v1.17: Make the quic server connection table use an async lock, reducing thrashing (backport of #293) (#299)

Make the quic server connection table use an async lock

Author: mergify[bot]

streamer/src/nonblocking/quic.rs

@@ -30,13 +30,26 @@ use {
     std::{
         iter::repeat_with,
         net::{IpAddr, SocketAddr, UdpSocket},
+        // CAUTION: be careful not to introduce any awaits while holding an RwLock.
         sync::{
             atomic::{AtomicBool, AtomicU64, Ordering},
-            Arc, Mutex, MutexGuard, RwLock,
+            Arc, RwLock,
         },
         time::{Duration, Instant},
     },
-    tokio::{task::JoinHandle, time::timeout},
+    tokio::{
+        // CAUTION: It's kind of sketch that we're mixing async and sync locks (see the RwLock above).
+        // This is done so that sync code can also access the stake table.
+        // Make sure we don't hold a sync lock across an await - including the await to
+        // lock an async Mutex. This does not happen now and should not happen as long as we
+        // don't hold an async Mutex and sync RwLock at the same time (currently true)
+        // but if we do, the scope of the RwLock must always be a subset of the async Mutex
+        // (i.e. lock order is always async Mutex -> RwLock). Also, be careful not to
+        // introduce any other awaits while holding the RwLock.
+        sync::{Mutex, MutexGuard},
+        task::JoinHandle,
+        time::timeout,
+    },
 };
 
 /// Limit to 500K PPS
@@ -383,7 +396,7 @@ fn handle_and_cache_new_connection(
     }
 }
 
-fn prune_unstaked_connections_and_add_new_connection(
+async fn prune_unstaked_connections_and_add_new_connection(
     connection: Connection,
     connection_table: Arc<Mutex<ConnectionTable>>,
     max_connections: usize,
@@ -393,7 +406,7 @@ fn prune_unstaked_connections_and_add_new_connection(
     let stats = params.stats.clone();
     if max_connections > 0 {
         let connection_table_clone = connection_table.clone();
-        let mut connection_table = connection_table.lock().unwrap();
+        let mut connection_table = connection_table.lock().await;
         prune_unstaked_connection_table(&mut connection_table, max_connections, stats);
         handle_and_cache_new_connection(
             connection,
@@ -496,7 +509,8 @@ async fn setup_connection(
                 );
 
                 if params.stake > 0 {
-                    let mut connection_table_l = staked_connection_table.lock().unwrap();
+                    let mut connection_table_l = staked_connection_table.lock().await;
+
                     if connection_table_l.total_size >= max_staked_connections {
                         let num_pruned =
                             connection_table_l.prune_random(PRUNE_RANDOM_SAMPLE_SIZE, params.stake);
@@ -525,7 +539,9 @@ async fn setup_connection(
                             max_unstaked_connections,
                             &params,
                             wait_for_chunk_timeout,
-                        ) {
+                        )
+                        .await
+                        {
                             stats
                                 .connection_added_from_staked_peer
                                 .fetch_add(1, Ordering::Relaxed);
@@ -544,7 +560,9 @@ async fn setup_connection(
                     max_unstaked_connections,
                     &params,
                     wait_for_chunk_timeout,
-                ) {
+                )
+                .await
+                {
                     stats
                         .connection_added_from_unstaked_peer
                         .fetch_add(1, Ordering::Relaxed);
@@ -807,7 +825,7 @@ async fn handle_connection(
         }
     }
 
-    let removed_connection_count = connection_table.lock().unwrap().remove_connection(
+    let removed_connection_count = connection_table.lock().await.remove_connection(
         ConnectionTableKey::new(remote_addr.ip(), params.remote_pubkey),
         remote_addr.port(),
         stable_id,