Function scan_serving_cached_pr 

pub async fn scan_serving_cached_pr<S>(
    cwd: &Path,
    env: &HashMap<String, String, S>,
    prev: Option<&GitDiffSnapshot>,
) -> GitDiffSnapshot
where
    S: BuildHasher,

scan for a read a client is waiting on: a PR cache past its window is served for the branch and sha it belongs to rather than looked up, so the read never waits on the network; a cache from another branch or push answers nothing. The seat’s own loop scans with scan and pushes the refreshed row.