Re: Add LZ4 compression in pg_dump

On 11.03.23 07:00, Alexander Lakhin wrote:
> Hello,
> 23.02.2023 23:24, Tomas Vondra wrote:
>> On 2/23/23 16:26, Tomas Vondra wrote:
>>> Thanks for v30 with the updated commit messages. I've pushed 0001 after
>>> fixing a comment typo and removing (I think) an unnecessary change in an
>>> error message.
>>>
>>> I'll give the buildfarm a bit of time before pushing 0002 and 0003.
>>>
>> I've now pushed 0002 and 0003, after minor tweaks (a couple typos etc.),
>> and marked the CF entry as committed. Thanks for the patch!
>>
>> I wonder how difficult would it be to add the zstd compression, so that
>> we don't have the annoying "unsupported" cases.
>
> With the patch 0003 committed, a single warning -Wtype-limits appeared
> in the
> master branch:
> $ CPPFLAGS="-Og -Wtype-limits" ./configure --with-lz4 -q && make -s -j8
> compress_lz4.c: In function ‘LZ4File_gets’:
> compress_lz4.c:492:19: warning: comparison of unsigned expression in ‘<
> 0’ is always false [-Wtype-limits]
>   492 |         if (dsize < 0)
>       |
> (I wonder, is it accidental that there no other places that triggers
> the warning, or some buildfarm animals had this check enabled before?)

I think there is an underlying problem in this code that it dances back
and forth between size_t and int in an unprincipled way.

In the code that triggers the warning, dsize is size_t. dsize is the
return from LZ4File_read_internal(), which is declared to return int.
The variable that LZ4File_read_internal() returns in the success case is
size_t, but in case of an error it returns -1. (So the code that is
warning is meaning to catch this error case, but it won't ever work.)
Further below LZ4File_read_internal() calls LZ4File_read_overflow(),
which is declared to return int, but in some cases it returns
fs->overflowlen, which is size_t.

This should be cleaned up.

AFAICT, the upstream API in lz4.h uses int for size values, but
lz4frame.h uses size_t, so I don't know what the correct approach is.