Absolutely awful hack to sync data before reboot

I really don't want to talk about it, OK? Just read the patch and leave
me alone so I can drink until I forget about this.

90system-upgrade/upgrade-post.sh

@@ -15,4 +15,20 @@ getarg 'rd.upgrade.break=post' 'rd.break=upgrade-post' && \
     emergency_shell -n upgrade-post "Break before upgrade-post hook"
 source_hook upgrade-post
 
+# absolutely godawful hack workaround garbage that I am ashamed of:
+# Since we can't figure out how to unmount the root device properly
+# (it's inaccessable because we moved system-upgrade-root on top of it)
+# we need some other way to make sure all the data that might be pending
+# actually gets written to disk. So...
+
+startmsg="upgrade-post pid $$ doing emergency sync"
+echo 1 > /proc/sys/kernel/sysrq  # enable sysrq
+echo "$startmsg" > /dev/kmsg     # add marker to dmesg
+echo s > /proc/sysrq-trigger     # start sync
+for ((i=0; i<20; i++)); do       # wait up to 20s for sync to finish
+    dmesg | sed "1,/$startmsg/d" | grep -q 'Emergency Sync complete' && break
+    sleep 1
+done
+sleep 2                          # sleep a little more just to be sure
+
 exit 0