That is, to detect if a chunk of bytes has the zstd magic number as a prefix.

There are Go modules like https://github.com/h2non/filetype, which support lots of file types. But at the end of the day, I just need to check four bytes, and it seems unnecessary to add an extra module dependency when I already depend on a zstd package :)

For example, here's a sample implementation I wrote, which could be used like if zstd.HasMagic(chunk) { ...:

// HasMagic reports whether src begins with zstd v1's magic number,
// 0xFD2FB528 in little-endian format.
func HasMagic(src []byte) bool {
        return len(src) >= 4 &&
                src[0] == 0x28 && src[1] == 0xB5 &&
                src[2] == 0x2f && src[3] == 0xFD
}

I only raised this as an issue instead of a PR because the patch is tiny and I feel like there's a lot of room for bikeshedding, or discussion around whether or not we want this.