Function crawl_once 

pub(crate) async fn crawl_once<S>(
    crawl_service: &mut RateLimitBySkipping<CrawlFanout<S>>,
    fanout_limit: Option<usize>,
) -> Result<Option<MorePeers>, BoxError>
where
    S: Service<Request, Response = Response, Error = BoxError> + Clone + Send + 'static,
    S::Future: Send + 'static,

Crawl the network for more peer addresses, limiting the fanout to fanout_limit if it is Some.

• Ask a few live Responded peers to send us more peers.
• Process all completed peer responses, adding new peers in the NeverAttemptedGossiped state.

Returns Some(MorePeers) if the crawl was successful and the crawler should ask for more peers. Returns None if there are no new peers.

Correctness

Pass the initial peer set size as fanout_limit during initialization, so that Zebra does not send duplicate requests to the same peer.

The crawler exits when a crawl returns an error, so errors are only returned on permanent failures.

The handshaker sets up the peer message receiver so it also sends a Responded peer address update.

next_reconnect_peer puts peers into the AttemptPending state.

Security

This call is rate-limited to prevent sending a burst of repeated requests for new peer addresses. Each call will only crawl the network if more time than MIN_PEER_GET_ADDR_INTERVAL has passed since the last crawl. Otherwise, the crawl is skipped.