[3.6] bpo-26819: Prevent proactor double read on resume (GH-6921) (#7110)

The proactor event loop has a race condition when reading with
pausing/resuming. `resume_reading()` unconditionally schedules the read
function to read from the current future. If `resume_reading()` was
called before the previously scheduled done callback fires, this results
in two attempts to get the data from the most recent read and an
assertion failure. This commit tracks whether or not `resume_reading`
needs to reschedule the callback to restart the loop, preventing a
second attempt to read the data..
(cherry picked from commit 4151061855)

Co-authored-by: CtrlZvi <viz+github@flippedperspective.com>

Lib/asyncio/proactor_events.py

@@ -156,6 +156,7 @@ class _ProactorReadPipeTransport(_ProactorBasePipeTransport,
                  extra=None, server=None):
         super().__init__(loop, sock, protocol, waiter, extra, server)
         self._paused = False
+        self._reschedule_on_resume = False
         self._loop.call_soon(self._loop_reading)
 
     def pause_reading(self):
@@ -173,12 +174,15 @@ class _ProactorReadPipeTransport(_ProactorBasePipeTransport,
         self._paused = False
         if self._closing:
             return
+        if self._reschedule_on_resume:
             self._loop.call_soon(self._loop_reading, self._read_fut)
+            self._reschedule_on_resume = False
         if self._loop.get_debug():
             logger.debug("%r resumes reading", self)
 
     def _loop_reading(self, fut=None):
         if self._paused:
+            self._reschedule_on_resume = True
             return
         data = None
 

Lib/test/test_asyncio/test_proactor_events.py

@@ -330,7 +330,7 @@ class ProactorSocketTransportTests(test_utils.TestCase):
     def test_pause_resume_reading(self):
         tr = self.socket_transport()
         futures = []
-        for msg in [b'data1', b'data2', b'data3', b'data4', b'']:
+        for msg in [b'data1', b'data2', b'data3', b'data4', b'data5', b'']:
             f = asyncio.Future(loop=self.loop)
             f.set_result(msg)
             futures.append(f)
@@ -352,6 +352,13 @@ class ProactorSocketTransportTests(test_utils.TestCase):
         self.protocol.data_received.assert_called_with(b'data3')
         self.loop._run_once()
         self.protocol.data_received.assert_called_with(b'data4')
+
+        tr.pause_reading()
+        tr.resume_reading()
+        self.loop.call_exception_handler = mock.Mock()
+        self.loop._run_once()
+        self.loop.call_exception_handler.assert_not_called()
+        self.protocol.data_received.assert_called_with(b'data5')
         tr.close()
 
 

Misc/NEWS.d/next/Library/2018-05-16-05-24-43.bpo-26819.taxbVT.rst

@@ -0,0 +1,2 @@
+Fix race condition with `ReadTransport.resume_reading` in Windows proactor
+event loop.