For logical replication there is no need to implement this, but others are using the pgoutput plugin for Change Data Capture. The reason they are using pgoutput is because it is guaranteed to be available as it is in core postgres.
Implementing LogicalDecodeMessageCB provides some synchronization facility that is not easily replicated.
Thoughts ?
Attached is a draft patch that adds this functionality into the pgoutput plugin. A slot consumer can pass 'messages' as an option to include logical messages from pg_logical_emit_message in the replication flow.
FWIW, we have been using pg_logical_emit_message to send application-level events alongside our change-data-capture for about two years, and we would move this part of our stack to pgoutput if message support was available.
