Allow cancellation of all capability contexts

test/test_capability.py

@@ -1,4 +1,5 @@
 import pytest
+import asyncio
 
 import capnp
 import test_capability_capnp as capability
@@ -389,3 +390,22 @@ async def test_generic():
     obj = capnp._MallocMessageBuilder().get_root_as_any()
     obj.set_as_text("anypointer_")
     assert (await client.foo(obj)).b == "anypointer_test"
+
+
+class CancelServer(capability.TestInterface.Server):
+    def __init__(self, val=1):
+        self.val = val
+
+    async def foo(self, i, j, **kwargs):
+        with pytest.raises(asyncio.CancelledError):
+            await asyncio.sleep(10)
+
+
+async def test_cancel2():
+    client = capability.TestInterface._new_client(CancelServer())
+
+    task = asyncio.ensure_future(client.foo(1, True))
+    await asyncio.sleep(0)  # Make sure that the task runs
+    task.cancel()
+    with pytest.raises(asyncio.CancelledError):
+        await task