Add test to verify remote task cancellation

tests/test_streaming.py

@@ -13,6 +13,12 @@ async def stream_seq(sequence):
         yield i
         await trio.sleep(0.1)
 
+    # block indefinitely waiting to be cancelled by ``aclose()`` call
+    with trio.open_cancel_scope() as cs:
+        await trio.sleep(float('inf'))
+        assert 0
+    assert cs.cancelled_caught
+
 
 async def stream_from_single_subactor():
     """Verify we can spawn a daemon actor and retrieve streamed data.
@@ -37,15 +43,20 @@ async def stream_from_single_subactor():
                 )
                 # it'd sure be nice to have an asyncitertools here...
                 iseq = iter(seq)
+                ival = next(iseq)
                 async for val in agen:
-                    assert val == next(iseq)
+                    assert val == ival
-                    # TODO: test breaking the loop (should it kill the
+                    try:
-                    # far end?)
+                        ival = next(iseq)
-                    # break
+                    except StopIteration:
-                    # terminate far-end async-gen
+                        # should cancel far end task which will be
-                    # await gen.asend(None)
+                        # caught and no error is raised
-                    # break
+                        await agen.aclose()
 
+                await trio.sleep(0.3)
+                try:
+                    await agen.__anext__()
+                except StopAsyncIteration:
                     # stop all spawned subactors
                     await portal.cancel_actor()
                 # await nursery.cancel()