Function pump_session 

pub(super) fn pump_session(
    client: &mut Client<JournaledSession>,
    outbound: &mut PacketWriter<GrowBuffer>,
    pending: &mut Pending,
    outbound_aliases: &mut OutboundAliases,
)

Drains pending subscription groups, the outbox, and pending PUBRELs onto the outbound buffer via the client’s poll_transmit, mapping its per-packet events onto the user-facing futures: a written PUBLISH reports Sent progress, every written packet (re-)arms its ack-timeout deadline, and an over-large packet fails its completion with Error::PacketTooLarge.