fixing race in Binder that can cause NullReferenceExceptions

src/Microsoft.Azure.WebJobs.Host/Bindings/Runtime/Binder.cs

@@ -19,6 +19,7 @@ namespace Microsoft.Azure.WebJobs
     /// </summary>
     public class Binder : IBinder, IWatchable, IDisposable
     {
+        private readonly object _bindersLock = new object();
         private readonly IAttributeBindingSource _bindingSource;
         private readonly IList<IValueBinder> _binders = new List<IValueBinder>();
         private readonly RuntimeBindingWatcher _watcher = new RuntimeBindingWatcher();
@@ -54,6 +55,11 @@ internal Binder(IAttributeBindingSource bindingSource)
             }
         }
 
+        /// <summary>
+        /// For testing only.
+        /// </summary>
+        internal IList<IValueBinder> Binders => _binders;
+
         /// <summary>
         /// Gets the binding data.
         /// </summary>
@@ -133,7 +139,10 @@ IWatcher IWatchable.Watcher
             IValueBinder binder = provider as IValueBinder;
             if (binder != null)
             {
-                _binders.Add(binder);
+                lock (_bindersLock)
+                {
+                    _binders.Add(binder);
+                }
             }
 
             IDisposable disposableProvider = provider as IDisposable;
@@ -153,7 +162,13 @@ IWatcher IWatchable.Watcher
         /// <returns></returns>
         internal async Task Complete(CancellationToken cancellationToken)
         {
-            foreach (IValueBinder binder in _binders)
+            IValueBinder[] binders;
+            lock (_bindersLock)
+            {
+                binders = _binders.ToArray();
+            }
+
+            foreach (IValueBinder binder in binders)
             {
                 // Binding can only be uses for non-Out parameters, and their binders ignore this argument.
                 await binder.SetValueAsync(value: null, cancellationToken: cancellationToken);

test/Microsoft.Azure.WebJobs.Host.UnitTests/Bindings/Runtime/BinderTests.cs

@@ -0,0 +1,79 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT License. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Azure.WebJobs.Host.Bindings;
+using Microsoft.Azure.WebJobs.Host.Bindings.Runtime;
+using Microsoft.Azure.WebJobs.Host.Protocols;
+using Moq;
+using Xunit;
+
+namespace Microsoft.Azure.WebJobs.Host.UnitTests.Bindings.Runtime;
+
+public class BinderTests
+{
+    [Fact]
+    public async Task Complete_AfterConcurrentBinds_ThrowsNullReferenceException()
+    {
+        // Arrange
+        var mockValueBinder = new Mock<IValueBinder>();
+        mockValueBinder.Setup(m => m.Type).Returns(typeof(object));
+
+        var mockBinding = new Mock<IBinding>();
+        mockBinding.Setup(b => b.BindAsync(It.IsAny<BindingContext>())).ReturnsAsync(mockValueBinder.Object);
+        mockBinding.Setup(b => b.ToParameterDescriptor()).Returns(new ParameterDescriptor());
+
+        var mockBindingSource = new Mock<IAttributeBindingSource>();
+        mockBindingSource.Setup(s => s.BindAsync<object>(It.IsAny<Attribute>(), It.IsAny<Attribute[]>(), It.IsAny<CancellationToken>()))
+                         .ReturnsAsync(mockBinding.Object);
+
+        var functionContext = new FunctionBindingContext(Guid.NewGuid(), CancellationToken.None, null);
+        mockBindingSource.Setup(s => s.AmbientBindingContext).Returns(new AmbientBindingContext(functionContext, new Dictionary<string, object>().AsReadOnly()));
+
+        var binder = new Binder(mockBindingSource.Object);
+        var threads = new List<Thread>();
+
+        const int numConcurrentThreads = 100;
+        const int numTasksPerThread = 100;
+
+        // Call BindAsync simultaneously from many threads. There was a race where this would corrupt
+        // the internal _binders list.
+        for (int i = 0; i < numConcurrentThreads; i++)
+        {
+            threads.Add(new Thread(() =>
+            {
+                var tasks = new List<Task>();
+                for (int t = 0; t < numTasksPerThread; t++)
+                {
+                    tasks.Add(binder.BindAsync<object>(new TestAttribute()));
+                }
+
+                Task.WaitAll(tasks.ToArray());
+            }));
+        }
+
+        foreach (var thread in threads)
+        {
+            thread.Start();
+        }
+
+        foreach (var thread in threads)
+        {
+            thread.Join();
+        }
+
+        // Now that all concurrent binds are done, call Complete.
+        // If the list's internal state was corrupted by the concurrent Add calls,
+        // this iteration will likely hit a null element.
+        await binder.Complete(CancellationToken.None);
+
+        // Because List resizing is not deterministic, another side effect is having a
+        // different count of binders than expected.
+        Assert.Equal(numConcurrentThreads * numTasksPerThread, binder.Binders.Count);
+    }
+
+    private class TestAttribute : Attribute { }
+}