Enum DraftPeek 

enum DraftPeek {
    Named(DraftVersion),
    NeedMore,
    NotSetup,
}

What peek_draft made of a control stream’s opening bytes.

Three answers rather than an Option, because “not yet” and “not ever” have opposite consequences: one says keep buffering and keep the tasks waiting, the other says stop both. Collapsing them is what made a stream that opens with anything but a SETUP buffer 64 KiB before giving up, and a stream that never sends that much never gave up at all.

Variants

Named(DraftVersion)

The first message names this draft.

NeedMore

Too few bytes so far. Buffer more and ask again.

NotSetup

The first message is not a SETUP this peek can read, and no number of further bytes will change that: the type varint is already whole and it is not one of the four this function knows.