Add debouncing to FileSystemWatcher events (#129)

* debounces FileSystemWatcher events
* makes debounce interval configurable
* ignores SQL timeout in unit tests (the purpose is to load the native libraries, not the actual connection to the database)

src/Plugins/Internal/Debouncer.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace McMaster.NETCore.Plugins.Internal
+{
+    internal class Debouncer : IDisposable
+    {
+        private readonly CancellationTokenSource _cts = new CancellationTokenSource();
+        private readonly TimeSpan _waitTime;
+        private int _counter;
+
+        public Debouncer(TimeSpan waitTime)
+        {
+            _waitTime = waitTime;
+        }
+
+        public void Execute(Action action)
+        {
+            var current = Interlocked.Increment(ref _counter);
+
+            Task.Delay(_waitTime).ContinueWith(task =>
+            {
+                // Is this the last task that was queued?
+                if (current == _counter && !_cts.IsCancellationRequested)
+                {
+                    action();
+                }
+
+                task.Dispose();
+            }, _cts.Token);
+        }
+
+        public void Dispose()
+        {
+            _cts.Cancel();
+        }
+    }
+}

src/Plugins/PluginConfig.cs

@@ -86,6 +86,12 @@ public bool IsUnloadable
         /// Use the event <see cref="PluginLoader.Reloaded" /> to be notified of changes.
         /// </summary>
         public bool EnableHotReload { get; set; }
+
+        /// <summary>
+        /// Specifies the delay to reload a plugin, after file changes have been detected.
+        /// Default value is 200 milliseconds.
+        /// </summary>
+        public TimeSpan ReloadDelay { get; set; } = TimeSpan.FromMilliseconds(200);
 #endif
     }
 }

src/Plugins/PluginLoader.cs

@@ -5,6 +5,7 @@
 using System.IO;
 using System.Reflection;
 using System.Runtime.Loader;
+using McMaster.NETCore.Plugins.Internal;
 using McMaster.NETCore.Plugins.Loader;
 
 namespace McMaster.NETCore.Plugins
@@ -153,6 +154,7 @@ public static PluginLoader CreateFromAssemblyFile(string assemblyFile, Action<Pl
 
 #if FEATURE_UNLOAD
         private FileSystemWatcher? _fileWatcher;
+        private Debouncer? _debouncer;
 #endif
 
         /// <summary>
@@ -223,13 +225,14 @@ This is a very simple implementation.
             Some improvements that could be made in the future:
 
                 * Watch all directories which contain assemblies that could be loaded
-                * Debounce changes. When files are written in-place, there can be multiple events within a few milliseconds.
                 * Support a polling file watcher.
                 * Handle delete/recreate better.
 
             If you're interested in making improvements, feel free to send a pull request.
             */
 
+            _debouncer = new Debouncer(_config.ReloadDelay);
+
             _fileWatcher = new FileSystemWatcher();
             _fileWatcher.Path = Path.GetDirectoryName(_config.MainAssemblyPath);
             _fileWatcher.Changed += OnFileChanged;
@@ -242,7 +245,7 @@ private void OnFileChanged(object source, FileSystemEventArgs e)
         {
             if (!_disposed)
             {
-                Reload();
+                _debouncer?.Execute(Reload);
             }
         }
 #endif
@@ -322,6 +325,8 @@ public void Dispose()
                 _fileWatcher.Dispose();
             }
 
+            _debouncer?.Dispose();
+
             if (_context.IsCollectible)
             {
                 _context.Unload();

src/Plugins/PublicAPI.Unshipped.txt

@@ -1 +1,3 @@
-McMaster.NETCore.Plugins.Loader.AssemblyLoadContextBuilder.ShadowCopyNativeLibraries() -> McMaster.NETCore.Plugins.Loader.AssemblyLoadContextBuilder
+McMaster.NETCore.Plugins.Loader.AssemblyLoadContextBuilder.ShadowCopyNativeLibraries() -> McMaster.NETCore.Plugins.Loader.AssemblyLoadContextBuilder
+McMaster.NETCore.Plugins.PluginConfig.ReloadDelay.get -> System.TimeSpan
+McMaster.NETCore.Plugins.PluginConfig.ReloadDelay.set -> void

test/Plugins.Tests/DebouncerTests.cs

@@ -0,0 +1,57 @@
+using System;
+using System.Threading.Tasks;
+using McMaster.NETCore.Plugins.Internal;
+using Xunit;
+
+namespace McMaster.NETCore.Plugins.Tests
+{
+    public class DebouncerTests
+    {
+        [Fact]
+        public async Task InvocationIsDelayed()
+        {
+            var executionCounter = 0;
+
+            var debouncer = new Debouncer(TimeSpan.FromSeconds(.1));
+            debouncer.Execute(() => executionCounter++);
+
+            Assert.Equal(0, executionCounter);
+
+            await Task.Delay(TimeSpan.FromSeconds(.5));
+
+            Assert.Equal(1, executionCounter);
+        }
+
+        [Fact]
+        public async Task ActionsAreDebounced()
+        {
+            var executionCounter = 0;
+
+            var debouncer = new Debouncer(TimeSpan.FromSeconds(.1));
+            debouncer.Execute(() => executionCounter++);
+            debouncer.Execute(() => executionCounter++);
+            debouncer.Execute(() => executionCounter++);
+
+            await Task.Delay(TimeSpan.FromSeconds(.5));
+
+            Assert.Equal(1, executionCounter);
+        }
+
+        [Fact]
+        public async Task OnlyLastActionIsInvoked()
+        {
+            string? invokedAction = null;
+
+            var debouncer = new Debouncer(TimeSpan.FromSeconds(.1));
+            foreach (var action in new[]{"a", "b", "c"})
+            {
+                debouncer.Execute(() => invokedAction = action);
+            }
+
+            await Task.Delay(TimeSpan.FromSeconds(.5));
+
+            Assert.NotNull(invokedAction);
+            Assert.Equal("c", invokedAction);
+        }
+    }
+}

test/TestProjects/SqlClientApp/Program.cs

@@ -12,10 +12,19 @@ public static void Main(string[] args)
 
         public static bool Run()
         {
-            using (var client = new SqlConnection(@"Data Source=(localdb)\mssqllocaldb;Integrated Security=True"))
+            try
             {
-                client.Open();
-                return !string.IsNullOrEmpty(client.ServerVersion);
+                using (var client = new SqlConnection(@"Data Source=(localdb)\mssqllocaldb;Integrated Security=True"))
+                {
+                    client.Open();
+                    return !string.IsNullOrEmpty(client.ServerVersion);
+                }
+            }
+            catch (SqlException ex) when (ex.Number == -2)  // -2 means SQL timeout
+            {
+                // When running the test in Azure DevOps build pipeline, we'll get a SqlException with "Connection Timeout Expired".
+                // We can ignore this safely in unit tests.
+                return true;
             }
         }
     }