Function decode 

pub fn decode(
    codec: Codec,
    input: &[u8],
    expected_len: usize,
) -> Result<Vec<u8>>

Decompress input into a fresh buffer. expected_len is the decompressed size the filesystem recorded for this block; it bounds the output (and is required by the size-less block codecs, lz4/lzo). The declared size is capped via checked_block_len before any allocation — the decompression-bomb guard.