Horton

Re: Add LZ4 compression in pg_dump

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: gkokolatos@pm.me, shiy.fnst@fujitsu.com, Michael Paquier <michael@paquier.xyz>, pgsql-hackers@lists.postgresql.org, Rachel Heaton <rachelmheaton@gmail.com>
Date: 2023-03-08T01:45:20Z
Lists: pgsql-hackers

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On Wed, Mar 01, 2023 at 04:52:49PM +0100, Tomas Vondra wrote:
> Thanks. That seems correct to me, but I find it somewhat confusing,
> because we now have
> 
>  DeflateCompressorInit vs. InitCompressorGzip
> 
>  DeflateCompressorEnd vs. EndCompressorGzip
> 
>  DeflateCompressorData - The name doesn't really say what it does (would
>                          be better to have a verb in there, I think).
> 
> I wonder if we can make this somehow clearer?

To move things along, I updated Georgios' patch:

Rename DeflateCompressorData() to DeflateCompressorCommon();
Rearrange functions to their original order allowing a cleaner diff to the prior code;
Change pg_fatal() to an assertion+comment;
Update the commit message and fix a few typos;

> Also, InitCompressorGzip says this:
> 
>    /*
>     * If the caller has defined a write function, prepare the necessary
>     * state. Avoid initializing during the first write call, because End
>     * may be called without ever writing any data.
>     */
>     if (cs->writeF)
>         DeflateCompressorInit(cs);
>
> Does it actually make sense to not have writeF defined in some cases?

InitCompressor is being called for either reading or writing, either of
which could be null:

src/bin/pg_dump/pg_backup_custom.c:     ctx->cs = AllocateCompressor(AH->compression_spec,
src/bin/pg_dump/pg_backup_custom.c-                                                              NULL,
src/bin/pg_dump/pg_backup_custom.c-                                                              _CustomWriteFunc);
--
src/bin/pg_dump/pg_backup_custom.c:     cs = AllocateCompressor(AH->compression_spec,
src/bin/pg_dump/pg_backup_custom.c-                                                     _CustomReadFunc, NULL);

It's confusing that the comment says "Avoid initializing...".  What it
really means is "Initialize eagerly...".  But that makes more sense in
the context of the commit message for this bugfix than in a comment.  So
I changed that too.

+       /* If deflation was initialized, finalize it */                                                                                      
+       if (cs->private_data)                                                                                                                
+               DeflateCompressorEnd(AH, cs);                                                                                                

Maybe it'd be more clear if this used "if (cs->writeF)", like in the
init function ?

-- 
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,