[Spice-devel] RFC: remove Adler checksum and zlib header/trailer from zlib compressed images

[Alon Levy]

On Sun, Jan 29, 2012 at 09:28:34AM +0200, Yaniv Kaul wrote:
> These small changes to server and (gtk) client seem to work, and
> remove both the zlib header/trailer from the deflate stream as well
> as the Adler checksum, which removes several bytes from the stream
> as well as speedup (unnoticeable) of the compression/decompression
> due to the removal of the checksum calculation.
> 
> Problem is - not sure how to test it - don't know how to get an
> image once with and once without the patch - and ensure it's the
> same image.
> The fact that the first image is never zlib compressed complicates
> things. I could not reliably get a 2nd image sent to the client
> which looks the same as one in a different connection.
> 
> Nevertheless it does no harm - and images pass well - verified by
> connecting with a modified Spice GTK client to a modified Spice
> server, as well as looking at the network capture via wireshark.
> 
> It's of course missing the display channel specific capability
> negotiation for this change.
> Comments are welcome.

Other then that adding a link to the docs page of
inlateInit2/deflateInit2 in the commit message would be nice for those
seeking to find the explanation for the magic numbers, no comment,
thanks for doing this!

If you want a repeatable test you can write one or use one of the
existing artificial tests in server/tests.

> 
> Client change:
> diff --git a/gtk/decode-zlib.c b/gtk/decode-zlib.c
> index a692020..997f350 100644
> --- a/gtk/decode-zlib.c
> +++ b/gtk/decode-zlib.c
> @@ -63,7 +63,7 @@ SpiceZlibDecoder *zlib_decoder_new(void)
>      d->_z_strm.opaque = Z_NULL;
>      d->_z_strm.next_in = Z_NULL;
>      d->_z_strm.avail_in = 0;
> -    z_ret = inflateInit(&d->_z_strm);
> +    z_ret = inflateInit2(&d->_z_strm, -15);
>      if (z_ret != Z_OK) {
>          g_warning("zlib decoder init failed, error %d", z_ret);
>          goto fail;
> 
> 
> server change:
> diff --git a/server/zlib_encoder.c b/server/zlib_encoder.c
> index c51466b..66a039a 100644
> --- a/server/zlib_encoder.c
> +++ b/server/zlib_encoder.c
> @@ -47,7 +47,7 @@ ZlibEncoder*
> zlib_encoder_create(ZlibEncoderUsrContext *usr, int level)
>      enc->strm.zfree = Z_NULL;
>      enc->strm.opaque = Z_NULL;
> 
> -    z_ret = deflateInit(&enc->strm, level);
> +    z_ret = deflateInit2(&enc->strm, level, Z_DEFLATED, -15, 8,
> Z_DEFAULT_STRATEGY);
>      enc->last_level = level;
>      if (z_ret != Z_OK) {
>          red_printf("zlib error");
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