Read smaller frames to workaround OpenSSL bug

As older versions of OpenSSL (in particular 1.0.2) have limitations on
the size of buffers they can work with, take small views into our larger
buffer and read those in instead. This should keep the buffer sizes more
manageable for OpenSSL.

distributed/comm/tcp.py

@@ -1,3 +1,4 @@
+import ctypes
 import errno
 import functools
 import logging
@@ -14,6 +15,7 @@
 except ImportError:
     ssl = None
 
+from tlz import sliding_window
 from tornado import netutil
 from tornado.iostream import StreamClosedError
 from tornado.tcpclient import TCPClient
@@ -35,6 +37,7 @@
 
 
 MAX_BUFFER_SIZE = MEMORY_LIMIT / 2
+C_MAX_INT = 256 ** ctypes.sizeof(ctypes.c_int) // 2 - 1
 
 
 def set_tcp_timeout(comm):
@@ -193,9 +196,15 @@ async def read(self, deserializers=None):
             frames_nbytes = await stream.read_bytes(fmt_size)
             (frames_nbytes,) = struct.unpack(fmt, frames_nbytes)
 
-            frames = bytearray(frames_nbytes)
-            n = await stream.read_into(frames)
-            assert n == frames_nbytes, (n, frames_nbytes)
+            frames = memoryview(bytearray(frames_nbytes))
+            # Workaround for OpenSSL 1.0.2 (can drop with OpenSSL 1.1.1)
+            for i, j in sliding_window(
+                2, range(0, frames_nbytes + C_MAX_INT, C_MAX_INT)
+            ):
+                chunk = frames[i:j]
+                chunk_nbytes = len(chunk)
+                n = await stream.read_into(chunk)
+                assert n == chunk_nbytes, (n, chunk_nbytes)
         except StreamClosedError as e:
             self.stream = None
             self._closed = True