Re: allowing broader use of simplehash

Hi,

On 2019-12-11 10:05:00 -0500, Robert Haas wrote:
> On Tue, Dec 10, 2019 at 4:59 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > > A significant problem in either case is that a simplehash wants to
> > > live in a memory context; no such thing exists either for data in
> > > shared memory nor in frontend code. However, it seems to be quite easy
> > > to provide a way for simplehash to be defined so that it doesn't care
> > > about memory contexts. See 0001.
> >
> > I wonder if we shouldn't instead just go for an "implicit" memory
> > context instead. It's a bit ugly to have a growing set of different
> > signatures.
>
> I don't really know what you mean by this. I don't actually think the
> different signatures are a big deal. It affects a pretty limited
> number of functions, and that seems less ugly than trying to create
> some sort of dummy not-really-a-context object that can live in
> frontend code, and a lot easier than actually making contexts work in
> frontend code. The latter might be the better long-term solution, but
> I don't think we should insist on doing it first.

I was basically just thinking that we could pass the context to use via
CurrentMemoryContext, instead of explicitly passing it in.

> Another way forward would be to replace the MemoryContext references
> with a void * that happens, in the case of the backend, to be a
> MemoryContext, and could be NULL when none is required. However, that
> would give up some type-checking for no current benefit. If simplehash
> becomes more widely used and at some point it's clear that this would
> be a net win, we can change it then.

Yea, that seems worse. I'd rather work on the MemoryContext
infrastructure being available for frontend code.

Greetings,

Andres Freund