Proposed fix for GC rooting + extensive notes on how/why are in Timer…

…Token (#2413)

Co-authored-by: Nick Craver <[email protected]>

docs/ReleaseNotes.md

@@ -8,7 +8,7 @@ Current package versions:
 
 ## Unreleased
 
-No pending changes.
+- Fix [#2392](https://github.com/StackExchange/StackExchange.Redis/issues/2392): critical (but rare) GC bug that can lead to async tasks never completing if the multiplexer is not held by the consumer ([#2413 by mgravell](https://github.com/StackExchange/StackExchange.Redis/pull/2413))
 
 ## 2.6.96
 

src/StackExchange.Redis/ConnectionMultiplexer.cs

@@ -34,7 +34,7 @@ public sealed partial class ConnectionMultiplexer : IInternalConnectionMultiplex
         internal long syncOps, asyncOps;
         private long syncTimeouts, fireAndForgets, asyncTimeouts;
         private string? failureMessage, activeConfigCause;
-        private IDisposable? pulse;
+        private TimerToken? pulse;
 
         private readonly Hashtable servers = new Hashtable();
         private volatile ServerSnapshot _serverSnapshot = ServerSnapshot.Empty;
@@ -874,7 +874,7 @@ public ServerSnapshotFiltered(ServerEndPoint[] endpoints, int count, Func<Server
             }
         }
 
-        [return: NotNullIfNotNull("endpoint")]
+        [return: NotNullIfNotNull(nameof(endpoint))]
         internal ServerEndPoint? GetServerEndPoint(EndPoint? endpoint, LogProxy? log = null, bool activate = true)
         {
             if (endpoint == null) return null;
@@ -909,40 +909,130 @@ public ServerSnapshotFiltered(ServerEndPoint[] endpoints, int count, Func<Server
             return server;
         }
 
-        private sealed class TimerToken
+        internal void Root() => pulse?.Root(this);
+
+        // note that this also acts (conditionally) as the GC root for the multiplexer
+        // when there are in-flight messages; the timer can then acts as the heartbeat
+        // to make sure that everything *eventually* completes
+        private sealed class TimerToken : IDisposable
         {
             private TimerToken(ConnectionMultiplexer muxer)
             {
-                _ref = new WeakReference(muxer);
+                _weakRef = new(muxer);
             }
             private Timer? _timer;
             public void SetTimer(Timer timer) => _timer = timer;
-            private readonly WeakReference _ref;
+
+            private readonly WeakReference<ConnectionMultiplexer> _weakRef;
+
+            private object StrongRefSyncLock => _weakRef; // private and readonly? it'll do
+            private ConnectionMultiplexer? _strongRef;
+            private int _strongRefToken;
 
             private static readonly TimerCallback Heartbeat = state =>
             {
                 var token = (TimerToken)state!;
-                var muxer = (ConnectionMultiplexer?)(token._ref?.Target);
-                if (muxer != null)
+                if (token._weakRef.TryGetTarget(out var muxer))
                 {
                     muxer.OnHeartbeat();
                 }
                 else
                 {
                     // the muxer got disposed from out of us; kill the timer
-                    var tmp = token._timer;
-                    token._timer = null;
-                    if (tmp != null) try { tmp.Dispose(); } catch { }
+                    token.Dispose();
                 }
             };
 
-            internal static IDisposable Create(ConnectionMultiplexer connection)
+            internal static TimerToken Create(ConnectionMultiplexer connection)
             {
                 var token = new TimerToken(connection);
                 var heartbeatMilliseconds = (int)connection.RawConfig.HeartbeatInterval.TotalMilliseconds;
                 var timer = new Timer(Heartbeat, token, heartbeatMilliseconds, heartbeatMilliseconds);
                 token.SetTimer(timer);
-                return timer;
+                return token;
+            }
+
+            public void Dispose()
+            {
+                var tmp = _timer;
+                _timer = null;
+                if (tmp is not null) try { tmp.Dispose(); } catch { }
+
+                _strongRef = null; // note that this shouldn't be relevant since we've unrooted the TimerToken
+            }
+
+
+            // explanation of rooting model:
+            //
+            // the timer has a reference to the TimerToken; this *always* has a weak-ref,
+            // and *may* sometimes have a strong-ref; this is so that if a consumer
+            // drops a multiplexer, it can be garbage collected, i.e. the heartbeat timer
+            // doesn't keep the entire thing alive forever; instead, if the heartbeat detects
+            // the weak-ref has been collected, it can cancel the timer and *itself* go away;
+            // however: this leaves a problem where there is *in flight work* when the consumer
+            // drops the multiplexer; in particular, if that happens when disconnected, there
+            // could be consumer-visible pending TCS items *in the backlog queue*; we don't want
+            // to leave those incomplete, as that fails the contractual expectations of async/await;
+            // instead we need to root ourselves. The natural place to do this is by rooting the
+            // multiplexer, allowing the heartbeat to keep poking things, so that the usual
+            // message-processing and timeout rules apply. This is why we *sometimes* also keep
+            // a strong-ref to the same multiplexer.
+            //
+            // The TimerToken is rooted by the timer callback; this then roots the multiplexer,
+            // which keeps our bridges and connections in scope - until we're sure we're done
+            // with them.
+            //
+            // 1) any bridge or connection will trigger rooting by calling Root when
+            // they change from "empty" to "non-empty" i.e. whenever there
+            // in-flight items; this always changes the token; this includes both the
+            // backlog and awaiting-reply queues.
+            //
+            // 2) the heartbeat is responsible for unrooting, after processing timeouts
+            // etc; first it checks whether it is needed (IsRooted), which also gives
+            // it the current token.
+            //
+            // 3) if so, the heartbeat will (outside of the lock) query all sources to
+            // see if they still have outstanding work; if everyone reports negatively,
+            // then the heartbeat calls UnRoot passing in the old token; if this still
+            // matches (i.e. no new work came in while we were looking away), then the
+            // strong reference is removed; note that "has outstanding work" ignores
+            // internal-call messages; we are only interested in consumer-facing items
+            // (but we need to check this *here* rather than when adding, as otherwise
+            // the definition of "is empty, should root" becomes more complicated, which
+            // impacts the write path, rather than the heartbeat path.
+            //
+            // This means that the multiplexer (via the timer) lasts as long as there are
+            // outstanding messages; if the consumer has dropped the multiplexer, then
+            // there will be no new incoming messages, and after timeouts: everything
+            // should drop.
+
+            public void Root(ConnectionMultiplexer multiplexer)
+            {
+                lock (StrongRefSyncLock)
+                {
+                    _strongRef = multiplexer;
+                    _strongRefToken++;
+                }
+            }
+
+            public bool IsRooted(out int token)
+            {
+                lock (StrongRefSyncLock)
+                {
+                    token = _strongRefToken;
+                    return _strongRef is not null;
+                }
+            }
+
+            public void UnRoot(int token)
+            {
+                lock (StrongRefSyncLock)
+                {
+                    if (token == _strongRefToken)
+                    {
+                        _strongRef = null;
+                    }
+                }
             }
         }
 
@@ -956,8 +1046,21 @@ private void OnHeartbeat()
                 Trace("heartbeat");
 
                 var tmp = GetServerSnapshot();
+                int token = 0;
+                bool isRooted = pulse?.IsRooted(out token) ?? false, hasPendingCallerFacingItems = false;
+
                 for (int i = 0; i < tmp.Length; i++)
+                {
                     tmp[i].OnHeartbeat();
+                    if (isRooted && !hasPendingCallerFacingItems)
+                    {
+                        hasPendingCallerFacingItems = tmp[i].HasPendingCallerFacingItems();
+                    }
+                }
+                if (isRooted && !hasPendingCallerFacingItems)
+                {   // release the GC root on the heartbeat *if* the token still matches
+                    pulse?.UnRoot(token);
+                }
             }
             catch (Exception ex)
             {
@@ -1935,7 +2038,7 @@ internal static void ThrowFailed<T>(TaskCompletionSource<T>? source, Exception u
             }
         }
 
-        [return: NotNullIfNotNull("defaultValue")]
+        [return: NotNullIfNotNull(nameof(defaultValue))]
         internal T? ExecuteSyncImpl<T>(Message message, ResultProcessor<T>? processor, ServerEndPoint? server, T? defaultValue = default)
         {
             if (_isDisposed) throw new ObjectDisposedException(ToString());
@@ -2049,7 +2152,7 @@ static async Task<T> ExecuteAsyncImpl_Awaited(ConnectionMultiplexer @this, Value
 
         internal Task<T?> ExecuteAsyncImpl<T>(Message? message, ResultProcessor<T>? processor, object? state, ServerEndPoint? server)
         {
-            [return: NotNullIfNotNull("tcs")]
+            [return: NotNullIfNotNull(nameof(tcs))]
             static async Task<T?> ExecuteAsyncImpl_Awaited(ConnectionMultiplexer @this, ValueTask<WriteResult> write, TaskCompletionSource<T?>? tcs, Message message, ServerEndPoint? server)
             {
                 var result = await write.ForAwait();

src/StackExchange.Redis/ExceptionFactory.cs

@@ -221,7 +221,7 @@ internal static Exception Timeout(ConnectionMultiplexer multiplexer, string? bas
             if (string.IsNullOrEmpty(baseErrorMessage) && result == WriteResult.TimeoutBeforeWrite)
             {
                 baseErrorMessage = message.IsBacklogged
-                    ? "The message timed out in the backlog attemtping to send because no connection became available"
+                    ? "The message timed out in the backlog attempting to send because no connection became available"
                     : "The timeout was reached before the message could be written to the output buffer, and it was not sent";
             }
 

src/StackExchange.Redis/PhysicalBridge.cs

@@ -173,15 +173,15 @@ private WriteResult QueueOrFailMessage(Message message)
 
             // Anything else goes in the bin - we're just not ready for you yet
             message.Cancel();
-            Multiplexer?.OnMessageFaulted(message, null);
+            Multiplexer.OnMessageFaulted(message, null);
             message.Complete();
             return WriteResult.NoConnectionAvailable;
         }
 
         private WriteResult FailDueToNoConnection(Message message)
         {
             message.Cancel();
-            Multiplexer?.OnMessageFaulted(message, null);
+            Multiplexer.OnMessageFaulted(message, null);
             message.Complete();
             return WriteResult.NoConnectionAvailable;
         }
@@ -485,7 +485,7 @@ private void AbandonPendingBacklog(Exception ex)
         {
             while (BacklogTryDequeue(out Message? next))
             {
-                Multiplexer?.OnMessageFaulted(next, ex);
+                Multiplexer.OnMessageFaulted(next, ex);
                 next.SetExceptionAndComplete(ex, this);
             }
         }
@@ -674,7 +674,7 @@ private WriteResult WriteMessageInsideLock(PhysicalConnection physical, Message
             var existingMessage = Interlocked.CompareExchange(ref _activeMessage, message, null);
             if (existingMessage != null)
             {
-                Multiplexer?.OnInfoMessage($"Reentrant call to WriteMessageTakingWriteLock for {message.CommandAndKey}, {existingMessage.CommandAndKey} is still active");
+                Multiplexer.OnInfoMessage($"Reentrant call to WriteMessageTakingWriteLock for {message.CommandAndKey}, {existingMessage.CommandAndKey} is still active");
                 return WriteResult.NoConnectionAvailable;
             }
 
@@ -819,9 +819,22 @@ private bool TryPushToBacklog(Message message, bool onlyIfExists, bool bypassBac
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private void BacklogEnqueue(Message message)
         {
+            bool wasEmpty = _backlog.IsEmpty;
+            // important that this *precedes* enqueue, to play well with HasPendingCallerFacingItems
+            Interlocked.Increment(ref _backlogCurrentEnqueued);
+            Interlocked.Increment(ref _backlogTotalEnqueued);
             _backlog.Enqueue(message);
             message.SetBacklogged();
-            Interlocked.Increment(ref _backlogTotalEnqueued);
+
+            if (wasEmpty)
+            {
+                // it is important to do this *after* adding, so that we can't
+                // get into a thread-race where the heartbeat checks too fast;
+                // the fact that we're accessing Multiplexer down here means that
+                // we're rooting it ourselves via the stack, so we don't need
+                // to worry about it being collected until at least after this
+                Multiplexer.Root();
+            }
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -1114,10 +1127,22 @@ private async Task ProcessBridgeBacklogAsync()
             }
         }
 
+        public bool HasPendingCallerFacingItems()
+        {
+            if (BacklogHasItems)
+            {
+                foreach (var item in _backlog) // non-consuming, thread-safe, etc
+                {
+                    if (!item.IsInternalCall) return true;
+                }
+            }
+            return physical?.HasPendingCallerFacingItems() ?? false;
+        }
+
         private WriteResult TimedOutBeforeWrite(Message message)
         {
             message.Cancel();
-            Multiplexer?.OnMessageFaulted(message, null);
+            Multiplexer.OnMessageFaulted(message, null);
             message.Complete();
             return WriteResult.TimeoutBeforeWrite;
         }

src/StackExchange.Redis/PhysicalConnection.cs

@@ -561,10 +561,31 @@ internal static void IdentifyFailureType(Exception? exception, ref ConnectionFai
 
         internal void EnqueueInsideWriteLock(Message next)
         {
+            var multiplexer = BridgeCouldBeNull?.Multiplexer;
+            if (multiplexer is null)
+            {
+                // multiplexer already collected? then we're almost certainly doomed;
+                // we can still process it to avoid making things worse/more complex,
+                // but: we can't reliably assume this works, so: shout now!
+                next.Cancel();
+                next.Complete();
+            }
+
+            bool wasEmpty;
             lock (_writtenAwaitingResponse)
             {
+                wasEmpty = _writtenAwaitingResponse.Count == 0;
                 _writtenAwaitingResponse.Enqueue(next);
             }
+            if (wasEmpty)
+            {
+                // it is important to do this *after* adding, so that we can't
+                // get into a thread-race where the heartbeat checks too fast;
+                // the fact that we're accessing Multiplexer down here means that
+                // we're rooting it ourselves via the stack, so we don't need
+                // to worry about it being collected until at least after this
+                multiplexer?.Root();
+            }
         }
 
         internal void GetCounters(ConnectionCounters counters)
@@ -1975,5 +1996,35 @@ private static RawResult ParseInlineProtocol(Arena<RawResult> arena, in RawResul
             }
             return new RawResult(block, false);
         }
+
+        internal bool HasPendingCallerFacingItems()
+        {
+            bool lockTaken = false;
+            try
+            {
+                Monitor.TryEnter(_writtenAwaitingResponse, 0, ref lockTaken);
+                if (lockTaken)
+                {
+                    if (_writtenAwaitingResponse.Count != 0)
+                    {
+                        foreach (var item in _writtenAwaitingResponse)
+                        {
+                            if (!item.IsInternalCall) return true;
+                        }
+                    }
+                    return false;
+                }
+                else
+                {
+                    // don't contend the lock; *presume* that something
+                    // qualifies; we can check again next heartbeat
+                    return true;
+                }
+            }
+            finally
+            {
+                if (lockTaken) Monitor.Exit(_writtenAwaitingResponse);
+            }
+        }
     }
 }

src/StackExchange.Redis/ServerEndPoint.cs

@@ -769,7 +769,7 @@ internal void OnHeartbeat()
 
             var source = TaskResultBox<T?>.Create(out var tcs, null);
             message.SetSource(processor, source);
-            if (bridge == null) bridge = GetBridge(message);
+            bridge ??= GetBridge(message);
 
             WriteResult result;
             if (bridge == null)
@@ -990,5 +990,12 @@ internal void SimulateConnectionFailure(SimulatedFailureType failureType)
             interactive?.SimulateConnectionFailure(failureType);
             subscription?.SimulateConnectionFailure(failureType);
         }
+
+        internal bool HasPendingCallerFacingItems()
+        {
+            // check whichever bridges exist
+            if (interactive?.HasPendingCallerFacingItems() == true) return true;
+            return subscription?.HasPendingCallerFacingItems() ?? false;
+        }
     }
 }