Re: Add LZ4 compression in pg_dump

------- Original Message -------
On Saturday, March 11th, 2023 at 7:00 AM, Alexander Lakhin <exclusion(at)gmail(dot)com> 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)
> |

Thank you Alexander. Please find attached an attempt to address it.

> (I wonder, is it accidental that there no other places that triggers
> the warning, or some buildfarm animals had this check enabled before?)

I can not answer about the buildfarms. Do you think that adding an explicit
check for this warning in meson would help? I am a bit uncertain as I think
that type-limits are included in extra.

@@ -1748,6 +1748,7 @@ common_warning_flags = [
'-Wshadow=compatible-local',
# This was included in -Wall/-Wformat in older GCC versions
'-Wformat-security',
+ '-Wtype-limits',
]

>
> It is not a false positive as can be proved by the 002_pg_dump.pl modified as
> follows:
> - program => $ENV{'LZ4'},
>
> + program => 'mv',
>
> args => [
>
> - '-z', '-f', '--rm',
> "$tempdir/compression_lz4_dir/blobs.toc",
> "$tempdir/compression_lz4_dir/blobs.toc.lz4",
> ],
> },

Correct, it is not a false positive. The existing testing framework provides
limited support for exercising error branches. Especially so when those are
dependent on generated output.

> A diagnostic logging added shows:
> LZ4File_gets() after LZ4File_read_internal; dsize: 18446744073709551615
>
> and pg_restore fails with:
> error: invalid line in large object TOC file
> ".../src/bin/pg_dump/tmp_check/tmp_test_22ri/compression_lz4_dir/blobs.toc": "????"

It is a good thing that the restore fails with bad input. Yet it should
have failed earlier. The attached makes certain it does fail earlier.

Cheers,
//Georgios

>
> Best regards,
> Alexander