Fix background thread leaks from rejected worker construction

CHANGELOG.md

@@ -1,5 +1,6 @@
 # 1.1.1
 
+- Fixed background thread leaks when worker construction is rejected
 - Fixed canonical DOM node identity and concrete prototypes through `DomSameObject` getters (#140)
 
 # 1.1.0

src/AngleSharp.Js.Tests/WorkerTests.cs

@@ -1,9 +1,11 @@
 namespace AngleSharp.Js.Tests
 {
+    using AngleSharp.Dom;
     using AngleSharp.Io;
     using AngleSharp.Js.Tests.Mocks;
     using NUnit.Framework;
     using System;
+    using System.Diagnostics;
     using System.Threading.Tasks;
 
     [TestFixture]
@@ -90,6 +92,34 @@ public async Task WorkerShouldRequireResourceLoading()
             Assert.AreEqual("true", result);
         }
 
+        [Test]
+        [NonParallelizable]
+        public async Task RejectedWorkersShouldNotLeaveRunningThreads()
+        {
+            using (var context = BrowsingContext.New(Configuration.Default.WithJs()))
+            {
+                var document = await context.OpenAsync(request => request.Content("<!doctype html>")).ConfigureAwait(false);
+                Assert.Throws<DomException>(() => new AngleSharp.Js.Dom.Worker(document.DefaultView, "/worker.js"));
+                var before = CountProcessThreads();
+
+                for (var i = 0; i < 32; i++)
+                {
+                    Assert.Throws<DomException>(() => new AngleSharp.Js.Dom.Worker(document.DefaultView, "/worker.js"));
+                }
+
+                // Allow unrelated runtime threads without hiding one surviving loop per failure.
+                Assert.Less(CountProcessThreads() - before, 16);
+            }
+        }
+
+        private static Int32 CountProcessThreads()
+        {
+            using (var process = Process.GetCurrentProcess())
+            {
+                return process.Threads.Count;
+            }
+        }
+
         [Test]
         public async Task WorkerDirectPostToOwnerShouldRaiseMessage()
         {
@@ -133,4 +163,4 @@ private static async Task WaitForResultAsync(AngleSharp.Dom.IDocument document,
             }
         }
     }
-}
+}

src/AngleSharp.Js/Dom/Worker.cs

@@ -60,28 +60,45 @@ public Worker(IWindow window, String source)
 
             var parentContext = _window.Document.Context;
             _scripting = parentContext.GetService<JsScriptingService>() ?? throw new DomException(DomError.NotSupported);
-            _workerLoop = new JsEventLoop();
-            var workerConfig = Configuration.Default
-                .With(_scripting)
-                .WithOnly(_workerLoop);
-            _workerContext = BrowsingContext.New(workerConfig);
-
+            EnsureLoaderAvailable();
+            var workerUrl = ResolveUrl(source);
             _pendingMessages = new Queue<Object>();
 
-            EnsureLoaderAvailable();
+            // Check the parent's capabilities before starting a thread that a failed constructor
+            // would leave unreachable. Release it if context creation or scheduling fails too.
+            _workerLoop = new JsEventLoop();
+            try
+            {
+                var workerConfig = Configuration.Default
+                    .With(_scripting)
+                    .WithOnly(_workerLoop);
+                _workerContext = BrowsingContext.New(workerConfig);
 
-            var workerUrl = ResolveUrl(source);
-            Enqueue(_workerLoop, TaskPriority.Critical, () =>
+                Enqueue(_workerLoop, TaskPriority.Critical, () =>
+                {
+                    try
+                    {
+                        InitializeWorker(workerUrl);
+                    }
+                    catch (Exception ex)
+                    {
+                        _startupError = ex;
+                    }
+                });
+            }
+            catch
             {
                 try
                 {
-                    InitializeWorker(workerUrl);
+                    _workerContext?.Dispose();
                 }
-                catch (Exception ex)
+                finally
                 {
-                    _startupError = ex;
+                    ((IDisposable)_workerLoop).Dispose();
                 }
-            });
+
+                throw;
+            }
         }
 
         #endregion
@@ -289,4 +306,4 @@ private static void Enqueue(IEventLoop loop, TaskPriority priority, Action actio
         #endregion
 
     }
-}
+}