crashkernel: check if crashmemory is available

If a kernel was started with crashmemory enabled, create
/node/startup-crashmemory/available.

This in turn will allow a conditional startup of the
startup-crashmemory.service.

crashkernel/crashkernel.service

@@ -1,5 +1,8 @@
 [Unit]
-Description=load crashkernel
+Description=load crashkernel if enabled
+Wants=startup-crashmemory.service
+After=startup-crashmemory.service
+ConditionPathExists=/node/startup-crashmemory/available
 ConditionPathExists=/boot/bzImage.crash
 
 [Service]

crashkernel/startup-crashmemory

@@ -0,0 +1,21 @@
+#! /bin/sh
+
+# Workaround for systemd versions missing certain ConditionXY= features.
+# This helper is called by startup-crashmemory.service and creates
+#   /node/startup-crashmemory/available
+# So other services can use
+#   Wants=startup-crashmemory.service
+#   After=startup-crashmemory.service
+#   ConditionPathExists=/node/startup-crashmemory/available
+# to start a unit only if the memory can be used
+#
+
+kexec_crash_size='0'
+if [ -e /sys/kernel/kexec_crash_size ]; then
+  kexec_crash_size=$(cat /sys/kernel/kexec_crash_size)
+fi
+mkdir -p /node/startup-crashmemory
+rm /node/startup-crashmemory/* 2>/dev/null
+if [ "$kexec_crash_size" != "0" ] ; then
+  touch /node/startup-crashmemory/available
+fi

crashkernel/startup-crashmemory.service

@@ -0,0 +1,6 @@
+[Unit]
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=/usr/sbin/startup-crashmemory