asyncio: sync with Tulip

* _UnixSubprocessTransport: fix file mode of stdin. Open stdin in write mode,
  not in read mode
* Examples: close the event loop at exit
* More reliable CoroWrapper.__del__. If the constructor is interrupted by
  KeyboardInterrupt or the coroutine objet is destroyed lately, some the
  _source_traceback attribute doesn't exist anymore.
* repr(Task): include also the future the task is waiting for

Lib/asyncio/coroutines.py

@@ -111,12 +111,14 @@ class CoroWrapper:
         frame = getattr(gen, 'gi_frame', None)
         if frame is not None and frame.f_lasti == -1:
             func = events._format_callback(self.func, ())
-            tb = ''.join(traceback.format_list(self._source_traceback))
-            message = ('Coroutine %s was never yielded from\n'
-                       'Coroutine object created at (most recent call last):\n'
-                       '%s'
-                       % (func, tb.rstrip()))
-            logger.error(message)
+            msg = 'Coroutine %s was never yielded from' % func
+            tb = getattr(self, '_source_traceback', ())
+            if tb:
+                tb = ''.join(traceback.format_list(tb))
+                msg += ('\nCoroutine object created at '
+                        '(most recent call last):\n')
+                msg += tb.rstrip()
+            logger.error(msg)
 
 
 def coroutine(func):

Lib/asyncio/tasks.py

@@ -109,6 +109,9 @@ class Task(futures.Future):
         if self._callbacks:
             info.append(self._format_callbacks())
 
+        if self._fut_waiter is not None:
+            info.append('wait_for=%r' % self._fut_waiter)
+
         return '<%s %s>' % (self.__class__.__name__, ' '.join(info))
 
     def get_stack(self, *, limit=None):

Lib/asyncio/unix_events.py

@@ -494,7 +494,7 @@ class _UnixSubprocessTransport(base_subprocess.BaseSubprocessTransport):
             universal_newlines=False, bufsize=bufsize, **kwargs)
         if stdin_w is not None:
             stdin.close()
-            self._proc.stdin = open(stdin_w.detach(), 'rb', buffering=bufsize)
+            self._proc.stdin = open(stdin_w.detach(), 'wb', buffering=bufsize)
 
 
 class AbstractChildWatcher: