bpo-32622: Native sendfile on windows (GH-5565)

* Support sendfile on Windows Proactor event loop naively.
(cherry picked from commit a19fb3c6aa)

Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

Lib/asyncio/windows_events.py

@@ -4,6 +4,7 @@ import _overlapped
 import _winapi
 import errno
 import math
+import msvcrt
 import socket
 import struct
 import weakref
@@ -527,6 +528,27 @@ class IocpProactor:
 
         return self._register(ov, conn, finish_connect)
 
+    def sendfile(self, sock, file, offset, count):
+        self._register_with_iocp(sock)
+        ov = _overlapped.Overlapped(NULL)
+        offset_low = offset & 0xffff_ffff
+        offset_high = (offset >> 32) & 0xffff_ffff
+        ov.TransmitFile(sock.fileno(),
+                        msvcrt.get_osfhandle(file.fileno()),
+                        offset_low, offset_high,
+                        count, 0, 0)
+
+        def finish_sendfile(trans, key, ov):
+            try:
+                return ov.getresult()
+            except OSError as exc:
+                if exc.winerror in (_overlapped.ERROR_NETNAME_DELETED,
+                                    _overlapped.ERROR_OPERATION_ABORTED):
+                    raise ConnectionResetError(*exc.args)
+                else:
+                    raise
+        return self._register(ov, sock, finish_sendfile)
+
     def accept_pipe(self, pipe):
         self._register_with_iocp(pipe)
         ov = _overlapped.Overlapped(NULL)