pdist-bootcheck: Don't loop when system disk is full

This script uses a file `/var/run/updatecheck.reboot-triggered` to
prevent boot loops on pdist failures when pdist+reboot is required.
However, when the system disk is full, the creation of that file might
fail.

Check for reasonable free space before attempting pdist and create the
flag file before pdist is started.

pdist/pdist-bootcheck

@@ -35,10 +35,15 @@ elif [ "$need_pdist_and_reboot" ]; then
         rm /var/run/updatecheck.reboot-triggered
         exit 1
     fi
+    if (( $(stat -f -c %f /) * $(stat -f -c %s /) /1024/1024/1024 < 10)); then
+        echo "Update and reboot required but prevented because of insufficient free disk space (<10G)"
+        exit 1
+    fi
     echo "Update and reboot required"
+    touch /var/run/updatecheck.reboot-triggered
+    sync /
     while true; do sleep 5; echo "System update in progress. Please be patient..."; done &
     netcat $(distmaster) 237
     mxgrub default
-    touch /var/run/updatecheck.reboot-triggered
     systemctl start reboot.target --job-mode=replace-irreversibly
 fi