Constant OVER_LONG_KEY 

pub const OVER_LONG_KEY: &str = "over-long";

The single bucket every key over MAX_KEY_LENGTH shares.

The same answer caller_key already gives a caller it cannot tell apart from another: one shared budget rather than a free one. Sharing is strictly the stricter reading — the keys collapsed into it cannot name an account, so nothing legitimate is being made to queue behind anything else — and it is what keeps a key’s content out of this map.