Is custom MemoryContext prohibited?

Hello,

I noticed MemoryContextIsValid() called by various kinds of memory context
routines checks its node-tag as follows:

#define MemoryContextIsValid(context) \
((context) != NULL && \
(IsA((context), AllocSetContext) || \
IsA((context), SlabContext) || \
IsA((context), GenerationContext)))

It allows only "known" memory context methods, even though the memory context
mechanism enables to implement custom memory allocator by extensions.
Here is a node tag nobody used: T_MemoryContext.
It looks to me T_MemoryContext is a neutral naming for custom memory context,
and here is no reason why memory context functions prevents custom methods.

https://github.com/heterodb/pg-strom/blob/master/src/shmbuf.c#L1243
I recently implemented a custom memory context for shared memory allocation
with portable pointers. It shall be used for cache of pre-built gpu
binary code and
metadata cache of apache arrow files.
However, the assertion check above requires extension to set a fake node-tag
to avoid backend crash. Right now, it is harmless to set T_AllocSetContext, but
feel a bit bad.

Best regards,
--
HeteroDB, Inc / The PG-Strom Project
KaiGai Kohei <kaigai(at)heterodb(dot)com>