Re: Add LZ4 compression in pg_dump

On Mon, Jan 16, 2023 at 11:54:46AM +0900, Michael Paquier wrote:
> On Sun, Jan 15, 2023 at 07:56:25PM -0600, Justin Pryzby wrote:
> > On Mon, Jan 16, 2023 at 10:28:50AM +0900, Michael Paquier wrote:
> >> The functions changed by 0001 are cfopen[_write](),
> >> AllocateCompressor() and ReadDataFromArchive(). Why is it a good idea
> >> to change these interfaces which basically exist to handle inputs?
> >
> > I changed to pass pg_compress_specification as a pointer, since that's
> > the usual convention for structs, as followed by the existing uses of
> > pg_compress_specification.
>
> Okay, but what do we gain here? It seems to me that this introduces
> the risk that a careless change in one of the internal routines if
> they change slight;ly compress_spec, hence impacting any of their
> callers? Or is that fixing an actual bug (except if I am missing your
> point, that does not seem to be the case)?

To circle back to this: I was not saying there's any bug. The proposed
change was only to follow normal and existing normal conventions for
passing structs. It could also be a pointer to const. It's fine with
me if you say that it's intentional how it's written already.

> >> Is there some benefit in changing compression_spec within the
> >> internals of these routines before going back one layer down to their
> >> callers? Changing the compression_spec on-the-fly in these internal
> >> paths could be risky, actually, no?
> >
> > I think what you're saying is that if the spec is passed as a pointer,
> > then the called functions shouldn't set spec->algorithm=something.
>
> Yes. HEAD makes sure of that, 0001 would not prevent that. So I am a
> bit confused in seeing how this is a benefit.