Date: Thu, 26 Jan 2023 21:56:32 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: gkokolatos@pm.me, pgsql-hackers@lists.postgresql.org,
	Rachel Heaton <rachelmheaton@gmail.com>
Subject: Re: Add LZ4 compression in pg_dump
Message-ID: <20230127035631.GV22427@telsasoft.com>
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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.