[stream-refactor] parent: fix crash on graceful shutdown

Now it's possible for stream.protocol to not refer to MitogenProtocol,
move the signal handler to a MitogenProtocol subclass instead.

Fixes a crash where CTRL+C during child bootstrap would print
AttributeError.

mitogen/parent.py

@@ -638,10 +638,10 @@ def _upgrade_broker(broker):
     # This function is deadly! The act of calling start_receive() generates log
     # messages which must be silenced as the upgrade progresses, otherwise the
     # poller state will change as it is copied, resulting in write fds that are
-    # lost. (Due to LogHandler->Router->Stream->Broker->Poller, where Stream
+    # lost. (Due to LogHandler->Router->Stream->Protocol->Broker->Poller, where
-    # only calls start_transmit() when transitioning from empty to non-empty
+    # Stream only calls start_transmit() when transitioning from empty to
-    # buffer. If the start_transmit() is lost, writes from the child hang
+    # non-empty buffer. If the start_transmit() is lost, writes from the child
-    # permanently).
+    # hang permanently).
     root = logging.getLogger()
     old_level = root.level
     root.setLevel(logging.CRITICAL)
@@ -810,7 +810,8 @@ class CallSpec(object):
 class PollPoller(mitogen.core.Poller):
     """
     Poller based on the POSIX poll(2) interface. Not available on some versions
-    of OS X, otherwise it is the preferred poller for small FD counts.
+    of OS X, otherwise it is the preferred poller for small FD counts, as there
+    is no setup/teardown/configuration system call overhead.
     """
     SUPPORTED = hasattr(select, 'poll')
     _repr = 'PollPoller()'
@@ -1106,8 +1107,8 @@ class BootstrapProtocol(RegexProtocol):
     """
     Respond to stdout of a child during bootstrap. Wait for EC0_MARKER to be
     written by the first stage to indicate it can receive the bootstrap, then
-    await EC1_MARKER to indicate success, and
+    await EC1_MARKER to indicate success, and :class:`MitogenProtocol` can be
-    :class:`mitogen.core.MitogenProtocol` can be enabled.
+    enabled.
     """
     #: Sentinel value emitted by the first stage to indicate it is ready to
     #: receive the compressed bootstrap. For :mod:`mitogen.ssh` this must have
@@ -1161,6 +1162,26 @@ class LogProtocol(LineLoggingProtocolMixin, mitogen.core.DelimitedProtocol):
         LOG.info(u'%s: %s', self.stream.name, line.decode('utf-8', 'replace'))
 
 
+class MitogenProtocol(mitogen.core.MitogenProtocol):
+    """
+    Extend core.MitogenProtocol to cause SHUTDOWN to be sent to the child
+    during graceful shutdown.
+    """
+    def on_shutdown(self, broker):
+        """
+        Respond to the broker's request for the stream to shut down by sending
+        SHUTDOWN to the child.
+        """
+        LOG.debug('%r: requesting child shutdown', self)
+        self._send(
+            mitogen.core.Message(
+                src_id=mitogen.context_id,
+                dst_id=self.remote_id,
+                handle=mitogen.core.SHUTDOWN,
+            )
+        )
+
+
 class Options(object):
     name = None
 
@@ -1407,7 +1428,7 @@ class Connection(object):
         if not self.exception:
             self._router.register(self.context, self.stdio_stream)
             self.stdio_stream.set_protocol(
-                mitogen.core.MitogenProtocol(
+                MitogenProtocol(
                     router=self._router,
                     remote_id=self.context.context_id,
                 )
@@ -1428,19 +1449,6 @@ class Connection(object):
                 stream.on_disconnect(self._router.broker)
         self._complete_connection()
 
-    def on_stream_shutdown(self):
-        """
-        Request the slave gracefully shut itself down.
-        """
-        LOG.debug('%r: requesting child shutdown', self)
-        self.stdio_stream.protocol._send(
-            mitogen.core.Message(
-                src_id=mitogen.context_id,
-                dst_id=self.stdio_stream.protocol.remote_id,
-                handle=mitogen.core.SHUTDOWN,
-            )
-        )
-
     eof_error_msg = 'EOF on stream; last 100 lines received:\n'
 
     def on_stdio_disconnect(self):
@@ -1511,7 +1519,6 @@ class Connection(object):
         stream.name = self.options.name or self._get_name()
         stream.accept(self.proc.stdout, self.proc.stdin)
 
-        mitogen.core.listen(stream, 'shutdown', self.on_stream_shutdown)
         mitogen.core.listen(stream, 'disconnect', self.on_stdio_disconnect)
         self._router.broker.start_receive(stream)
         return stream