Horton

Re: Add LZ4 compression in pg_dump

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: gkokolatos@pm.me
Cc: Andrew Dunstan <andrew@dunslane.net>, Alexander Lakhin <exclusion@gmail.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, Justin Pryzby <pryzby@telsasoft.com>, shiy.fnst@fujitsu.com, pgsql-hackers@lists.postgresql.org, Rachel Heaton <rachelmheaton@gmail.com>
Date: 2023-05-08T00:31:06Z
Lists: pgsql-hackers 
On Fri, May 05, 2023 at 02:13:28PM +0000, gkokolatos@pm.me wrote:
> Good point. I thought about it before submitting the patch. I
> concluded that given the complexity and operations involved in
> LZ4Stream_read_internal() and the rest of t he pg_dump/pg_restore
> code, the memset() call will be negligible. However from the
> readability point of view, the function is a bit cleaner with the
> memset(). 
> 
> I will not object to any suggestion though, as this is a very
> trivial point. Please find attached a v2 of the patch following the
> suggested approach.

Hmm.  I was looking at this patch, and what you are trying to do
sounds rather right to keep a parallel with the gzip and zstd code
paths.

Looking at the code of gzread.c, gzgets() enforces a null-termination
on the string read.  Still, isn't that something we'd better enforce
in read_none() as well?  compress_io.h lists this as a requirement of
the callback, and Zstd_gets() does so already.  read_none() does not
enforce that, unfortunately.

+   /* No work needs to be done for a zero-sized output buffer */
+   if (size <= 0)
+       return 0;

Indeed.  This should be OK.

-   ret = LZ4Stream_read_internal(state, ptr, size, true);
+   Assert(size > 1);

The addition of this assertion is a bit surprising, and this is
inconsistent with Zstd_gets where a length of 1 is authorized.  We
should be more consistent across all the callbacks, IMO, not less, so
as we apply the same API contract across all the compression methods.

While testing this patch, I have triggered an error pointing out that
the decompression path of LZ4 is broken for table data.  I can
reproduce that with a dump of the regression database, as of:
make installcheck
pg_dump --format=d --file=dump_lz4 --compress=lz4 regression
createdb regress_lz4
pg_restore --format=d -d regress_lz4 dump_lz4
pg_restore: error: COPY failed for table "clstr_tst": ERROR:  extra data after last expected column
CONTEXT:  COPY clstr_tst, line 15: "32	6	seis	xyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzyxyzzy..."
pg_restore: warning: errors ignored on restore: 1

This does not show up with gzip or zstd, and the patch does not
influence the result.  In short it shows up with and without the
patch, on HEAD.  That does not look really stable :/
--
Michael

Commits

  1. Advance input pointer when LZ4 compressing data

  2. Null-terminate the output buffer of LZ4Stream_gets

  3. Rework code defining default compression for dir/custom formats in pg_dump

  4. pg_dump: Use only LZ4 frame format for compression

  5. Minor comment improvements for compress_lz4

  6. Unify buffer sizes in pg_dump compression API

  7. Improve type handling in pg_dump's compress file API

  8. Improve wording in pg_dump compression docs

  9. Fix condition in pg_dump TAP test

  10. Add LZ4 compression to pg_dump

  11. Introduce a generic pg_dump compression API

  12. Prepare pg_dump internals for additional compression methods

  13. Fix behavior with pg_restore -l and compressed dumps

  14. Add 250c8ee07ed to git-blame-ignore-revs

  15. Provide test coverage in pg_dump for default behaviors with compression

  16. Switch pg_dump to use compression specifications

  17. Refactor code parsing compression option values (-Z/--compress)

  18. meson: Add some missing env settings for tests of pg_dump and pg_verifybackup

  19. Extend TAP tests of pg_dump to test for compression with gzip

  20. Clean up some dead code in pg_dump with tar format and gzip compression

  21. Add TAP test in pg_dump with --format=tar and --compress

  22. Replace BASE_BACKUP COMPRESSION_LEVEL option with COMPRESSION_DETAIL.

  23. Refactor the pg_dump zlib code from pg_backup_custom.c to a separate file,