Ok so I have a proof of concept patch here.
This is proof of concept only. It odes not change documentation or the like.
The purpose of the patch is discussion on the "do we want this" side.
The patch is fairly trivial but I have not added test cases or changed docs yet.
Intention of the patch:
pg_rewind is an important backbone tool for recovering data directories following a switchover. However currently it is over inclusive as to what it copies. This patch excludes any file ending in "serverlog", ".conf", and ".log" because these are never directly related and add a great deal of complexity to switchovers.
.conf files are excluded for two major reasons. The first is that often we may want to put postgresql.conf and other files in the data directory, and if we change these during switchover this can change, for example, the port the database is running on or other things that can break production or testing environments. This is usually a problem with testing environments, but it could happen with production environments as well.
A much larger concern with .conf files though is the recovery.conf. This file MUST be put in the data directory, and it helps determine the replication topology regarding cascading replication and the like. If you rewind from an upstream replica, you suddenly change the replication topology and that can have wide-ranging impacts.
I think we are much better off insisting that .conf files should be copied separately because the scenarios where you want to do so are more limited and the concern often separate from rewinding the timeline itself.
The second major exclusion added are files ending in "serverlog" and ".log." I can find no good reason why server logs from the source should *ever* clobber those on the target. If you do this, you lose historical information relating to problems and introduce management issues.
Backwards-incompatibility scenarios:
If somehow one has a workflow that depends on copying .conf files, this would break that. I cannot think of any cases where anyone would actually want to do this but that doesn't mean they aren't out there. If people really want to, then they need to copy the configuration files they want separately.
Next Steps:
If people like this idea I will add test cases and edit documentation as appropriate.