Report cancellation as cancellation on .NET Framework

src/DiffEngine/Protocol/ViewerClient.cs

@@ -137,6 +137,12 @@ public static async Task<bool> TrySendAsync(
 #else
             token.ThrowIfCancellationRequested();
             await client.ConnectAsync(IPAddress.Loopback, endpointPort);
+            // The abort registration cancels by closing the client, and .NET Framework's
+            // TcpClient.Dispose nulls its Client - so a token that fires around here leaves
+            // Configure and HalfClose dereferencing null rather than reporting cancellation.
+            // Asking the token directly is how that becomes the OperationCanceledException the
+            // caller is written against
+            token.ThrowIfCancellationRequested();
 #endif
             Configure(client, timeToWait);
             var stream = client.GetStream();
@@ -211,6 +217,11 @@ exception is
             SocketException or
             IOException or
             ObjectDisposedException or
+            // .NET Framework's TcpClient.Dispose nulls Client, so the abort registration closing
+            // the socket mid exchange leaves Configure or HalfClose dereferencing null. It is a
+            // torn down connection wearing the wrong exception type, and letting it escape turned
+            // an absent owner into a crash in the caller's test
+            NullReferenceException or
             AggregateException
             {
                 InnerException: SocketException or IOException

src/DiffEngine/Tray/PiperClient.cs

@@ -97,8 +97,18 @@ static async Task SendAsync(string payload, Cancel cancel)
         {
             await InnerSendAsync(payload, cancel);
         }
-        // Let cancellation surface to the caller; only genuine send failures are swallowed.
-        catch (Exception exception) when (exception is not OperationCanceledException)
+        // Let cancellation surface to the caller; only genuine send failures are swallowed. A
+        // NullReferenceException under a cancelled token is cancellation too - .NET Framework's
+        // Dispose nulls Client - and reporting it as a send failure would lose the cancel
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (NullReferenceException) when (cancel.IsCancellationRequested)
+        {
+            throw new OperationCanceledException(cancel);
+        }
+        catch (Exception exception)
         {
             HandleSendException(payload, exception);
         }
@@ -152,6 +162,11 @@ static async Task InnerSendAsync(string payload, Cancel cancel)
             using (cancel.Register(client.Close))
             {
                 await client.ConnectAsync(endpoint.Address, endpoint.Port);
+                // Dispose nulls Client on .NET Framework, so a token that fires around the connect
+                // leaves GetStream dereferencing null instead of reporting cancellation. Asking
+                // the token directly is what makes that an OperationCanceledException, which the
+                // caller lets through rather than swallowing as a send failure
+                cancel.ThrowIfCancellationRequested();
                 using var stream = client.GetStream();
                 using var writer = new StreamWriter(stream);
                 await writer.WriteAsync(payload);