events: try to wait for available connection slots

nzr-api/src/event/server.rs

@@ -1,3 +1,4 @@
+use futures::Future;
 use std::{fmt, io, pin::Pin};
 
 use futures::SinkExt;
@@ -114,6 +115,17 @@ impl EventServer {
         Self { channel }
     }
 
+    /// Returns a future that returns [`Poll::Pending`] until the client count falls below the threshold.
+    pub fn until_available(&self) -> EventServerAvailability<'_> {
+        EventServerAvailability { parent: self }
+    }
+
+    /// Whether we're able to take connections.
+    #[inline]
+    fn is_available(&self) -> bool {
+        self.channel.receiver_count() < MAX_RECEIVERS
+    }
+
     /// Spawns a new [`EventEmitter`] where events will be sent to.
     pub async fn spawn<T: AsyncWrite + Send + 'static>(
         &self,
@@ -122,8 +134,8 @@ impl EventServer {
     ) -> io::Result<()> {
         // Sender<T> doesn't have a try_subscribe, so this is our last-ditch
         // effort to avoid a panic
-        if self.channel.receiver_count() + 1 > MAX_RECEIVERS {
-            todo!("Too many connections!");
+        if !self.is_available() {
+            return Err(io::Error::new(io::ErrorKind::Other, "Too many connections"));
         }
 
         EventEmitter::new(inner, client_addr.into(), self.channel.subscribe()).run();
@@ -152,3 +164,24 @@ impl Default for EventServer {
         Self::new()
     }
 }
+
+pub struct EventServerAvailability<'a> {
+    parent: &'a EventServer,
+}
+
+impl<'a> Future for EventServerAvailability<'a> {
+    type Output = ();
+
+    fn poll(
+        self: Pin<&mut Self>,
+        _cx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Self::Output> {
+        use std::task::Poll;
+
+        if self.parent.is_available() {
+            Poll::Ready(())
+        } else {
+            Poll::Pending
+        }
+    }
+}

nzrd/src/event.rs

@@ -8,6 +8,8 @@ pub async fn event_server(ctx: Context) -> io::Result<()> {
     let sock = UnixListener::bind(&ctx.config.rpc.events_sock)?;
 
     loop {
+        // Wait until we have an available slot for a connection
+        ctx.events.until_available().await;
         let (client, addr) = sock.accept().await?;
         ctx.events.spawn(client, addr).await?;
     }