Generalize and harden ST-sync-context

\n\nCommit migrated from 247bb34a63

src/Shared/test/Shared.Tests/SingleThreadedSynchronizationContext.cs

@@ -0,0 +1,45 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Concurrent;
+using System.Threading;
+
+namespace Microsoft.Extensions.Internal
+{
+    internal class SingleThreadedSynchronizationContext : SynchronizationContext
+    {
+        private readonly BlockingCollection<(SendOrPostCallback Callback, object State)> _queue = new BlockingCollection<(SendOrPostCallback Callback, object State)>();
+
+        public override void Send(SendOrPostCallback d, object state) // Sync operations
+        {
+            throw new NotSupportedException($"{nameof(SingleThreadedSynchronizationContext)} does not support synchronous operations.");
+        }
+
+        public override void Post(SendOrPostCallback d, object state) // Async operations
+        {
+            _queue.Add((d, state));
+        }
+
+        public static void Run(Action action)
+        {
+            var previous = Current;
+            var context = new SingleThreadedSynchronizationContext();
+            SetSynchronizationContext(context);
+            try
+            {
+                action();
+
+                while (context._queue.TryTake(out var item))
+                {
+                    item.Callback(item.State);
+                }
+            }
+            finally
+            {
+                context._queue.CompleteAdding();
+                SetSynchronizationContext(previous);
+            }
+        }
+    }
+}