dtprobed: make sure the daemon is restarted

Unfortunately relying on the presets and a %systemd_postun_with_restart to
enable and restart dtprobed is not enough: older installations used
%systemd_postun, so when we upgrade from one of those, the daemon is not
restarted.  The older installations themselves enabled and restarted it by
hand.  Presets take the burden of enabling from us, but not the burden of
restarting: we still have to do that, and doing it in %postun is only good
enough when this is the package being upgraded *from*.

So forcibly restart it in %post (after %systemd_post installs its service
files, etc).  This does mean that reinstallations may restart the daemon
twice, but this is harmless, and in any case only applies to OL8 and below:
OL9 amortizes all restarts and does them in %posttrans.

Doing this is tangled and ridiculous: the actual underlying method used to
restart differs on OL8-and-before and on OL9+, but the
%systemd_postun_with_restart macro does the right thing on both distros.
Unfortunately because it's meant to run from %postun it checks for the wrong
value of $1 (>= 1, meaning everything but install, while we want >= 2 which
means the same thing in %post).  So wrap the whole thing in a conditional to
prevent it double-restarting on new installations.

On OL8 and below we have another problem: the unit file has changed, but the
systemd macros on OL8 and below don't reload it except on initial
installation: and the change is essential because without it dtprobed
doesn't have permission to write anything to /run/dtrace.  So on OL7 and
OL8, do a daemon-reload before restarting.

The result appears to survive initial installation, upgrade from .12 and
.13.1, and rpm --reinstall on both OL8 and OL9.

While we're changing this, take out the mention of dtrace-usdt.target in
%systemd_preun: as a .target, it's meaningless to restart it or do anything
else %preun does to it, and naming it there causes horrible warning messages
on uninstallation.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

dtrace.spec

@@ -207,6 +207,20 @@ rm -rf $RPM_BUILD_DIR/%{name}-%{version}
 /sbin/ldconfig
 %udev_rules_update
 %systemd_post dtprobed.service
+# Force a daemon restart on upgrade even if the previous package did
+# not use presets and did not request a restart on uninstallation.
+# (When upgrades from 1.13.1 and below are no longer supported, we can
+# remove this.)
+# systemd_postun_with_restart does the right thing here, though we need
+# to wrap it in an extra conditional to make it not run on install.
+if [ $1 -ge 2 ] ; then
+    # OL8 and below don't reload the unit file properly.
+    %if "%{?dist}" == ".el7" || "%{?dist}" == ".el8"
+    systemctl daemon-reload || :
+    %endif
+
+    %systemd_postun_with_restart dtprobed.service
+fi
 
 # if sdt-systemtap.h doesn't exist then we can move the existing dtrace sdt.h
 SYSINCDIR=/usr/include/sys
@@ -218,7 +232,7 @@ elif [ ! -e $SYSINCDIR/sdt.h ]; then
 fi
 
 %preun
-%systemd_preun dtprobed.service dtrace-usdt.target
+%systemd_preun dtprobed.service
 
 %postun
 /sbin/ldconfig