4.x: Optimize Concat(IObservable<IObservable<T>>) (#491)

* 4.x: Optimize Concat(IObservable<IObservable<T>>)

* Add license header.

* Fix error handling

Author: akarnokd

Rx.NET/Source/src/System.Reactive/Linq/Observable/ConcatMany.cs

@@ -0,0 +1,237 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information. 
+
+using System.Collections.Concurrent;
+using System.Reactive.Disposables;
+using System.Threading;
+
+namespace System.Reactive.Linq.ObservableImpl
+{
+    internal sealed class ConcatMany<T> : IObservable<T>
+    {
+        readonly IObservable<IObservable<T>> sources;
+
+        internal ConcatMany(IObservable<IObservable<T>> sources)
+        {
+            this.sources = sources;
+        }
+
+        public IDisposable Subscribe(IObserver<T> observer)
+        {
+            var parent = new ConcatManyOuterObserver(observer);
+            var d = sources.SubscribeSafe(parent);
+            parent.OnSubscribe(d);
+            return parent;
+        }
+
+        internal sealed class ConcatManyOuterObserver : IObserver<IObservable<T>>, IDisposable
+        {
+            readonly IObserver<T> downstream;
+
+            readonly ConcurrentQueue<IObservable<T>> queue;
+
+            readonly InnerObserver innerObserver;
+
+            IDisposable upstream;
+
+            int trampoline;
+
+            Exception error;
+
+            bool done;
+
+            int active;
+
+            internal ConcatManyOuterObserver(IObserver<T> downstream)
+            {
+                this.downstream = downstream;
+                this.queue = new ConcurrentQueue<IObservable<T>>();
+                this.innerObserver = new InnerObserver(this);
+            }
+
+            internal void OnSubscribe(IDisposable d)
+            {
+                if (Interlocked.CompareExchange(ref upstream, d, null) != null)
+                {
+                    d?.Dispose();
+                }
+            }
+
+            public void Dispose()
+            {
+                innerObserver.Dispose();
+                DisposeMain();
+            }
+
+            void DisposeMain()
+            {
+                Interlocked.Exchange(ref upstream, BooleanDisposable.True)?.Dispose();
+            }
+
+            bool IsDisposed()
+            {
+                return Volatile.Read(ref upstream) == BooleanDisposable.True;
+            }
+
+            public void OnCompleted()
+            {
+                Volatile.Write(ref done, true);
+                Drain();
+            }
+
+            public void OnError(Exception error)
+            {
+                if (Interlocked.CompareExchange(ref this.error, error, null) == null)
+                {
+                    Volatile.Write(ref done, true);
+                    Drain();
+                }
+            }
+
+            public void OnNext(IObservable<T> value)
+            {
+                queue.Enqueue(value);
+                Drain();
+            }
+
+            void InnerNext(T item)
+            {
+                downstream.OnNext(item);
+            }
+
+            void InnerError(Exception error)
+            {
+                if (innerObserver.Finish())
+                {
+                    if (Interlocked.CompareExchange(ref this.error, error, null) == null)
+                    {
+                        Volatile.Write(ref done, true);
+                        Volatile.Write(ref active, 0);
+                        Drain();
+                    }
+                }
+            }
+
+            void InnerComplete()
+            {
+                if (innerObserver.Finish())
+                {
+                    Volatile.Write(ref active, 0);
+                    Drain();
+                }
+            }
+
+            void Drain()
+            {
+                if (Interlocked.Increment(ref trampoline) != 1)
+                {
+                    return;
+                }
+
+                do
+                {
+                    if (IsDisposed())
+                    {
+                        while (queue.TryDequeue(out var _)) ;
+                    }
+                    else
+                    {
+                        if (Volatile.Read(ref active) == 0)
+                        {
+                            var isDone = Volatile.Read(ref done);
+
+                            if (isDone)
+                            {
+                                var ex = Volatile.Read(ref error);
+                                if (ex != null)
+                                {
+                                    downstream.OnError(ex);
+                                    DisposeMain();
+                                    continue;
+                                }
+                            }
+
+                            if (queue.TryDequeue(out var source))
+                            {
+                                var sad = new SingleAssignmentDisposable();
+                                if (innerObserver.SetDisposable(sad))
+                                {
+                                    Interlocked.Exchange(ref active, 1);
+                                    sad.Disposable = source.SubscribeSafe(innerObserver);
+                                }
+                            }
+                            else
+                            {
+                                if (isDone)
+                                {
+                                    downstream.OnCompleted();
+                                    DisposeMain();
+                                }
+                            }
+                        }
+                    }
+                } while (Interlocked.Decrement(ref trampoline) != 0);
+            }
+
+            internal sealed class InnerObserver : IObserver<T>, IDisposable
+            {
+                readonly ConcatManyOuterObserver parent;
+
+                internal SingleAssignmentDisposable upstream;
+
+                static readonly SingleAssignmentDisposable DISPOSED;
+
+                static InnerObserver()
+                {
+                    DISPOSED = new SingleAssignmentDisposable();
+                    DISPOSED.Dispose();
+                }
+
+                internal InnerObserver(ConcatManyOuterObserver parent)
+                {
+                    this.parent = parent;
+                }
+
+                internal bool SetDisposable(SingleAssignmentDisposable sad)
+                {
+                    return Interlocked.CompareExchange(ref upstream, sad, null) == null;
+                }
+
+                internal bool Finish()
+                {
+                    var sad = Volatile.Read(ref upstream);
+                    if (sad != DISPOSED)
+                    {
+                        if (Interlocked.CompareExchange(ref upstream, null, sad) == sad)
+                        {
+                            sad.Dispose();
+                            return true;
+                        }
+                    }
+                    return false;
+                }
+
+                public void Dispose()
+                {
+                    Interlocked.Exchange(ref upstream, DISPOSED)?.Dispose();
+                }
+
+                public void OnCompleted()
+                {
+                    parent.InnerComplete();
+                }
+
+                public void OnError(Exception error)
+                {
+                    parent.InnerError(error);
+                }
+
+                public void OnNext(T value)
+                {
+                    parent.InnerNext(value);
+                }
+            }
+        }
+    }
+}

Rx.NET/Source/src/System.Reactive/Linq/QueryLanguage.Multiple.cs

@@ -230,7 +230,7 @@ public virtual IObservable<TSource> Concat<TSource>(IObservable<Task<TSource>> s
 
         private IObservable<TSource> Concat_<TSource>(IObservable<IObservable<TSource>> sources)
         {
-            return Merge(sources, 1);
+            return new ConcatMany<TSource>(sources);
         }
 
         #endregion