Date: Tue, 11 Apr 2023 19:41:11 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>, gkokolatos@pm.me
Cc: Michael Paquier <michael@paquier.xyz>, shiy.fnst@fujitsu.com,
	pgsql-hackers@lists.postgresql.org,
	Rachel Heaton <rachelmheaton@gmail.com>
Subject: Re: Add LZ4 compression in pg_dump
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On Mon, Feb 27, 2023 at 02:33:04PM +0000, gkokolatos@pm.me wrote:
> > > - Finally, the "Nothing to do in the default case" comment comes from
> > > Michael's commit 5e73a6048:
> > > 
> > > + /*
> > > + * Custom and directory formats are compressed by default with gzip when
> > > + * available, not the others.
> > > + /
> > > + if ((archiveFormat == archCustom || archiveFormat == archDirectory) &&
> > > + !user_compression_defined)
> > > {
> > > #ifdef HAVE_LIBZ
> > > - if (archiveFormat == archCustom || archiveFormat == archDirectory)
> > > - compressLevel = Z_DEFAULT_COMPRESSION;
> > > - else
> > > + parse_compress_specification(PG_COMPRESSION_GZIP, NULL,
> > > + &compression_spec);
> > > +#else
> > > + / Nothing to do in the default case */
> > > #endif
> > > - compressLevel = 0;
> > > }
> > > 
> > > As the comment says: for -Fc and -Fd, the compression is set to zlib, if
> > > enabled, and when not otherwise specified by the user.
> > > 
> > > Before 5e73a6048, this set compressLevel=0 for -Fp and -Ft, and when
> > > zlib was unavailable.
> > > 
> > > But I'm not sure why there's now an empty "#else". I also don't know
> > > what "the default case" refers to.
> > > 
> > > Maybe the best thing here is to move the preprocessor #if, since it's no
> > > longer in the middle of a runtime conditional:
> > > 
> > > #ifdef HAVE_LIBZ
> > > + if ((archiveFormat == archCustom || archiveFormat == archDirectory) &&
> > > + !user_compression_defined)
> > > + parse_compress_specification(PG_COMPRESSION_GZIP, NULL,
> > > + &compression_spec);
> > > #endif
> > > 
> > > ...but that elicits a warning about "variable set but not used"...
> > 
> > 
> > Not sure, I need to think about this a bit.

> /* Nothing to do for the default case when LIBZ is not available */
> is easier to understand.

Maybe I would write it as: "if zlib is unavailable, default to no
compression".  But I think that's best done in the leading comment, and
not inside an empty preprocessor #else.

I was hoping Michael would comment on this.
The placement and phrasing of the comment makes no sense to me.

-- 
Justin