Re: Add bump memory context type and use it for tuplesorts

Thanks for taking an interest in this.

On Sat, 17 Feb 2024 at 11:46, Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
> I wasn't paying much attention to these memcontext reworks in 2022, so
> my instinct was simply to use one of those "UNUSED" IDs. But after
> looking at the 80ef92675823 a bit more, are those IDs really unused? I
> mean, we're relying on those values to detect bogus pointers, right? So
> if we instead start using those values for a new memory context, won't
> we lose the ability to detect those issues?

I wouldn't say we're "relying" on them. Really there just there to
improve debugability. If we call any code that tries to look at the
MemoryChunk header of a malloc'd chunk, then we can expect bad things
to happen. We no longer have any code which does this.
MemoryContextContains() did, and it's now gone.

> Maybe I'm completely misunderstanding the implication of those limits,
> but doesn't this mean the claim that we can support 8 memory context
> types is not quite true, and the limit is 4, because the 4 IDs are
> already used for malloc stuff?

I think we all expected a bit more pain from the memory context
change. I was happy that Tom did the extra work to look at the malloc
patterns of glibc, but I think there's been very little gone wrong.
The reserved MemoryContextMethodIDs do seem to have allowed [1] to be
found, but I guess there'd have been a segfault instead of an ERROR
without the reserved IDs.

I've attached version 2, now split into 2 patches.

0001 for the bump allocator
0002 to use the new allocator for tuplesorts

David

[1] https://postgr.es/m/796b65c3-57b7-bddf-b0d5-a8afafb8b627@gmail.com