ENH: webagg: Handle ioloop shutdown correctly

This change correctly cleans up after the ioloop is
interrupted, allowing the user to call plt.show(),
then resume control with a SIGINT, then call plt.show() again.

lib/matplotlib/backends/backend_webagg.py

@@ -15,14 +15,15 @@
 
 from matplotlib.externals import six
 
-import datetime
 import errno
 import json
 import os
 import random
 import sys
+import signal
 import socket
 import threading
+from contextlib import contextmanager
 
 try:
     import tornado
@@ -323,26 +324,39 @@ def start(cls):
         if cls.started:
             return
 
-        # Set the flag to True *before* blocking on IOLoop.instance().start()
-        cls.started = True
-
         """
         IOLoop.running() was removed as of Tornado 2.4; see for example
         https://groups.google.com/forum/#!topic/python-tornado/QLMzkpQBGOY
         Thus there is no correct way to check if the loop has already been
         launched. We may end up with two concurrently running loops in that
         unlucky case with all the expected consequences.
         """
-        print("Press Ctrl+C to stop WebAgg server")
-        sys.stdout.flush()
-        try:
-            tornado.ioloop.IOLoop.instance().start()
-        except KeyboardInterrupt:
+        ioloop = tornado.ioloop.IOLoop.instance()
+
+        def shutdown():
+            ioloop.stop()
             print("Server is stopped")
             sys.stdout.flush()
-        finally:
             cls.started = False
 
+        @contextmanager
+        def catch_sigint():
+            old_handler = signal.signal(
+                signal.SIGINT,
+                lambda sig, frame: ioloop.add_callback_from_signal(shutdown))
+            try:
+                yield
+            finally:
+                signal.signal(signal.SIGINT, old_handler)
+
+        # Set the flag to True *before* blocking on ioloop.start()
+        cls.started = True
+
+        print("Press Ctrl+C to stop WebAgg server")
+        sys.stdout.flush()
+        with catch_sigint():
+            ioloop.start()
+
 
 def ipython_inline_display(figure):
     import tornado.template