handling exceptions from FileWatcherEventSource

Author: brettsam

src/WebJobs.Script/Eventing/File/FileWatcherEventSource.cs

@@ -13,6 +13,7 @@ public sealed class FileWatcherEventSource : IDisposable
         private readonly AutoRecoveringFileSystemWatcher _fileWatcher;
         private readonly IScriptEventManager _eventManager;
         private readonly string _source;
+        private readonly TraceWriter _traceWriter;
         private bool _disposed = false;
 
         public FileWatcherEventSource(IScriptEventManager eventManager,
@@ -27,12 +28,22 @@ public FileWatcherEventSource(IScriptEventManager eventManager,
             _eventManager = eventManager;
             _fileWatcher = new AutoRecoveringFileSystemWatcher(path, filter, includeSubdirectories, changeTypes, traceWriter);
             _fileWatcher.Changed += FileChanged;
+            _traceWriter = traceWriter;
         }
 
         private void FileChanged(object sender, FileSystemEventArgs e)
         {
-            var fileEvent = new FileEvent(_source, e);
-            _eventManager.Publish(fileEvent);
+            // This handler is called on a background thread, so any exceptions thrown will crash
+            // the process. Handle and log all errors instead.
+            try
+            {
+                var fileEvent = new FileEvent(_source, e);
+                _eventManager.Publish(fileEvent);
+            }
+            catch (Exception ex)
+            {
+                _traceWriter?.Error($"Error handling '{e.ChangeType}' notification for '{e.FullPath}'.", ex);
+            }
         }
 
         private void Dispose(bool disposing)

test/WebJobs.Script.Tests/Eventing/File/FileWatcherEventSourceTests.cs

@@ -3,10 +3,10 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
 using System.Reactive.Linq;
-using System.Text;
 using System.Threading.Tasks;
 using Microsoft.Azure.WebJobs.Script.Eventing;
 using Microsoft.Azure.WebJobs.Script.Eventing.File;
@@ -66,5 +66,29 @@ public async Task DisposedSource_DoesNotPublishEvents()
                 Assert.Equal(2, events.Count);
             }
         }
+
+        [Fact]
+        public async Task FileChangedHandlerExceptions_LogError_AndDoNotThrow()
+        {
+            // Note: The FileSystemWatcher handler is called on a background thread. This means that when the handler
+            //       throws an exception, it crashes the process. The Visual Studio test runner handles that scenario and
+            //       does not crash. Before this handler was fixed, this test would only fail in the console runner.
+
+            TestTraceWriter traceWriter = new TestTraceWriter(TraceLevel.Verbose);
+            using (var directory = new TempDirectory())
+            using (var eventManager = new ScriptEventManager())
+            using (var eventSource = new FileWatcherEventSource(eventManager, "TestSource", directory.Path, traceWriter: traceWriter))
+            {
+                var expectedException = new InvalidOperationException("This should not crash the process!");
+                eventManager.Subscribe(p => throw expectedException);
+
+                string fullPath = Path.Combine(directory.Path, "test.txt");
+                File.WriteAllText(fullPath, "Test");
+
+                await TestHelpers.Await(
+                    () => traceWriter.GetTraces().Any(p => p.Level == TraceLevel.Error && p.Message.Contains(fullPath) && p.Exception == expectedException),
+                    timeout: 2000, pollingInterval: 250);
+            }
+        }
     }
 }