Re: Add LZ4 compression in pg_dump

On Wed, Jan 25, 2023 at 07:57:18PM +0000, gkokolatos@pm.me wrote:
> On Wednesday, January 25th, 2023 at 7:00 PM, Justin Pryzby <pryzby@telsasoft.com> wrote:
> > While looking at this, I realized that commit 5e73a6048 introduced a
> > regression:
> > 
> > @@ -3740,19 +3762,24 @@ ReadHead(ArchiveHandle *AH)
> > 
> > - if (AH->compression != 0)
> > 
> > - pg_log_warning("archive is compressed, but this installation does not support compression -- no data will be
available");
> > + if (AH->compression_spec.algorithm == PG_COMPRESSION_GZIP)
> > 
> > + pg_fatal("archive is compressed, but this installation does not support compression");
> > 
> > Before, it was possible to restore non-data chunks of a dump file, even
> > if the current build didn't support its compression. But that's now
> > impossible - and it makes the code we're discussing in RestoreArchive()
> > unreachable.

On Thu, Jan 26, 2023 at 08:53:28PM +0900, Michael Paquier wrote:
> On Thu, Jan 26, 2023 at 11:24:47AM +0000, gkokolatos@pm.me wrote:
> > I gave this a little bit of thought. I think that ReadHead should not
> > emit a warning, or at least not this warning as it is slightly misleading.
> > It implies that it will automatically turn off data restoration, which is
> > false. Further ahead, the code will fail with a conflicting error message
> > stating that the compression is not available.
> > 
> > Instead, it would be cleaner both for the user and the maintainer to
> > move the check in RestoreArchive and make it the sole responsible for
> > this logic.
> 
> -    pg_fatal("cannot restore from compressed archive (compression not supported in this installation)");
> +    pg_fatal("cannot restore data from compressed archive (compression not supported in this installation)");
> Hmm.  I don't mind changing this part as you suggest.
> 
> -#ifndef HAVE_LIBZ
> -       if (AH->compression_spec.algorithm == PG_COMPRESSION_GZIP)
> -               pg_fatal("archive is compressed, but this installation does not support compression");
> -#endif
> However I think that we'd better keep the warning, as it can offer a
> hint when using pg_restore -l not built with compression support if
> looking at a dump that has been compressed.

Yeah.  But the original log_warning text was better, and should be
restored:

-       if (AH->compression != 0)
-               pg_log_warning("archive is compressed, but this installation does not support compression -- no data
willbe available");
 

That commit also added this to pg-dump.c:

+               case PG_COMPRESSION_ZSTD:
+                       pg_fatal("compression with %s is not yet supported", "ZSTD");
+                       break;
+               case PG_COMPRESSION_LZ4:
+                       pg_fatal("compression with %s is not yet supported", "LZ4");
+                       break;

In 002, that could be simplified by re-using the supports_compression()
function.  (And maybe the same in WriteDataToArchive()?)

-- 
Justin