Fixed a race condition: Cancelling a question (at client side) which is about to be answered (from server side) might prevent releasing returned capabilities.

Capnp.Net.Runtime.Tests/TcpRpcInterop.cs

@@ -265,13 +265,13 @@ namespace Capnp.Net.Runtime.Tests
             }
         }
 
-        [TestMethod, Timeout(120000)]
+        [TestMethod]
         public void ReleaseOnCancelClient()
         {
             // Since we have a threaded model, there is no way to deterministically provoke the situation
             // where Cancel and Finish message cross paths. Instead, we'll do a lot of such requests and
             // later on verify that the handle count is 0.
-            int iterationCount = 1000;
+            int iterationCount = 5000;
 
             LaunchCompatTestProcess("server:MoreStuff", stdout =>
             {
@@ -305,9 +305,6 @@ namespace Capnp.Net.Runtime.Tests
                                         --handleCount;
                                         break;
 
-                                    case "getCallSequence":
-                                        break;
-
                                     default:
                                         Assert.Fail("Unexpected output");
                                         break;
@@ -323,35 +320,22 @@ namespace Capnp.Net.Runtime.Tests
                         for (int i = 0; i < iterationCount; i++)
                         {
                             var task = main.GetHandle(default);
-                            Impatient.GetAnswer(task).Dispose();
                             taskList.Add(task.ContinueWith(t =>
                             {
                                 try
                                 {
                                     t.Result.Dispose();
-                                    // Scenario 1: Cancellation happened after computing the answer, but before client-side completion.
                                 }
-                                catch (TaskCanceledException)
+                                catch (AggregateException ex) when (ex.InnerException is TaskCanceledException)
                                 {
-                                    // Scenario 2: Cancellation happened before or while computing the answer.
-                                }
-                                catch (AggregateException ex) when (ex.InnerException is ObjectDisposedException)
-                                {
-                                    // Scenario 3: Cancellation happened after computing the answer, and after client-side completion.
                                 }
                             }));
+                            Impatient.GetAnswer(task).Dispose();
                         }
 
                         // Ensure that all answers return (probably in canceled state)
                         Assert.IsTrue(Task.WhenAll(taskList).Wait(LargeNonDbgTimeout));
 
-                        // Not part of original test. "Terminate" sequence with
-                        // call to some different operation: getCallSequence
-                        // Motivation: Cancelling the GetHandle request immediately after sending it might seduce the
-                        // remote RPC implementation to immediately discard the request, before even calling the server
-                        // method. => Cannot rely on receiving *exactly* 2*iterationCount outputs.
-                        var term = main.GetCallSequence(0, default);
-
                         Assert.IsTrue(verifyOutputTask.Wait(LargeNonDbgTimeout));
 
                         // Not part of original test. Ensure that there is no unwanted extra output

Capnp.Net.Runtime.Tests/TcpRpcPorted.cs

@@ -139,8 +139,14 @@ namespace Capnp.Net.Runtime.Tests
                         var task = main.GetHandle(cts.Token);
                         cts.Cancel();
                         task.ContinueWith(t =>
+                        {
+                            try
                             {
                                 t.Result.Dispose();
+                            }
+                            catch (AggregateException ex) when (ex.InnerException is TaskCanceledException)
+                            {
+                            }
                             cts.Dispose();
                         });
                     }

Capnp.Net.Runtime/Rpc/PendingQuestion.cs

@@ -136,7 +136,10 @@ namespace Capnp.Rpc
             }
             else
             {
-                _tcs.TrySetResult(results);
+                if (!_tcs.TrySetResult(results))
+                {
+                    ReleaseOutCaps(results);
+                }
             }
         }
 
@@ -272,6 +275,14 @@ namespace Capnp.Rpc
             }
         }
 
+        static void ReleaseOutCaps(DeserializerState outParams)
+        {
+            foreach (var cap in outParams.Caps)
+            {
+                cap?.Release();
+            }
+        }
+
         internal void Send()
         {
             SerializerState inParams;
@@ -314,6 +325,7 @@ namespace Capnp.Rpc
         {
             SerializerState inParams;
             ConsumedCapability target;
+            bool justDisposed = false;
 
             lock (ReentrancyBlocker)
             {
@@ -327,6 +339,7 @@ namespace Capnp.Rpc
                     if (!StateFlags.HasFlag(State.Disposed))
                     {
                         StateFlags |= State.Disposed;
+                        justDisposed = true;
 
                         AutoFinish();
                     }
@@ -338,6 +351,11 @@ namespace Capnp.Rpc
             }
 
             ReleaseCaps(target, inParams);
+
+            if (justDisposed)
+            {
+                _tcs.TrySetCanceled();
+            }
         }
 
         /// <summary>

Capnp.Net.Runtime/Rpc/RefCountingCapability.cs

@@ -74,7 +74,7 @@ namespace Capnp.Rpc
 
         internal sealed override void Release()
         {
-            if (1 == Interlocked.Decrement(ref _refCount))
+            if (1 >= Interlocked.Decrement(ref _refCount))
             {
                 Dispose(true);
                 GC.SuppressFinalize(this);

Capnp.Net.Runtime/Rpc/RemoteAnswerCapability.cs

@@ -52,8 +52,12 @@ namespace Capnp.Rpc
         protected override void Dispose(bool disposing)
         {
             base.Dispose(disposing);
+
+            lock (_question.ReentrancyBlocker)
+            {
                 _resolvedCap?.Dispose();
             }
+        }
 
         protected override Proxy ResolvedCap
         {