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//! Following a port's NPDU limit while the router runs (#1696).
//!
//! A port whose transport offers `TransportPort::max_npdu_changes`, as
//! BACnet/SC does, gets a task that gives the engine each limit the
//! transport publishes. It changes the port's limit under the engine's
//! lock, as every other engine change is made, so every frame a dispatch
//! task routes after that goes by the new limit. Nothing waits under the
//! lock: the limit is copied out of the watch before the lock is taken,
//! and the lock is let go before the task waits again.
//!
//! Each change also updates the smallest limits among the ports, which the
//! router's application ports report (#2070).
use std::sync::Arc;
use bacnet_npdu::NpduLimits;
use bacnet_router_core::PortId;
use bacnet_router_core::router::PortConfig;
use tokio::sync::watch;
use tracing::{info, warn};
use super::dispatch::Shared;
use super::engine::Engine;
/// Give the engine's `port` each limit `changes` holds, from the one it
/// holds now, until the transport drops its sender.
///
/// The one it holds now comes first, so a change between the start's read
/// of `max_npdu` and this subscription isn't missed.
pub(super) async fn follow(
shared: Arc<Shared>,
port: PortId,
mut changes: watch::Receiver<NpduLimits>,
) {
loop {
let reported = *changes.borrow_and_update();
shared.set_port_limits(port, reported);
if changes.changed().await.is_err() {
return;
}
}
}
impl Shared {
/// Give the engine's `port` the limits its transport reports now, as
/// [`Engine::limits`] takes them, and log a change.
pub(super) fn set_port_limits(&self, port: PortId, reported: NpduLimits) {
let limits = Engine::limits(reported);
let changed = {
let mut router = self.engine.lock();
let changed = match router.ports().get(usize::from(port.0)) {
Some(config) if config.max_npdu == limits => Ok(false),
_ => router.set_port_max_npdu(port, limits).map(|()| true),
};
// Published under the engine's lock, so the ports' tasks publish
// in the order they change the engine. A watch send never waits,
// and nothing that borrows the watch takes the engine's lock.
let smallest = smallest(router.ports());
self.application_limits
.send_if_modified(|current| std::mem::replace(current, smallest) != smallest);
changed
};
match changed {
Ok(true) => info!(
port = port.0,
unicast = limits.unicast,
broadcast = limits.broadcast,
"Router port's NPDU limit changed"
),
Ok(false) => {}
// The port is the router's and the limits are within the
// engine's bounds, so this is never expected.
Err(error) => warn!(port = port.0, %error, "Router kept a port's NPDU limit"),
}
}
}
/// The smallest unicast and broadcast limits among `ports`: what a request
/// from the router's application fits on whichever port it leaves by.
pub(super) fn smallest<G>(ports: &[PortConfig<G>]) -> NpduLimits {
let smallest = |limit: fn(&NpduLimits) -> u16| {
ports
.iter()
.map(|port| limit(&port.max_npdu))
.min()
// A started router has at least one port.
.unwrap_or(NpduLimits::STANDARD.unicast)
};
NpduLimits {
unicast: smallest(|limits| limits.unicast),
broadcast: smallest(|limits| limits.broadcast),
}
}