Changes necessary to get async kernel startup working

Inspired by #402, but needing support for python 2.7, these changes
essentially apply the same model used in Notebook for supporting
coroutines with appropriately placed yield statements in order to
start (and restart) multiple kernels simultaneously within the same
server instance.

jupyter_client/manager.py

@@ -13,7 +13,7 @@
 import time
 
 import zmq
-
+from tornado import gen
 from ipython_genutils.importstring import import_item
 from .localinterfaces import is_local_ip, local_ips
 from traitlets import (
@@ -205,6 +205,7 @@ def _close_control_socket(self):
         self._control_socket.close()
         self._control_socket = None
 
+    @gen.coroutine
     def start_kernel(self, **kw):
         """Starts a kernel on this host in a separate process.
 
@@ -246,8 +247,7 @@ def start_kernel(self, **kw):
 
         # launch the kernel subprocess
         self.log.debug("Starting kernel: %s", kernel_cmd)
-        self.kernel = self._launch_kernel(kernel_cmd, env=env,
-                                    **kw)
+        self.kernel = yield gen.maybe_future(self._launch_kernel(kernel_cmd, env=env, **kw))
         self.start_restarter()
         self._connect_control_socket()
 
@@ -324,6 +324,7 @@ def shutdown_kernel(self, now=False, restart=False):
 
         self.cleanup(connection_file=not restart)
 
+    @gen.coroutine
     def restart_kernel(self, now=False, newports=False, **kw):
         """Restarts a kernel with the arguments that were used to launch it.
 
@@ -361,7 +362,7 @@ def restart_kernel(self, now=False, newports=False, **kw):
 
             # Start new kernel.
             self._launch_args.update(kw)
-            self.start_kernel(**self._launch_args)
+            yield gen.maybe_future(self.start_kernel(**self._launch_args))
 
     @property
     def has_kernel(self):

jupyter_client/multikernelmanager.py

@@ -7,9 +7,9 @@
 
 import os
 import uuid
-
 import zmq
 
+from tornado import gen
 from traitlets.config.configurable import LoggingConfigurable
 from ipython_genutils.importstring import import_item
 from traitlets import (
@@ -82,6 +82,7 @@ def __len__(self):
     def __contains__(self, kernel_id):
         return kernel_id in self._kernels
 
+    @gen.coroutine
     def start_kernel(self, kernel_name=None, **kwargs):
         """Start a new kernel.
 
@@ -107,9 +108,9 @@ def start_kernel(self, kernel_name=None, **kwargs):
                     parent=self, log=self.log, kernel_name=kernel_name,
                     **constructor_kwargs
         )
-        km.start_kernel(**kwargs)
+        yield gen.maybe_future(km.start_kernel(**kwargs))
         self._kernels[kernel_id] = km
-        return kernel_id
+        raise gen.Return(kernel_id)
 
     @kernel_method
     def shutdown_kernel(self, kernel_id, now=False, restart=False):
@@ -186,15 +187,25 @@ def signal_kernel(self, kernel_id, signum):
         """
         self.log.info("Signaled Kernel %s with %s" % (kernel_id, signum))
 
-    @kernel_method
+    @gen.coroutine
     def restart_kernel(self, kernel_id, now=False):
         """Restart a kernel by its uuid, keeping the same ports.
 
         Parameters
         ==========
         kernel_id : uuid
-            The id of the kernel to interrupt.
+            The id of the kernel to restart.
+
+        now : bool, optional
+            If True, the kernel is forcefully restarted *immediately*, without
+            having a chance to do any cleanup action.  Otherwise the kernel is
+            given 1s to clean up before a forceful restart is issued.
+
+            In all cases the kernel is restarted, the only difference is whether
+            it is given a chance to perform a clean shutdown or not.
         """
+        km = self.get_kernel(kernel_id)
+        yield gen.maybe_future(km.restart_kernel(now))
         self.log.info("Kernel restarted: %s" % kernel_id)
 
     @kernel_method