dev-tools/packaging/templates/rpm: add warning when /etc/rc.d/init.d/…

…functions is missing (#36188)

On RHEL9, the functions script is no longer present resulting in failure
when init-script based service management commands are used. We still
support at least one distribution that provides init-script based
service management, so we need to retain these scripts, but provide a
warning to users on systems where these will not work pointing to the
new approach.

CHANGELOG.next.asciidoc

@@ -211,6 +211,7 @@ automatic splitting at root level, if root level element is an array. {pull}3415
 
 - Added append Processor which will append concrete values or values from a field to target. {issue}29934[29934] {pull}33364[33364]
 - When running under Elastic-Agent the status is now reported per Unit instead of the whole Beat {issue}35874[35874] {pull}36183[36183]
+- Add warning message to SysV init scripts for RPM-based systems that lack `/etc/rc.d/init.d/functions`. {issue}35708[35708] {pull}36188[36188]
 
 *Auditbeat*
 

dev-tools/packaging/templates/rpm/elastic-agent.init.sh.tmpl

@@ -17,6 +17,13 @@
 #                    family. Please see: https://www.elastic.co/beats
 ### END INIT INFO
 
+# Exit with an informative message if functions does not exist.
+# This has been removed from RHEL9, but exists on other RPM-based
+# distributions in the support matrix.
+if [ ! -f /etc/rc.d/init.d/functions ]; then
+	echo "SysV-style init script service management is not supported on this system, use systemctl instead"
+	exit 1
+fi
 
 
 PATH=/usr/bin:/sbin:/bin:/usr/sbin

dev-tools/packaging/templates/rpm/init.sh.tmpl

@@ -17,6 +17,13 @@
 #                    family. Please see: https://www.elastic.co/beats
 ### END INIT INFO
 
+# Exit with an informative message if functions does not exist.
+# This has been removed from RHEL9, but exists on other RPM-based
+# distributions in the support matrix.
+if [ ! -f /etc/rc.d/init.d/functions ]; then
+	echo "SysV-style init script service management is not supported on this system, use systemctl instead"
+	exit 1
+fi
 
 
 PATH=/usr/bin:/sbin:/bin:/usr/sbin