One concern that I have about this --link mode, which Euler Taveira also got concerned about: the fact that it can invalidate the original backups if the user modifies or starts the synthetic backup without moving it to another file system or machine.
At the moment, pg_combinebackup issues a warning about that problem as soon as the execution of the tool is about to finish. There is also a warning in the docs. But some users don't pay proper attention to the output and/or docs. I wonder if we have some way, like pg_upgrade does (rename a global file in the original cluster), so we could increase the protection against that issue?
