Hallo Andres,
And you're basically adding it because Fabien doesn't like postmaster.pid and wants to invent another lockout mechanism in this thread.
I did not suggest to rename the control file, but as it is already done by another command it did not look like a bad idea in itself, or at least an already used bad idea:-)
I'd be okay with anything that works consistently accross all commands that may touch a cluster and are mutually exclusive (postmater, pg_rewind, pg_resetwal, pg_upgrade, pg_checksums…), without underlying race conditions. It could be locking, a control file state, a special file (which one ? what is the procedure to create/remove it safely and avoid potential race conditions ?), possibly "postmaster.pid", whatever really.
I'll admit that I'm moderately enthousiastic about "posmaster.pid" because it does not do anymore what the file names says, but if it really works and is used consistently by all commands, why not. In case of unexpected problems, the file will probably have to be removed/fixed by hand. I also think that the implemented mechanism should be made available in "control_utils.c", not duplicated in every command.
-- Fabien.
