TestTaskRunner Now to move forward only.

Author: akarnokd

async-enumerable-dotnet-test/TestTaskRunnerTest.cs

@@ -322,5 +322,23 @@ public async void RightTaskQueued_Absolute()
                 Assert.False(ttr.HasTasks);
             }
         }
+
+        [Fact]
+        public async void Time_Moves_Forward()
+        {
+            var ttr = new TestTaskRunner(1000);
+
+            var t1 = ttr.CreateLambdaTask<long>(tcs => tcs.SetResult(ttr.Now), 0, true);
+            var t2 = ttr.CreateLambdaTask<long>(tcs => tcs.SetResult(ttr.Now), 500, true);
+            var t3 = ttr.CreateLambdaTask<long>(tcs => tcs.SetResult(ttr.Now), 1000, true);
+            var t4 = ttr.CreateLambdaTask<long>(tcs => tcs.SetResult(ttr.Now), 1500, true);
+            
+            ttr.AdvanceTimeBy(500);
+            
+            Assert.Equal(1000, await t1);
+            Assert.Equal(1000, await t2);
+            Assert.Equal(1000, await t3);
+            Assert.Equal(1500, await t4);
+        }
     }
 }

async-enumerable-dotnet/TestTaskRunner.cs

@@ -129,7 +129,8 @@ public void AdvanceTimeBy(long milliseconds)
 
                 if (has)
                 {
-                    Volatile.Write(ref _now, t.DueTime);
+                    // don't allow time to go back for past-scheduled work
+                    Volatile.Write(ref _now, Math.Max(Now, t.DueTime));
                     t.Signal();
                 }
                 else