Use custom thread pool for saving the type system caches.

src/Libraries/NRefactory/ICSharpCode.NRefactory/TypeSystem/CecilLoader.cs

@@ -227,6 +227,7 @@ namespace ICSharpCode.NRefactory.TypeSystem
 			}
 			
 			AddToTypeSystemTranslationTable(this.currentAssembly, assemblyDefinition);
+			currentAssembly.Freeze();
 			
 			var result = this.currentAssembly;
 			this.currentAssembly = null;

src/Main/Base/Project/ICSharpCode.SharpDevelop.csproj

@@ -745,11 +745,13 @@
     <Compile Include="Src\Services\ProjectService\ProjectLoader.cs" />
     <Compile Include="Src\Services\ViewContentServiceAttribute.cs" />
     <Compile Include="Src\Util\AtomicBoolean.cs" />
+    <Compile Include="Src\Util\CustomThreadPoolTaskScheduler.cs" />
     <Compile Include="Src\Util\DotnetDetection.cs" />
     <Compile Include="Src\Util\FakeXmlViewContent.cs" />
     <Compile Include="Src\Util\IOTaskScheduler.cs" />
     <Compile Include="Src\Util\ReactiveExtensions.cs" />
     <Compile Include="Src\Util\SharpDevelopServiceContainer.cs" />
+    <Compile Include="Src\Util\SimpleTaskScheduler.cs" />
     <Compile Include="Src\Util\TreeNode.cs" />
     <Compile Include="Src\Util\NativeMethods.cs" />
     <Compile Include="Src\Util\ProcessRunnerException.cs" />

src/Main/Base/Project/Src/Services/IShutdownService.cs

@@ -49,9 +49,20 @@ namespace ICSharpCode.SharpDevelop
 		
 		/// <summary>
 		/// Gets a cancellation token that gets signalled when SharpDevelop is shutting down.
+		/// 
+		/// This cancellation token may be used to stop background calculations.
 		/// </summary>
 		CancellationToken ShutdownToken { get; }
 		
+		/// <summary>
+		/// Gets a cancellation token that gets signalled a couple of seconds after the ShutdownToken.
+		/// 
+		/// This cancellation token may be used to stop background calculations that should run
+		/// for a limited time after SharpDevelop is closed (e.g. saving state in caches
+		/// - work that should better run even though we're shutting down, but shouldn't take too long either)
+		/// </summary>
+		CancellationToken DelayedShutdownToken { get; }
+		
 		/// <summary>
 		/// Adds a background task on which SharpDevelop should wait on shutdown.
 		/// 

src/Main/Base/Project/Src/Services/ParserService/ParseProjectContent.cs

@@ -121,7 +121,7 @@ namespace ICSharpCode.SharpDevelop.Parser
 					} else {
 						RemoveCache(cacheFileName);
 					}
-				}, SD.ShutdownService.ShutdownToken);
+				}, SD.ShutdownService.DelayedShutdownToken);
 			SD.ShutdownService.AddBackgroundTask(task);
 			return task;
 		}

src/Main/Base/Project/Src/Util/CustomThreadPoolTaskScheduler.cs

@@ -0,0 +1,67 @@
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ICSharpCode.SharpDevelop.Util
+{
+	/// <summary>
+	/// A task scheduler that manages its own thread pool.
+	/// </summary>
+	public class CustomThreadPoolTaskScheduler : SimpleTaskScheduler
+	{
+		int currentThreadCount;
+		readonly int maxThreadCount;
+		
+		public CustomThreadPoolTaskScheduler(int maxThreadCount)
+		{
+			this.maxThreadCount = maxThreadCount;
+		}
+		
+		public override int MaximumConcurrencyLevel {
+			get { return maxThreadCount; }
+		}
+		
+		protected override void QueueTask(Task task)
+		{
+			base.QueueTask(task);
+			if (IncrementThreadCount()) {
+				// Successfully incremented the thread count, we may start a thread
+				StartThread(RunThread);
+				return;
+			}
+		}
+		
+		protected virtual void StartThread(ThreadStart start)
+		{
+			var t = new Thread(RunThread);
+			t.IsBackground = true;
+			t.Start();
+		}
+		
+		bool IncrementThreadCount()
+		{
+			int c = Volatile.Read(ref currentThreadCount);
+			while (c < maxThreadCount) {
+				if (Interlocked.CompareExchange(ref currentThreadCount, c + 1, c) == c) {
+					return true;
+				}
+			}
+			return false;
+		}
+		
+		void RunThread()
+		{
+			do {
+				// Run tasks while they are available:
+				while (TryRunNextTask());
+				// Decrement the thread count:
+				Interlocked.Decrement(ref currentThreadCount);
+				// Tasks might have been added while we were decrementing the thread count,
+				// so if the queue isn't empty anymore, resume this thread
+			} while(ScheduledTaskCount > 0 && IncrementThreadCount());
+		}
+	}
+}

src/Main/Base/Project/Src/Util/IOTaskScheduler.cs

@@ -12,9 +12,17 @@ namespace ICSharpCode.SharpDevelop.Util
 	/// </summary>
 	public class IOTaskScheduler
 	{
-		// TODO: use a limited-concurrency scheduler instead
+		static readonly CustomThreadPoolTaskScheduler scheduler = new CustomThreadPoolTaskScheduler(
+			Math.Min(Environment.ProcessorCount, 2));
+		
+		static readonly TaskFactory factory = new TaskFactory(scheduler);
+		
+		public static TaskScheduler Scheduler {
+			get { return scheduler; }
+		}
+		
 		public static TaskFactory Factory {
-			get { return Task.Factory; }
+			get { return factory; }
 		}
 	}
 }

src/Main/Base/Project/Src/Util/SimpleTaskScheduler.cs

@@ -0,0 +1,82 @@
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
+
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ICSharpCode.SharpDevelop.Util
+{
+	/// <summary>
+	/// A simple scheduler that adds tasks to a queue.
+	/// This scheduler does not create any worker threads on its own,
+	/// but requires external code to call <see cref="RunNextTask"/>.
+	/// </summary>
+	public class SimpleTaskScheduler : TaskScheduler, IDisposable
+	{
+		[ThreadStatic]
+		static SimpleTaskScheduler activeScheduler;
+		
+		BlockingCollection<Task> queue = new BlockingCollection<Task>();
+		
+		protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
+		{
+			return activeScheduler == this && base.TryExecuteTask(task);
+		}
+		
+		protected override void QueueTask(Task task)
+		{
+			queue.Add(task);
+		}
+		
+		protected override IEnumerable<Task> GetScheduledTasks()
+		{
+			return queue;
+		}
+		
+		protected int ScheduledTaskCount {
+			get { return queue.Count; }
+		}
+		
+		/// <summary>
+		/// Runs the next task in the queue.
+		/// If no task is available, this method will block.
+		/// </summary>
+		/// <param name="cancellationToken">Cancellation token that can be used to cancel
+		/// waiting for a task to become available. It cannot be used to cancel task execution!</param>
+		public void RunNextTask(CancellationToken cancellationToken = default(CancellationToken))
+		{
+			Task task = queue.Take(cancellationToken);
+			RunTask(task);
+		}
+		
+		public bool TryRunNextTask()
+		{
+			Task task;
+			if (queue.TryTake(out task)) {
+				RunTask(task);
+				return true;
+			} else {
+				return false;
+			}
+		}
+		
+		void RunTask(Task task)
+		{
+			var oldActiveScheduler = activeScheduler;
+			activeScheduler = this;
+			try {
+				base.TryExecuteTask(task);
+			} finally {
+				activeScheduler = oldActiveScheduler;
+			}
+		}
+		
+		public virtual void Dispose()
+		{
+			queue.Dispose();
+		}
+	}
+}

src/Main/SharpDevelop/Parser/AssemblyParserService.cs

@@ -160,10 +160,9 @@ namespace ICSharpCode.SharpDevelop.Parser
 				}
 			}
 			l.CancellationToken = cancellationToken;
-			l.InterningProvider = null;
 			pc = l.LoadAssembly(asm);
-			//SaveToCacheAsync(cacheFileName, lastWriteTime, pc).FireAndForget();
-			SaveToCache(cacheFileName, lastWriteTime, pc);
+			SaveToCacheAsync(cacheFileName, lastWriteTime, pc).FireAndForget();
+			//SaveToCache(cacheFileName, lastWriteTime, pc);
 			return pc;
 		}
 		

src/Main/SharpDevelop/Workbench/ShutdownService.cs

@@ -13,15 +13,21 @@ namespace ICSharpCode.SharpDevelop.Workbench
 {
 	sealed class ShutdownService : IShutdownService
 	{
-		CancellationTokenSource cts = new CancellationTokenSource();
+		CancellationTokenSource shutdownCTS = new CancellationTokenSource();
+		CancellationTokenSource delayedShutdownCTS = new CancellationTokenSource();
 		
 		public CancellationToken ShutdownToken {
-			get { return cts.Token; }
+			get { return shutdownCTS.Token; }
+		}
+		
+		public CancellationToken DelayedShutdownToken {
+			get { return delayedShutdownCTS.Token; }
 		}
 		
 		internal void SignalShutdownToken()
 		{
-			cts.Cancel();
+			shutdownCTS.Cancel();
+			delayedShutdownCTS.CancelAfter(2000);
 		}
 		
 		public bool Shutdown()