Proposal: Add a callback data parameter to GetNamedDSMSegment

Hello hackers,

While developing an extension and trying to write some generic code
around DSM areas, I noticed a limitation: although GetNamedDSMSegment
accepts a callback function, there is no way to pass additional
arguments to that callback.

For example, the documentation for creating LWLocks after startup [1]
suggests creating locks in this callback. That works fine as long as
the callback only needs to create a hardcoded lock. But if the lock
name is a parameter to the function calling GetNamedDSMSegment, and
not fixed, I do not see a clean way to pass it through to the callback
(short of relying on global variables, for example).

As a proper solution for this, and possibly for other similar issues,
I propose adding a generic callback argument to GetNamedDSMSegment
that can be forwarded to the callback function.

What do you think about this?

[1] https://www.postgresql.org/docs/current/xfunc-c.html#XFUNC-ADDIN-LWLOCKS-AFTER-STARTUP