Re: Custom allocators in libpq

On Mon, Aug 28, 2017 at 03:11:26PM -0400, Tom Lane wrote:
> Aaron Patterson <tenderlove(at)ruby-lang(dot)org> writes:
> > I would like to be able to configure libpq with custom malloc functions.
>
> I can see the potential value of this ...
>
> > This patch doesn't replace all malloc calls to the configured ones, just
> > the mallocs related to creating result objects (which is what I'm
> > concerned with).
>
> ... but it seems like you're giving up a lot of the possible uses if
> you don't make it apply uniformly.

I'm happy to make the changes uniformly! I'll do that and update the
patch.

> I admit I'm not sure how we'd handle
> the initial creation of a connection object with a custom malloc. The
> obvious solution of requiring the functions to be specified at PQconnect
> time seems to require Yet Another PQconnect Variant, which is not very
> appetizing.

Other libraries I've worked with allow me to malloc a struct, then pass
it to an initialization function. This might take a bit of refactoring,
like introducing a new `PQconnectStart`, but might be worth while.

> I also wonder whether you wouldn't want a passthrough argument.
> For instance, one of the use-cases that springs to mind immediately is
> teaching postgres_fdw and dblink to use this so that their result objects
> are palloc'd not malloc'd, allowing removal of lots of PG_TRY overhead.
> While I suppose we could have the hook functions always allocate in
> CurrentMemoryContext, it'd likely be useful to be able to specify
> "use context X" at creation time.

We don't need this for the Ruby wrapper, but I've seen other libraries
do it. I'm happy to add it as well.

Thanks!

--
Aaron Patterson
http://tenderlovemaking.com/