Initial implementation of Amb.

Ix.NET/Source/System.Interactive.Async/System/Linq/Operators/Amb.cs

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+// 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.Generic;
+using System.Diagnostics;
+using System.Threading.Tasks;
+
+namespace System.Linq
+{
+    public static partial class AsyncEnumerableEx
+    {
+        public static IAsyncEnumerable<TSource> Amb<TSource>(this IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second)
+        {
+            if (first == null)
+                throw new ArgumentNullException(nameof(first));
+            if (second == null)
+                throw new ArgumentNullException(nameof(second));
+
+            return new AmbAsyncIterator<TSource>(first, second);
+        }
+
+        private sealed class AmbAsyncIterator<TSource> : AsyncIterator<TSource>
+        {
+            private readonly IAsyncEnumerable<TSource> first;
+            private readonly IAsyncEnumerable<TSource> second;
+
+            private IAsyncEnumerator<TSource> enumerator;
+
+            public AmbAsyncIterator(IAsyncEnumerable<TSource> first, IAsyncEnumerable<TSource> second)
+            {
+                Debug.Assert(first != null);
+                Debug.Assert(second != null);
+
+                this.first = first;
+                this.second = second;
+            }
+
+            public override AsyncIterator<TSource> Clone()
+            {
+                return new AmbAsyncIterator<TSource>(first, second);
+            }
+
+            public override async Task DisposeAsync()
+            {
+                if (enumerator != null)
+                {
+                    await enumerator.DisposeAsync().ConfigureAwait(false);
+                    enumerator = null;
+                }
+
+                await base.DisposeAsync().ConfigureAwait(false);
+            }
+
+            protected override async Task<bool> MoveNextCore()
+            {
+                switch (state)
+                {
+                    case AsyncIteratorState.Allocated:
+                        var firstEnumerator = first.GetAsyncEnumerator();
+                        var secondEnumerator = second.GetAsyncEnumerator();
+
+                        var firstMoveNext = firstEnumerator.MoveNextAsync();
+                        var secondMoveNext = secondEnumerator.MoveNextAsync();
+
+                        var winner = await Task.WhenAny(firstMoveNext, secondMoveNext).ConfigureAwait(false);
+
+                        //
+                        // REVIEW: An alternative option is to call DisposeAsync on the other and await it, but this has two drawbacks:
+                        //
+                        // 1. Concurrent DisposeAsync while a MoveNextAsync is in flight.
+                        // 2. The winner elected by Amb is blocked to yield results until the loser unblocks.
+                        //
+                        // The approach below has one drawback, namely that exceptions raised by loser are dropped on the floor.
+                        //
+
+                        if (winner == firstMoveNext)
+                        {
+                            enumerator = firstEnumerator;
+
+                            var ignored = secondMoveNext.ContinueWith(_ =>
+                            {
+                                secondEnumerator.DisposeAsync();
+                            });
+                        }
+                        else
+                        {
+                            enumerator = secondEnumerator;
+
+                            var ignored = firstMoveNext.ContinueWith(_ =>
+                            {
+                                firstEnumerator.DisposeAsync();
+                            });
+                        }
+
+                        state = AsyncIteratorState.Iterating;
+
+                        if (await winner.ConfigureAwait(false))
+                        {
+                            current = enumerator.Current;
+                            return true;
+                        }
+
+                        break;
+
+                    case AsyncIteratorState.Iterating:
+                        if (await enumerator.MoveNextAsync().ConfigureAwait(false))
+                        {
+                            current = enumerator.Current;
+                            return true;
+                        }
+
+                        break;
+                }
+
+                await DisposeAsync().ConfigureAwait(false);
+                return false;
+            }
+        }
+    }
+}