asyncio: Support Python 3.4.2 for compatibility with Debian jessie

The ensure_future function was not introduced until 3.4.4, and
some exceptions were changed from AssertionError to RuntimeError in
3.4.3.

Fixes #2301

tornado/gen.py

@@ -1292,7 +1292,12 @@ except ImportError:
                         break
         raise Return(_r)
 else:
-    _wrap_awaitable = asyncio.ensure_future
+    try:
+        _wrap_awaitable = asyncio.ensure_future
+    except AttributeError:
+        # asyncio.ensure_future was introduced in Python 3.4.4, but
+        # Debian jessie still ships with 3.4.2 so try the old name.
+        _wrap_awaitable = getattr(asyncio, 'async')
 
 
 def convert_yielded(yielded):

tornado/ioloop.py

@@ -281,7 +281,7 @@ class IOLoop(Configurable):
         else:
             try:
                 loop = asyncio.get_event_loop()
-            except RuntimeError:
+            except (RuntimeError, AssertionError):
                 if not instance:
                     return None
                 raise

tornado/platform/asyncio.py

@@ -104,7 +104,7 @@ class BaseAsyncIOLoop(IOLoop):
     def start(self):
         try:
             old_loop = asyncio.get_event_loop()
-        except RuntimeError:
+        except (RuntimeError, AssertionError):
             old_loop = None
         try:
             self._setup_logging()
@@ -211,7 +211,7 @@ class AsyncIOLoop(BaseAsyncIOLoop):
         if not self.is_current:
             try:
                 self.old_asyncio = asyncio.get_event_loop()
-            except RuntimeError:
+            except (RuntimeError, AssertionError):
                 self.old_asyncio = None
             self.is_current = True
         asyncio.set_event_loop(self.asyncio_loop)
@@ -270,7 +270,9 @@ class AnyThreadEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
     def get_event_loop(self):
         try:
             return super().get_event_loop()
-        except RuntimeError:
+        except (RuntimeError, AssertionError):
+            # This was an AssertionError in python 3.4.2 (which ships with debian jessie)
+            # and changed to a RuntimeError in 3.4.3.
             # "There is no current event loop in thread %r"
             loop = self.new_event_loop()
             self.set_event_loop(loop)

tornado/test/asyncio_test.py

@@ -150,7 +150,7 @@ class AnyThreadEventLoopPolicyTest(unittest.TestCase):
     def run_policy_test(self, accessor, expected_type):
         # With the default policy, non-main threads don't get an event
         # loop.
-        self.assertRaises(RuntimeError,
+        self.assertRaises((RuntimeError, AssertionError),
                           self.executor.submit(accessor).result)
         # Set the policy and we can get a loop.
         asyncio.set_event_loop_policy(AnyThreadEventLoopPolicy())