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

@@ -193,9 +193,14 @@ 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)
+            MAX = 1_000
+            for i, j in sliding_window(range(0, frame_nbytes + MAX, MAX)):
+                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