Enable the service that creates the file share symlink

This service is a systemd user-mode service for the vnc user.  Note
that we have to enable linger for the vnc user before enabling the
service, and we disable it afterwards.  This is just a trick to get
the vnc user's user-specific systemd session to start without actually
logging in as that user.

src/vnc.yml

@@ -23,6 +23,50 @@
             groups:
               - wheel
             name: "{{ username }}"
+    - name: >-
+        Enable and start the user-mode systemd service that creates
+        the file share symlink for the vnc user
+      block:
+        - name: Start and enable systemd-logind
+          service:
+            enabled: yes
+            name: systemd-logind
+            state: started
+        # This causes the vnc user's user-specific systemd session to
+        # start on boot instead of only when that user logs in.  We
+        # need the vnc user's session to be active so we can enable
+        # the user-mode systemd service, so we use the trick of
+        # enabling linger for the user, enabling the user-mode
+        # service, then disabling linger for the user.
+        - name: Enable linger for vnc user
+          ansible.builtin.command:
+            argv:
+              - /bin/loginctl
+              - enable-linger
+              - "{{ username }}"
+        - name: >-
+            Enable the user-mode systemd service that creates the file
+            share symlink for the vnc user
+          ansible.builtin.systemd:
+            daemon_reload: yes
+            enabled: yes
+            name: create-fileshare-symlink
+            scope: user
+          become_user: "{{ username }}"
+          vars:
+            # This is necessary to get around the difficult case where
+            # you ssh into a machine as an unprivileged user _and_
+            # become an unprivileged user:
+            # https://docs.ansible.com/ansible/latest/user_guide/become.html#risks-of-becoming-an-unprivileged-user
+            ansible_ssh_pipelining: yes
+        # Now that the user-mode service has been enabled we can
+        # disable linger for the vnc user.
+        - name: Disable linger for vnc user
+          ansible.builtin.command:
+            argv:
+              - /bin/loginctl
+              - disable-linger
+              - "{{ username }}"
   vars:
     # The username, password, and ssh keys for the VNC user
     username: "{{ lookup('aws_ssm', '/vnc/username') }}"