Horton

Re: [HACKERS] Custom compression methods (mac+lz4.h)

Dilip Kumar <dilipbalaut@gmail.com>

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From: Dilip Kumar <dilipbalaut@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Justin Pryzby <pryzby@telsasoft.com>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Alexander Korotkov <a.korotkov@postgrespro.ru>, David Steele <david@pgmasters.net>, Ildus Kurbangaliev <i.kurbangaliev@gmail.com>, Dmitry Dolgov <9erthalion6@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2021-03-22T07:50:47Z
Lists: pgsql-hackers 
On Mon, Mar 22, 2021 at 5:22 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Actually, after reading that closer, the problem only affects the
> case where the compressed-data-length passed to the function is
> a lie.  So it shouldn't be a problem for our usage.
>
> Also, after studying the documentation for LZ4_decompress_safe
> and LZ4_decompress_safe_partial, I realized that liblz4 is also
> counting on the *output* buffer size to not be a lie.  So we
> cannot pass it a number larger than the chunk's true decompressed
> size.  The attached patch resolves the issue I'm seeing.

Okay, the fix makes sense.  In fact, IMHO, in general also this fix
looks like an optimization, I mean when slicelength >=
VARRAWSIZE_4B_C(value), then why do we need to allocate extra memory
even in the case of pglz.  So shall we put this check directly in
toast_decompress_datum_slice instead of handling it at the lz4 level?

Like this.

diff --git a/src/backend/access/common/detoast.c
b/src/backend/access/common/detoast.c
index bed50e8..099ac15 100644
--- a/src/backend/access/common/detoast.c
+++ b/src/backend/access/common/detoast.c
@@ -506,6 +506,10 @@ toast_decompress_datum_slice(struct varlena
*attr, int32 slicelength)

        Assert(VARATT_IS_COMPRESSED(attr));

+       /* liblz4 assumes that slicelength is not an overestimate */
+       if (slicelength >= VARRAWSIZE_4B_C(attr))
+               return toast_decompress_datum(attr);
+
        /*
         * Fetch the compression method id stored in the compression header and
         * decompress the data slice using the appropriate
decompression routine.


-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

Commits

  1. docs: Update TOAST storage docs for configurable compression.

  2. Further tweaking of pg_dump's handling of default_toast_compression.

  3. Fix interaction of TOAST compression with expression indexes.

  4. Tidy up more loose ends related to configurable TOAST compression.

  5. Short-circuit slice requests that are for more than the object's size.

  6. Mostly-cosmetic adjustments of TOAST-related macros.

  7. Remove useless configure probe for <lz4/lz4.h>.

  8. Error on invalid TOAST compression in CREATE or ALTER TABLE.

  9. docs: Fix omissions related to configurable TOAST compression.

  10. More code cleanup for configurable TOAST compression.

  11. Bring configure support for LZ4 up to snuff.

  12. Make compression.sql regression test independent of default.

  13. Use valid compression method in brin_form_tuple

  14. Fix up pg_dump's handling of per-attribute compression options.

  15. Allow configurable LZ4 TOAST compression.

  16. Fix inconsistencies in the code

  17. Mostly-cosmetic improvements in memory chunk header alignment coding.

  18. Allow numeric to use a more compact, 2-byte header in many cases.