Re: refactoring basebackup.c

On Tue, Sep 21, 2021 at 9:35 AM Jeevan Ladhe
<jeevan(dot)ladhe(at)enterprisedb(dot)com> wrote:
> Here is a patch for lz4 based on the v5 set of patches. The patch adapts with the
> bbsink changes, and is now able to make the provision for the required length
> for the output buffer using the new callback function bbsink_lz4_begin_backup().
>
> Sample command to take backup:
> pg_basebackup -t server:/tmp/data_lz4 -Xnone --server-compression=lz4
>
> Please let me know your thoughts.

This pretty much looks right, with the exception of the autoFlush
thing about which I sent a separate email. I need to write docs for
all of this, and ideally test cases. It might also be good if
pg_basebackup had an option to un-gzip or un-lz4 archives, but I
haven't thought too hard about what would be required to make that
work.

+ if (opt->compression == BACKUP_COMPRESSION_LZ4)

else if

+ /* First of all write the frame header to destination buffer. */
+ Assert(CHUNK_SIZE >= LZ4F_HEADER_SIZE_MAX);
+ headerSize = LZ4F_compressBegin(mysink->ctx,
+ mysink->base.bbs_next->bbs_buffer,
+ CHUNK_SIZE,
+ prefs);

I think this is wrong. I think you should be passing bbs_buffer_length
instead of CHUNK_SIZE, and I think you can just delete CHUNK_SIZE. If
you think otherwise, why?

+ * sink's bbs_buffer of length that can accomodate the compressed input

Spelling.

+ * Make it next multiple of BLCKSZ since the buffer length is expected so.

The buffer length is expected to be a multiple of BLCKSZ, so round up.

+ * If we are falling short of available bytes needed by
+ * LZ4F_compressUpdate() per the upper bound that is decided by
+ * LZ4F_compressBound(), send the archived contents to the next sink to
+ * process it further.

If the number of available bytes has fallen below the value computed
by LZ4F_compressBound(), ask the next sink to process the data so that
we can empty the buffer.

--
Robert Haas
EDB: http://www.enterprisedb.com