Use threadpool for reading from file in sendfile fallback mode (#14076)

Lib/asyncio/base_events.py

@@ -1141,7 +1141,7 @@ class BaseEventLoop(events.AbstractEventLoop):
                     if blocksize <= 0:
                         return total_sent
                 view = memoryview(buf)[:blocksize]
-                read = file.readinto(view)
+                read = await self.run_in_executor(None, file.readinto, view)
                 if not read:
                     return total_sent  # EOF
                 await proto.drain()

Misc/NEWS.d/next/Library/2019-06-14-13-30-47.bpo-37280.Fxur0F.rst

+Use threadpool for reading from file for sendfile fallback mode.