Re: Add LZ4 compression in pg_dump

On Wed, Nov 30, 2022 at 05:11:44PM +0000, gkokolatos(at)pm(dot)me wrote:
> Fair enough. The atteched v11 does that. 0001 introduces compression
> specification and is using it throughout. 0002 paves the way to the
> new interface by homogenizing the use of cfp. 0003 introduces the new
> API and stores the compression algorithm in the custom format header
> instead of the compression level integer. Finally 0004 adds support for
> LZ4.

I have been looking at 0001, and.. Hmm. I am really wondering
whether it would not be better to just nuke this warning into orbit.
This stuff enforces non-compression even if -Z has been used to a
non-default value. This has been moved to its current location by
cae2bb1 as of this thread:
https://www.postgresql.org/message-id/20160526.185551.242041780.horiguchi.kyotaro%40lab.ntt.co.jp

However, this is only active if -Z is used when not building with
zlib. At the end, it comes down to whether we want to prioritize the
portability of pg_dump commands specifying a -Z/--compress across
environments knowing that these may or may not be built with zlib,
vs the amount of simplification/uniformity we would get across the
binaries in the tree once we switch everything to use the compression
specifications. Now that pg_basebackup and pg_receivewal are managed
by compression specifications, and that we'd want more compression
options for pg_dump, I would tend to do the latter and from now on
complain if attempting to do a pg_dump -Z under --without-zlib with a
compression level > 0. zlib is also widely available, and we don't
document the fact that non-compression is enforced in this case,
either. (Two TAP tests with the custom format had to be tweaked.)

As per the patch, it is true that we do not need to bump the format of
the dump archives, as we can still store only the compression level
and guess the method from it. I have added some notes about that in
ReadHead and WriteHead to not forget.

Most of the changes are really-straight forward, and it has resisted
my tests, so I think that this is in a rather-commitable shape as-is.
--
Michael