Technically, I didn't upgrade from 9.1 to 9.3, I built a new server with Ubuntu 14.04 and used my old 9.1 config files. PostgreSQL wouldn't start:
* Starting PostgreSQL 9.3 database server * The PostgreSQL server failed to start. Please check the log output.
but the logs were empty.
Eventually, after comparing the stock 9.3 postgresql.conf file and my 9.1 file I figured out that, at some point, the config variable
unix_socket_directory had been changed to
unix_socket_directories (not that easy to pick up in a diff with many changes and comments after each line).
Posted because I couldn't find anything on the net about this (until after I already knew what the issue was).