bpo-33263: Fix FD leak in _SelectorSocketTransport (GH-6450)

* bpo-33263 Fix FD leak in _SelectorSocketTransport. (GH-6450)

Under particular circumstances _SelectorSocketTransport can try to add a reader
even the transport is already being closed. This can lead to FD leak and
invalid stated of the following connections. Fixed the SelectorSocketTransport
to add the reader only if the trasport is still active.

Lib/asyncio/selector_events.py

@@ -706,6 +706,12 @@ class _SelectorTransport(transports._FlowControlMixin,
     def get_write_buffer_size(self):
         return len(self._buffer)
 
+    def _add_reader(self, fd, callback, *args):
+        if self._closing:
+            return
+
+        self._loop._add_reader(fd, callback, *args)
+
 
 class _SelectorSocketTransport(_SelectorTransport):
 
@@ -732,7 +738,7 @@ class _SelectorSocketTransport(_SelectorTransport):
 
         self._loop.call_soon(self._protocol.connection_made, self)
         # only start reading when connection_made() has been called
-        self._loop.call_soon(self._loop._add_reader,
+        self._loop.call_soon(self._add_reader,
                              self._sock_fd, self._read_ready)
         if waiter is not None:
             # only wake up the waiter when connection_made() has been called
@@ -754,7 +760,7 @@ class _SelectorSocketTransport(_SelectorTransport):
         if self._closing or not self._paused:
             return
         self._paused = False
-        self._loop._add_reader(self._sock_fd, self._read_ready)
+        self._add_reader(self._sock_fd, self._read_ready)
         if self._loop.get_debug():
             logger.debug("%r resumes reading", self)
 
@@ -930,7 +936,7 @@ class _SelectorDatagramTransport(_SelectorTransport):
         self._address = address
         self._loop.call_soon(self._protocol.connection_made, self)
         # only start reading when connection_made() has been called
-        self._loop.call_soon(self._loop._add_reader,
+        self._loop.call_soon(self._add_reader,
                              self._sock_fd, self._read_ready)
         if waiter is not None:
             # only wake up the waiter when connection_made() has been called

Lib/test/test_asyncio/test_selector_events.py

@@ -871,6 +871,21 @@ class SelectorTransportTests(test_utils.TestCase):
         self.assertIsNone(tr._protocol)
         self.assertIsNone(tr._loop)
 
+    def test__add_reader(self):
+        tr = self.create_transport()
+        tr._buffer.extend(b'1')
+        tr._add_reader(7, mock.sentinel)
+        self.assertTrue(self.loop.readers)
+
+        tr._force_close(None)
+
+        self.assertTrue(tr.is_closing())
+        self.assertFalse(self.loop.readers)
+
+        # can not add readers after closing
+        tr._add_reader(7, mock.sentinel)
+        self.assertFalse(self.loop.readers)
+
 
 class SelectorSocketTransportTests(test_utils.TestCase):
 

Misc/NEWS.d/next/Library/2018-04-11-20-29-19.bpo-33263.B56Hc1.rst

@@ -0,0 +1 @@
+Fix FD leak in `_SelectorSocketTransport`  Patch by Vlad Starostin.