Don't trigger udev if socket is not accessible

In container environments or rpm-ostree systems, scriptlets are not
directly run on a host system. Thus, there's no point in asking udev to
reload and retrigger: the host udev wouldn't see the new rules anyway.

Let's just generalize this by checking if the udev socket even exists.
If not, then we can't talk to `udevd` and we should just skip the reload
& trigger bits.

Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1753786

udisks2.spec

@@ -59,7 +59,7 @@
 Name:    udisks2
 Summary: Disk Manager
 Version: 2.8.4
-Release: 2%{?dist}
+Release: 3%{?dist}
 License: GPLv2+
 URL:     https://github.com/storaged-project/udisks
 Source0: https://github.com/storaged-project/udisks/releases/download/udisks-%{version}/udisks-%{version}.tar.bz2
@@ -307,8 +307,12 @@ chrpath --delete %{buildroot}/%{_libexecdir}/udisks2/udisksd
 %post -n %{name}
 %systemd_post udisks2.service
 %systemd_post clean-mount-point@.service
-udevadm control --reload
-udevadm trigger
+# skip retriggering if udevd isn't even accessible, e.g. containers or
+# rpm-ostree-based systems
+if [ -S /run/udev/control ]; then
+    udevadm control --reload
+    udevadm trigger
+fi
 
 %preun -n %{name}
 %systemd_preun udisks2.service
@@ -430,6 +434,9 @@ udevadm trigger
 %endif
 
 %changelog
+* Tue Oct 01 2019 Jonathan Lebon <jonathan@jlebon.com> - 2.8.4-3
+- Don't trigger udev if socket is not accessible
+
 * Sat Jul 27 2019 Fedora Release Engineering <releng@fedoraproject.org> - 2.8.4-2
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_31_Mass_Rebuild