Always close emsd data connection on close

piker/clearing/_client.py

@@ -191,7 +191,6 @@ async def maybe_open_emsd(
             yield portal
 
 
-
 @asynccontextmanager
 async def open_ems(
     broker: str,
@@ -248,4 +247,13 @@ async def open_ems(
         with trio.fail_after(10):
             await book._ready_to_receive.wait()
 
+        try:
             yield book, trades_stream
+
+        finally:
+            # TODO: we want to eventually keep this up (by having
+            # the exec loop keep running in the pikerd tree) but for
+            # now we have to kill the context to avoid backpressure
+            # build-up on the shm write loop.
+            with trio.CancelScope(shield=True):
+                await trades_stream.aclose()