Honor masking of systemd-sysctl.service

During package install on systemd-based systems, some sysctl settings
should be set (e.g. vm.max_map_count).

In some environments, changing sysctl settings plainly does not work;
previously a global environment variable named
ES_SKIP_SET_KERNEL_PARAMETERS was introduced to skip calling sysctl, but
this causes trouble for:
 - configuration management systems, which usually cannot apply an env
   var when running a package manager
 - package upgrades, which will not have the env var set any more, and
   thus leaving the package management system in a bad state (possibly
   half-way upgraded, can be very hard to recover)

This removes the env var again and instead of calling systemd-sysctl
manually, tells systemd to restart the wrapper unit - which itself can
be masked by system administrators or management tools if it is known
that sysctl does not work in a given environment.

The restart is not silent on systems in their default configuration, but
is ignored if the unit is masked.

Relates #24234

distribution/src/main/packaging/scripts/postinst

@@ -48,17 +48,8 @@ case "$1" in
 esac
 
 # to pick up /usr/lib/sysctl.d/elasticsearch.conf
-if [ "${ES_SKIP_SET_KERNEL_PARAMETERS:-false}" == "false" ]; then
-    if command -v /usr/lib/systemd/systemd-sysctl > /dev/null; then
-        /usr/lib/systemd/systemd-sysctl
-    elif command -v /lib/systemd/systemd-sysctl > /dev/null; then
-        /lib/systemd/systemd-sysctl
-    fi
-elif [ "$ES_SKIP_SET_KERNEL_PARAMETERS" == "true" ]; then
-    echo "skipping setting kernel parameters"
-else
-    echo "unrecognized value [$ES_SKIP_SET_KERNEL_PARAMETERS] for ES_SKIP_SET_KERNEL_PARAMETERS; must [false] (default) or [true]"
-    exit 1
+if command -v systemctl > /dev/null; then
+    systemctl restart systemd-sysctl.service
 fi
 
 if [ "x$IS_UPGRADE" != "xtrue" ]; then

docs/reference/setup/install/skip-set-kernel-parameters.asciidoc

@@ -1,2 +1,2 @@
 NOTE: On systemd-based distributions, the installation scripts will attempt to set kernel parameters (e.g.,
-`vm.max_map_count`); you can skip this by setting the environment variable `ES_SKIP_SET_KERNEL_PARAMETERS` to `true`.
+`vm.max_map_count`); you can skip this by masking the systemd-sysctl.service unit.