Struct GmailSession 

pub struct GmailSession { /* private fields */ }

A live Gmail OAuth2 session: holds the refresh token and the current in-memory access token, refreshing on demand.

Uses tokio::sync::Mutex (not std::sync::Mutex) held across the refresh network call — unlike SnowflakeSession::renew, which releases its lock before the network call and accepts concurrent refreshes racing each other. Gmail’s design requires single-flight refresh (issue #1465’s explicit “concurrent callers don’t stampede” requirement): a second concurrent caller blocks on this mutex and, once unblocked, observes the already-refreshed token instead of issuing a second POST.