Close fingergw connection handler on input termination

When a connection to the fingergw is terminated with no input, the
recv() method returns the empty string after poll returns an event
with the input flag set.

Handle that case by raising a BrokenPipeError which is already handled
by the calling code as a remote hang-up.

This has a noticeable impact on the test latency.

Change-Id: Ia56c94bc1e764b2f0f8ce3d6994689a3e2f7daff

zuul/lib/streamer_utils.py

@@ -50,7 +50,11 @@ class BaseFingerRequestHandler(socketserver.BaseRequestHandler):
                 raise Exception("Timeout while waiting for input")
             for fd, event in poll.poll(timeout):
                 if event & select.POLLIN:
-                    buffer += self.request.recv(self.MAX_REQUEST_LEN)
+                    x = self.request.recv(self.MAX_REQUEST_LEN)
+                    if not x:
+                        # This will cause the caller to quietly shut down
+                        raise BrokenPipeError
+                    buffer += x
                 else:
                     raise Exception("Received error event")
             if len(buffer) >= self.MAX_REQUEST_LEN: