Re: Add LZ4 compression in pg_dump

On 2/25/23 06:02, Justin Pryzby wrote:
> I have some fixes (attached) and questions while polishing the patch for
> zstd compression.  The fixes are small and could be integrated with the
> patch for zstd, but could be applied independently.
> 
> - I'm unclear about get_error_func().  That's called in three places
>   from pg_backup_directory.c, after failures from write_func(), to
>   supply an compression-specific error message to pg_fatal().  But it's
>   not being used outside of directory format, nor for errors for other
>   function pointers, or even for all errors in write_func().  Is there
>   some reason why each compression method's write_func() shouldn't call
>   pg_fatal() directly, with its compression-specific message ?
> 

I think there are a couple more places that might/should call
get_error_func(). For example ahwrite() in pg_backup_archiver.c now
simply does

    if (bytes_written != size * nmemb)
        WRITE_ERROR_EXIT;

but perhaps it should call get_error_func() too. There are probably
other places that call write_func() and should use get_error_func().

> - I still think supports_compression() should be renamed, or made into a
>   static function in the necessary file.  The main reason is that it's
>   more clear what it indicates - whether compression is "implemented by
>   pgdump" and not whether compression is "supported by this postgres
>   build".  It also seems possible that we'd want to add a function
>   called something like supports_compression(), indicating whether the
>   algorithm is supported by the current build.  It'd be better if pgdump
>   didn't subjugate that name.
> 

If we choose to rename this to have pgdump_ prefix, fine with me. But I
don't think there's a realistic chance of conflict, as it's restricted
to pgdump header etc. And it's not part of an API, so I guess we could
rename that in the future if needed.

> - Finally, the "Nothing to do in the default case" comment comes from
>   Michael's commit 5e73a6048:
> 
> +       /*
> +        * Custom and directory formats are compressed by default with gzip when
> +        * available, not the others.
> +        */
> +       if ((archiveFormat == archCustom || archiveFormat == archDirectory) &&
> +               !user_compression_defined)
>         {
>  #ifdef HAVE_LIBZ
> -               if (archiveFormat == archCustom || archiveFormat == archDirectory)
> -                       compressLevel = Z_DEFAULT_COMPRESSION;
> -               else
> +               parse_compress_specification(PG_COMPRESSION_GZIP, NULL,
> +                                                                        &compression_spec);
> +#else
> +               /* Nothing to do in the default case */
>  #endif
> -                       compressLevel = 0;
>         }
> 
> 
> As the comment says: for -Fc and -Fd, the compression is set to zlib, if
> enabled, and when not otherwise specified by the user.
> 
> Before 5e73a6048, this set compressLevel=0 for -Fp and -Ft, *and* when
> zlib was unavailable.
> 
> But I'm not sure why there's now an empty "#else".  I also don't know
> what "the default case" refers to.
> 
> Maybe the best thing here is to move the preprocessor #if, since it's no
> longer in the middle of a runtime conditional:
> 
>  #ifdef HAVE_LIBZ
> +       if ((archiveFormat == archCustom || archiveFormat == archDirectory) &&
> +               !user_compression_defined)
> +               parse_compress_specification(PG_COMPRESSION_GZIP, NULL,
> +                                            &compression_spec);
>  #endif
> 
> ...but that elicits a warning about "variable set but not used"...
> 

Not sure, I need to think about this a bit.

regards

-- 
Tomas Vondra
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