` or `manually`
+      * stops systemd socket if it is running
+      * and systemd service is running
+        * reloads systemd service
+      * and systemd service is not running
+        * starts systemd service
+  * when service does not support reload
+    * calls to "restart service action" (see restart section)