Add support for cancelling async value computation

app/Desktop/Main/Controls/MessageFilterPanelModel.cs

@@ -153,7 +153,7 @@ namespace DHT.Desktop.Main.Controls {
 			else {
 				exportedMessageCount = null;
 				UpdateFilterStatisticsText();
-				exportedMessageCountComputer.Compute(_ => db.CountMessages(filter));
+				exportedMessageCountComputer.Compute(() => db.CountMessages(filter));
 			}
 		}
 

app/Server/Database/Sqlite/SqliteDatabaseFile.cs

@@ -2,7 +2,6 @@ using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Text;
-using System.Threading;
 using System.Threading.Tasks;
 using DHT.Server.Data;
 using DHT.Server.Data.Filters;
@@ -458,7 +457,7 @@ LEFT JOIN replied_to rt ON m.message_id = rt.message_id" + filter.GenerateWhereC
 			Statistics.TotalUsers = conn.SelectScalar("SELECT COUNT(*) FROM users") as long? ?? 0;
 		}
 
-		private long ComputeMessageStatistics(CancellationToken token) {
+		private long ComputeMessageStatistics() {
 			using var conn = pool.Take();
 			return conn.SelectScalar("SELECT COUNT(*) FROM messages") as long? ?? 0L;
 		}

app/Utils/Tasks/AsyncValueComputer.cs

@@ -11,8 +11,10 @@ namespace DHT.Utils.Tasks {
 
 		private readonly object stateLock = new ();
 
-		private CancellationTokenSource? currentCancellationTokenSource;
+		private SoftHardCancellationToken? currentCancellationTokenSource;
-		private Func<CancellationToken, TValue>? currentComputeFunction;
+		private bool wasHardCancelled = false;
+		
+		private Func<TValue>? currentComputeFunction;
 		private bool hasComputeFunctionChanged = false;
 
 		private AsyncValueComputer(Action<TValue> resultProcessor, TaskScheduler resultTaskScheduler, bool processOutdatedResults) {
@@ -21,12 +23,21 @@ namespace DHT.Utils.Tasks {
 			this.processOutdatedResults = processOutdatedResults;
 		}
 
-		public void Compute(Func<CancellationToken, TValue> func) {
+		public void Cancel() {
 			lock (stateLock) {
+				wasHardCancelled = true;
+				currentCancellationTokenSource?.RequestHardCancellation();
+			}
+		}
+
+		public void Compute(Func<TValue> func) {
+			lock (stateLock) {
+				wasHardCancelled = false;
+				
 				if (currentComputeFunction != null) {
 					currentComputeFunction = func;
 					hasComputeFunctionChanged = true;
-					currentCancellationTokenSource?.Cancel();
+					currentCancellationTokenSource?.RequestSoftCancellation();
 				}
 				else {
 					EnqueueComputation(func);
@@ -35,19 +46,18 @@ namespace DHT.Utils.Tasks {
 		}
 
 		[SuppressMessage("ReSharper", "MethodSupportsCancellation")]
-		private void EnqueueComputation(Func<CancellationToken, TValue> func) {
+		private void EnqueueComputation(Func<TValue> func) {
-			var cancellationTokenSource = new CancellationTokenSource();
+			var cancellationTokenSource = new SoftHardCancellationToken();
-			var cancellationToken = cancellationTokenSource.Token;
 
-			currentCancellationTokenSource?.Cancel();
+			currentCancellationTokenSource?.RequestSoftCancellation();
 			currentCancellationTokenSource = cancellationTokenSource;
 			currentComputeFunction = func;
 			hasComputeFunctionChanged = false;
 
-			var task = Task.Run(() => func(cancellationToken));
+			var task = Task.Run(func);
 
 			task.ContinueWith(t => {
-				if (processOutdatedResults || !cancellationToken.IsCancellationRequested) {
+				if (!cancellationTokenSource.IsCancelled(processOutdatedResults)) {
 					resultProcessor(t.Result);
 				}
 			}, CancellationToken.None, TaskContinuationOptions.NotOnFaulted, resultTaskScheduler);
@@ -60,11 +70,12 @@ namespace DHT.Utils.Tasks {
 						currentCancellationTokenSource = null;
 					}
 
-					if (hasComputeFunctionChanged) {
+					if (hasComputeFunctionChanged && !wasHardCancelled) {
 						EnqueueComputation(currentComputeFunction);
 					}
 					else {
 						currentComputeFunction = null;
+						hasComputeFunctionChanged = false;
 					}
 				}
 			});
@@ -72,9 +83,9 @@ namespace DHT.Utils.Tasks {
 
 		public sealed class Single {
 			private readonly AsyncValueComputer<TValue> baseComputer;
-			private readonly Func<CancellationToken, TValue> resultComputer;
+			private readonly Func<TValue> resultComputer;
 
-			internal Single(AsyncValueComputer<TValue> baseComputer, Func<CancellationToken, TValue> resultComputer) {
+			internal Single(AsyncValueComputer<TValue> baseComputer, Func<TValue> resultComputer) {
 				this.baseComputer = baseComputer;
 				this.resultComputer = resultComputer;
 			}
@@ -107,7 +118,7 @@ namespace DHT.Utils.Tasks {
 				return new AsyncValueComputer<TValue>(resultProcessor, resultTaskScheduler, processOutdatedResults);
 			}
 
-			public Single BuildWithComputer(Func<CancellationToken, TValue> resultComputer) {
+			public Single BuildWithComputer(Func<TValue> resultComputer) {
 				return new Single(Build(), resultComputer);
 			}
 		}

app/Utils/Tasks/SoftHardCancellationToken.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Threading;
+
+namespace DHT.Utils.Tasks {
+	/// <summary>
+	/// Manages a pair of cancellation tokens that follow these rules:
+	/// <list type="number">
+	/// <item><description>If the soft token is cancelled, the hard token remains uncancelled.</description></item>
+	/// <item><description>If the hard token is cancelled, the soft token is also cancelled.</description></item>
+	/// </list>
+	/// </summary>
+	sealed class SoftHardCancellationToken : IDisposable {
+		private readonly CancellationTokenSource soft;
+		private readonly CancellationTokenSource hard;
+		
+		public SoftHardCancellationToken() {
+			this.soft = new CancellationTokenSource();
+			this.hard = new CancellationTokenSource();
+		}
+
+		public bool IsCancelled(bool onlyHardCancellation) {
+			return (onlyHardCancellation ? hard : soft).IsCancellationRequested;
+		}
+
+		public void RequestSoftCancellation() {
+			soft.Cancel();
+		}
+
+		public void RequestHardCancellation() {
+			soft.Cancel();
+			hard.Cancel();
+		}
+
+		public void Dispose() {
+			soft.Dispose();
+			hard.Dispose();
+		}
+	}
+}