With the recent upgrade of AppEngine Golang from go111 to go112 you might have to change
your database DSNs if you’re using CloudSQL and want to connect using the
DB DSN format for CloudSQL Postgres and Golang go112:
"user=<<USER>> password=<<PWD> host=/cloudsql/<<CONNECTION NAME>>/ dbname=<<NAME>>"
The instance connection name can be found on Instance Detail page of Google Cloud Console. Make sure your AppEngine service account has at least the CloudSQL Client role.