avoid premature _check_closed in _start_read

_start_read can resolve with _try_inline_read, which can succeed even if the stream has been closed
if the buffer has been populated by a prior read

preserve the fix for asserts being hit when dealing with closed sockets

diff --git a/tornado/iostream.py b/tornado/iostream.py
index 37aeb94f23d926894e91dcbcd80a7381ebeb0372..267a6c04b80907660033ffe4bbccd8d2d0212cc7 100644 (file)
--- a/tornado/iostream.py
+++ b/tornado/iostream.py
@@ -799,8 +799,11 @@ class BaseIOStream(object):
             self._read_from_buffer(pos)
 
     def _start_read(self) -> Future:
-        self._check_closed()  # Before reading, check that stream is not closed.
-        assert self._read_future is None, "Already reading"
+        if self._read_future is not None:
+            # raise StreamClosedError instead of assert
+            # in case of starting a second read after the stream is closed
+            self._check_closed()
+            assert self._read_future is None, "Already reading"
         self._read_future = Future()
         return self._read_future