deb: ensure to reload unit file before restarting (#512)

It fixes the following case:

      Failed to restart fluentd.service: Unit fluentd.service not found.
      dpkg: error processing package fluent-package (--configure):
installed fluent-package package post-installation script subprocess
returned error exit status 5

---------

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>
Co-authored-by: Daijiro Fukuda <fukuda@clear-code.com>

fluent-package/apt/systemd-test/install-newly.sh

@@ -11,4 +11,5 @@ systemctl status --no-pager fluentd
 
 sudo apt remove -y fluent-package
 
+test -h /etc/systemd/system/fluentd.service
 ! systemctl status fluentd

fluent-package/apt/systemd-test/update-from-v4.sh

@@ -27,8 +27,6 @@ sudo apt install -V -y \
 systemctl status --wait --no-pager fluentd
 ! systemctl status --wait --no-pager td-agent
 
-# TODO: There are some tests being commented out. They will be supported by future fixes.
-
 # Test: restoring td-agent service alias
 sudo systemctl unmask td-agent
 sudo systemctl enable --now fluentd
@@ -57,3 +55,6 @@ test $(eval $env_vars && echo $FLUENT_PACKAGE_LOG_FILE) = "/var/log/fluent/td-ag
 sudo apt remove -y fluent-package
 ! systemctl status --wait --no-pager td-agent
 ! systemctl status --wait --no-pager fluentd
+
+test -h /etc/systemd/system/td-agent.service
+test -h /etc/systemd/system/fluentd.service

fluent-package/templates/package-scripts/fluent-package/deb/postinst

@@ -128,6 +128,7 @@ migration_from_v4_post_process() {
     fi
     if [ "$v4migration_with_restart" = "y" ]; then
         if [ -n "$(command -v systemctl)" ]; then
+            systemctl daemon-reload
             systemctl restart <%= service_name %>
         fi
     fi