Added Endpoint::reject_new_connections

Closes quinn-rs#1584

This commit adds a new function that refuses new connections without
impacting existing connections. Internally, this just sets the
connection limit to 0, which causes incoming connections to be rejected.
This is the same approach that was taken in 0.8.5 when `Incoming` was
dropped.

quinn-proto/src/endpoint.rs

@@ -653,6 +653,13 @@ impl Endpoint {
         }
     }
 
+    /// Rejects new connections without affecting existing connections.
+    pub fn reject_new_connections(&mut self) {
+        if let Some(config) = self.server_config.as_mut() {
+            Arc::make_mut(config).concurrent_connections(0);
+        }
+    }
+
     /// Access the configuration used by this endpoint
     pub fn config(&self) -> &EndpointConfig {
         &self.config

quinn-proto/src/tests/mod.rs

@@ -2193,3 +2193,11 @@ fn stream_chunks(mut recv: RecvStream) -> Vec<u8> {
 
     buf
 }
+
+#[test]
+fn reject_new_connections() {
+    let _guard = subscribe();
+    let mut pair = Pair::default();
+    pair.server.reject_new_connections();
+    pair.assert_connection_refused();
+}

quinn-proto/src/tests/util.rs

@@ -166,6 +166,16 @@ impl Pair {
         }
     }
 
+    /// Attempts to connect, asserting that the server doesn't accept the
+    /// connection and the client marks the connection as closed.
+    pub(super) fn assert_connection_refused(&mut self) {
+        dbg!(&self.client.connections);
+        let client_ch = self.begin_connect(client_config());
+        self.drive();
+        self.server.assert_no_accept();
+        assert!(self.client.connections.get(&client_ch).unwrap().is_closed());
+    }
+
     pub(super) fn connect(&mut self) -> (ConnectionHandle, ConnectionHandle) {
         self.connect_with(client_config())
     }

quinn/src/endpoint.rs

@@ -236,6 +236,16 @@ impl Endpoint {
         self.inner.state.lock().unwrap().socket.local_addr()
     }
 
+    /// Rejects new connections without affecting existing connections.
+    pub fn reject_new_connections(&self) {
+        self.inner
+            .state
+            .lock()
+            .unwrap()
+            .inner
+            .reject_new_connections();
+    }
+
     /// Close all of this endpoint's connections immediately and cease accepting new connections.
     ///
     /// See [`Connection::close()`] for details.