spec: Empty compressed blocks not allowed about zstd

[klauspost]

Ah... It could be extra block data failing this check... Closing until I've clarified.

No. This doesn't appear to be it.

[klauspost]

This appears to be something that has changed, with a MIN_CBLOCK_SIZE. Latest version appear to decode this.

Closing, since it appears these blocks now decode.

[Cyan4973]

Indeed,
this is a known limitation of older decoder versions.

It's documented here :
https://github.com/facebook/zstd/blob/dev/doc/decompressor_errata.md#compressed-block-with-0-literals-and-0-sequences

It has been fixed since.

Even though newer versions of zstd decode such blocks, it's recommended to avoid generating such block, in order to improve compatibility with the installed base of zstd decoders. Besides, as you noticed, it's just a wasteful way to represent "empty", so it's good practice for the encoder to select a more compact representation.