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

Dilip Kumar <dilipbalaut(at)gmail(dot)com> writes:
> On Mon, Mar 22, 2021 at 5:22 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> 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?

Yeah, I thought about that too, but do we want to assume that
VARRAWSIZE_4B_C is the correct way to get the decompressed size
for all compression methods?

(If so, I think it would be better style to have a less opaque macro
name for the purpose.)

regards, tom lane