Re: display hot standby state in psql prompt

Hmmm... we can at least get default_transaction_read_only. As fe-connect.c points out:

                    /*

                     * "transaction_read_only = on" proves that at least one
                     * of default_transaction_read_only and in_hot_standby is
                     * on, but we don't actually know which.  We don't care
                     * though for the purpose of identifying a read-only
                     * session, so satisfy the CONNECTION_CHECK_TARGET code by
                     * claiming they are both on.  On the other hand, if it's
                     * a read-write session, they are certainly both off.
                     */

Maybe that's good enough? It won't detect people starting a new transaction and declaring it read-only, but it should be sufficient to warn people when a connection is starting out in a read-only state. And it will still toggle auto-magically on promotion.