Fixed zero timeout compatibility with IsTimedOut property

Author: sakno

src/DotNext.Tests/Threading/CancellationTokenMultiplexerTests.cs

@@ -72,13 +72,15 @@ public static void ExtraListOverflow()
         True(scope.Token.IsCancellationRequested);
     }
 
-    [Fact]
-    public static async Task TimeOut()
+    [Theory]
+    [InlineData(0)]
+    [InlineData(1)]
+    public static async Task TimeOut(int timeout)
     {
         using var cts = new CancellationTokenSource();
         var multiplexer = new CancellationTokenMultiplexer();
 
-        await using var scope = multiplexer.Combine(TimeSpan.FromMilliseconds(1), [cts.Token]);
+        await using var scope = multiplexer.Combine(TimeSpan.FromMilliseconds(timeout), [cts.Token]);
         await scope.Token.WaitAsync();
 
         Equal(scope.Token, scope.CancellationOrigin);
@@ -94,7 +96,7 @@ public static async Task LazyTimeout()
         await using var scope = multiplexer.CombineAndSetTimeoutLater([]);
         False(scope.Token.IsCancellationRequested);
 
-        scope.Timeout = TimeSpan.FromMilliseconds(1);
+        scope.Timeout = TimeSpan.FromMilliseconds(0);
         await scope.Token.WaitAsync();
 
         Equal(scope.Token, scope.CancellationOrigin);

src/DotNext.Threading/Threading/CancellationTokenMultiplexer.Scope.cs

@@ -66,7 +66,7 @@ private static CancellationToken GetToken(MultiplexerOrToken value)
         /// <summary>
         /// Gets a value indicating that the multiplexed token is cancelled by the timeout.
         /// </summary>
-        public bool IsTimedOut => source?.IsRootCause ?? false;
+        public bool IsTimedOut => source?.IsRootCause ?? GetToken(multiplexerOrToken) == TimedOutToken;
 
         internal void SetTimeout(TimeSpan value) => source?.CancelAfter(value);
 
@@ -158,4 +158,22 @@ public TimeSpan Timeout
         /// <inheritdoc/>
         public ValueTask DisposeAsync() => scope.DisposeAsync();
     }
+
+    private static CancellationToken TimedOutToken => TimedOutTokenSource.Token;
+}
+
+// This source represents a canceled token that is canceled by zero timeout.
+// It's not possible to use new CancellationToken(canceled: true) because the multiplexer
+// cannot distinguish between the canceled token passed by the user code and the token that represents the timeout.
+// This class is not accessible by the user code, and its token cannot be passed to the multiplexer directly.
+file static class TimedOutTokenSource
+{
+    public static readonly CancellationToken Token;
+
+    static TimedOutTokenSource()
+    {
+        using var source = new CancellationTokenSource();
+        Token = source.Token;
+        source.Cancel();
+    }
 }

src/DotNext.Threading/Threading/CancellationTokenMultiplexer.cs

@@ -75,7 +75,7 @@ public Scope Combine(ReadOnlySpan<CancellationToken> tokens) // TODO: use params
     /// <exception cref="ArgumentOutOfRangeException"><paramref name="timeout"/> is negative or too large.</exception>
     public Scope Combine(TimeSpan timeout, ReadOnlySpan<CancellationToken> tokens) => timeout.Ticks switch
     {
-        0L => new(new CancellationToken(canceled: true)),
+        0L => new(TimedOutToken),
         Timeout.InfiniteTicks => Combine(tokens),
         < 0L or > Timeout.MaxTimeoutParameterTicks => throw new ArgumentOutOfRangeException(nameof(timeout)),
         _ => new(this, timeout, tokens)