Horton

Re: Add LZ4 compression in pg_dump

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: gkokolatos@pm.me
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>, Michael Paquier <michael@paquier.xyz>, pgsql-hackers@lists.postgresql.org, Rachel Heaton <rachelmheaton@gmail.com>
Date: 2023-01-25T18:00:20Z
Lists: pgsql-hackers 
On Wed, Jan 25, 2023 at 03:37:12PM +0000, gkokolatos@pm.me wrote:
> Of course, one can throw the error before entering the loop, yet I think
> that it does not help the readability of the code. IMHO it is easier to
> follow if the error is thrown once during that check.

> If anything, I can suggest to throw an error much earlier, i.e. in ReadHead(),
> and remove altogether this check. On the other hand, I like the belts
> and suspenders approach because there are no more checks after this point.

While looking at this, I realized that commit 5e73a6048 introduced a
regression:

@@ -3740,19 +3762,24 @@ ReadHead(ArchiveHandle *AH)

-       if (AH->compression != 0)
-               pg_log_warning("archive is compressed, but this installation does not support compression -- no data will be available");
+       if (AH->compression_spec.algorithm == PG_COMPRESSION_GZIP)
+               pg_fatal("archive is compressed, but this installation does not support compression");

Before, it was possible to restore non-data chunks of a dump file, even
if the current build didn't support its compression.  But that's now
impossible - and it makes the code we're discussing in RestoreArchive()
unreachable.

I don't think we can currently test for that, since it requires creating a dump
using a build --with compression and then trying to restore using a build
--without compression.  The coverage report disagrees with me, though...
https://coverage.postgresql.org/src/bin/pg_dump/pg_backup_archiver.c.gcov.html#3901

> > I think it could be written to avoid the need to change for added
> > compression algorithms:
...
> 
> I am not certain how that would work in the example with ZSTD above.
> If I am not wrong, parse_compress_specification() will not throw an error
> if the codebase supports ZSTD, yet this specific pg_dump binary will not
> support it because ZSTD is not implemented. parse_compress_specification()
> is not aware of that and should not be aware of it, should it?

You're right.

I think the 001 patch should try to remove hardcoded references to
LIBZ/GZIP, such that the later patches don't need to update those same
places for LZ4.  For example in ReadHead() and RestoreArchive(), and
maybe other places dealing with file extensions.  Maybe that could be
done by adding a function specific to pg_dump indicating whether or not
an algorithm is implemented and supported.

-- 
Justin

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,