Do we have a way to dynamically make a callback stub?

Hi,

A lot of the APIs in PostgreSQL that accept a callback follow the
familiar idiom of an extra void* argument allowing a single static
callback address to be multiplexed. But not all of them do. For example,
if I wanted to expose the possibility of defining GUCs from code written
in my PL, I would run into the fact that none of GUC's check/assign/show
hooks have the void* extra arg.

(The GUC machinery allows a way for extra info to be passed from a check
hook to an assign hook, which for a moment I thought might be abusable to
multiplex hooks, but I don't believe it is.)

Making all such APIs follow the void *extra convention might have
the virtue of consistency, but that might not be worth disturbing APIs
that have been stable for many years, and an effort to do so
might not be guaranteed to catch every such instance anyway.

A more general solution might be a function that generates a callback
stub: "please give me a void (*foo)() at a distinct address that I can
pass into this API, and when called it will call bar(baz), passing
this value for baz."

In olden days I wasn't above writing C to just slam those instructions
on the stack and return their address, but that was without multiple
architectures to think about, and non-executable stacks, and so on.
Now, there'd be a bit more magic required. Maybe some such ability is
already present in LLVM and could be exposed in jit.c?

I see that Java is currently incubating such a feature [0], so if I wait
for that I will have another option that serves my specific purposes, but
I wonder if it would be useful for PostgreSQL itself to have such
a capability available (or if it already does, and I haven't found it).

Regards,
-Chap

[0]
https://docs.oracle.com/en/java/javase/16/docs/api/jdk.incubator.foreign/jdk/incubator/foreign/CLinker.html#upcallStub(java.lang.invoke.MethodHandle,jdk.incubator.foreign.FunctionDescriptor)