Support tribool for fallback

Author: asvetlov

Doc/library/asyncio-eventloop.rst

@@ -703,7 +703,7 @@ Low-level socket operations
 
 .. coroutinemethod:: AbstractEventLoop.sock_sendfile(sock, file, \
                                                      offset=0, count=None, \
-                                                     *, fallback=True)
+                                                     *, fallback=None)
 
    Send a file using high-performance :mod:`os.sendfile` if possible
    and return the total number of bytes which were sent.
@@ -722,13 +722,15 @@ Low-level socket operations
    <io.IOBase.tell>` can be used to figure out the number of bytes
    which were sent.
 
-   *fallback* set to ``True`` makes asyncio to manually read and send
-   the file when the platform does not support the sendfile syscall
-   (e.g. Windows or SSL socket on Unix).
+   *fallback* set to ``None`` makes asyncio to manually read and send
+   the file when the platform does not support the :func:`os.sendfile`
+   syscall (e.g. Windows or SSL socket on Unix).
 
    Raise :exc:`RuntimeError` if the system does not support
    *sendfile* syscall and *fallback* is ``False``.
 
+   Never try to use :func:`os.sendfile` if *fallback* is ``True``.
+
    .. versionadded:: 3.7
 
 

Lib/asyncio/base_events.py

@@ -652,19 +652,23 @@ async def getnameinfo(self, sockaddr, flags=0):
             None, socket.getnameinfo, sockaddr, flags)
 
     async def sock_sendfile(self, sock, file, offset=0, count=None,
-                            *, fallback=True):
+                            *, fallback=None):
         if self._debug and sock.gettimeout() != 0:
             raise ValueError("the socket must be non-blocking")
         self._check_sendfile_params(sock, file, offset, count)
-        try:
-            return await self._sock_sendfile_native(sock, file,
-                                                    offset, count)
-        except _SendfileNotAvailable as exc:
-            if fallback:
-                return await self._sock_sendfile_fallback(sock, file,
-                                                          offset, count)
-            else:
-                raise RuntimeError(exc.args[0]) from None
+        if fallback:
+            return await self._sock_sendfile_fallback(sock, file,
+                                                      offset, count)
+        else:
+            try:
+                return await self._sock_sendfile_native(sock, file,
+                                                        offset, count)
+            except _SendfileNotAvailable as exc:
+                if fallback is None:
+                    return await self._sock_sendfile_fallback(sock, file,
+                                                              offset, count)
+                else:
+                    raise RuntimeError(exc.args[0]) from None
 
     async def _sock_sendfile_native(self, sock, file, offset, count):
         raise _SendfileNotAvailable("Fast sendfile is not available")

Lib/test/test_asyncio/test_base_events.py

@@ -1868,6 +1868,21 @@ def test__sock_sendfile_native_failure(self):
         self.assertEqual(proto.data, b'')
         self.assertEqual(self.file.tell(), 0)
 
+    def test__sock_sendfile_always_fallback(self):
+        sock, proto = self.prepare()
+
+        self.loop._sock_sendfile_native = mock.Mock()
+        ret = self.run_loop(self.loop.sock_sendfile(sock, self.file,
+                                                    0, None, fallback=True))
+
+        sock.close()
+        self.run_loop(proto.wait_closed())
+
+        self.assertEqual(ret, len(self.DATA))
+        self.assertEqual(proto.data, self.DATA)
+        self.assertEqual(self.file.tell(), len(self.DATA))
+        self.loop._sock_sendfile_native.assert_not_called()
+
     def test_sock_sendfile_no_fallback(self):
         sock, proto = self.prepare()