I'm seeing a lot of this error from my GCE instance to my Cloud SQL instance:
lost connection to mysql server at 'reading authorization packet' system error: 4
Particularly this last Friday and Saturday. I am not sure why this would be happening, since I have my GCE instance fully allowed to access the Cloud SQL instance. I also don't see this on other setups where I am not using GCE/Cloud SQL, but using the same software app & setup (other than the Google hosting).
This doesn't seem normal, but maybe I've overlooked some setting or configuration.