The way I understand this, this would only work if DefineCustomXXXVariable could only be called from a global context (e.g., shared_preload_libraries). But AFAICT, you can define custom GUC parameters per session (e.g., LOAD 'auto_explain'), and so each session would have a different offset, and so the offset variable would itself have to be session-local.
I think that a session-local LOAD is something we're going to lose with threading anyway. A shared library is only going to be loaded once for the cluster, not once per backend. And to be clear: I think that's totally fine. (i.e. the benefit of these session-local extensions seems to small for us to find and maintain some workaround for this)
So having a DefineCustomXXXVariable only be callable from a global context seems fine to me.
I quite like the idea of this global offset array btw.
