Fix missing tokio context on client disconnect

The `PlayPacketController` might spawn a tokio task, but it was not in a
tokio context when that happened. This is because tokio contexts are
thread-local, but rayon tasks run in their own thread pool.

src/server.rs

@@ -88,8 +88,10 @@ struct SharedServerInner<C: Config> {
     max_connections: usize,
     incoming_capacity: usize,
     outgoing_capacity: usize,
+    /// The tokio handle used by the server.
     tokio_handle: Handle,
-    /// Store this here so we don't drop it.
+    /// Holding a runtime handle is not enough to keep tokio working. We need
+    /// to store the runtime here so we don't drop it.
     _tokio_runtime: Option<Runtime>,
     dimensions: Vec<Dimension>,
     biomes: Vec<Biome>,
@@ -254,8 +256,8 @@ impl<C: Config> SharedServer<C> {
 
 /// Consumes the configuration and starts the server.
 ///
-/// The function returns once the server has shut down, a runtime error
-/// occurs, or the configuration is found to be invalid.
+/// This function blocks the current thread and returns once the server has shut
+/// down, a runtime error occurs, or the configuration is found to be invalid.
 pub fn start_server<C: Config>(config: C, data: C::ServerState) -> ShutdownResult {
     let shared = setup_server(config)
         .context("failed to initialize server")
@@ -522,6 +524,7 @@ async fn handle_connection(
                     ctrl: ctrl.into_play_packet_controller(
                         server.0.incoming_capacity,
                         server.0.outgoing_capacity,
+                        server.tokio_handle().clone(),
                     ),
                 };
 

src/server/packet_controller.rs

@@ -3,6 +3,7 @@ use std::time::Duration;
 
 use tokio::io;
 use tokio::io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt};
+use tokio::runtime::Handle;
 use tokio::task::JoinHandle;
 use tokio::time::timeout;
 
@@ -91,6 +92,7 @@ where
         mut self,
         incoming_limit: usize,
         outgoing_limit: usize,
+        handle: Handle,
     ) -> PlayPacketController
     where
         R: Send + 'static,
@@ -144,6 +146,7 @@ where
             recv: incoming_receiver,
             reader_task,
             writer_task: Some(writer_task),
+            handle,
         }
     }
 }
@@ -158,6 +161,7 @@ pub struct PlayPacketController {
     recv: ByteReceiver,
     reader_task: JoinHandle<()>,
     writer_task: Option<JoinHandle<()>>,
+    handle: Handle,
 }
 
 impl PlayPacketController {
@@ -214,8 +218,11 @@ impl Drop for PlayPacketController {
 
         if let Some(writer_task) = self.writer_task.take() {
             if !writer_task.is_finished() {
+                let _guard = self.handle.enter();
+
                 // Give any unsent packets a moment to send before we cut the connection.
-                tokio::spawn(timeout(Duration::from_secs(1), writer_task));
+                self.handle
+                    .spawn(timeout(Duration::from_secs(1), writer_task));
             }
         }
     }