Re: Reducing the chunk header sizes on all memory context types

On Tue, 6 Sept 2022 at 14:43, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I think MemoryContextContains' charter is to return
>
>         GetMemoryChunkContext(pointer) == context
>
> *except* that instead of asserting what GetMemoryChunkContext asserts,
> it should treat those cases as reasons to return false.  So if you
> can still do GetMemoryChunkContext then you can still do
> MemoryContextContains.  The point of having the separate function
> is to be as forgiving as we can of bogus pointers.

Ok.  I've readded the Asserts that c6e0fe1f2 mistakenly removed from
GetMemoryChunkContext() and changed MemoryContextContains() to do
those same pre-checks before calling GetMemoryChunkContext().

I've also boosted the Assert in mcxt.c to
Assert(MemoryContextContains(context, ret)) in each place we call the
context's callback function to obtain a newly allocated pointer.  I
think this should cover the testing.

I felt the need to keep the adjustments I made to the header comment
in MemoryContextContains() to ward off anyone who thinks it's ok to
pass this any random pointer and have it do something sane. It's much
more prone to misbehaving/segfaulting now given the extra dereferences
that c6e0fe1f2 added to obtain a pointer to the owning context.

David