bpo-32622: Enforce sendfile fallback policy for FALLBACK transports (#5364)

Lib/asyncio/base_events.py

@@ -1035,7 +1035,12 @@ class BaseEventLoop(events.AbstractEventLoop):
             except events.SendfileNotAvailableError as exc:
                 if not fallback:
                     raise
-        # the mode is FALLBACK or fallback is True
+
+        if not fallback:
+            raise RuntimeError(
+                f"fallback is disabled and native sendfile is not "
+                f"supported for transport {transport!r}")
+
         return await self._sendfile_fallback(transport, file,
                                              offset, count)
 

Lib/test/test_asyncio/test_events.py

@@ -27,6 +27,7 @@ if sys.platform != 'win32':
 
 import asyncio
 from asyncio import base_events
+from asyncio import constants
 from asyncio import coroutines
 from asyncio import events
 from asyncio import proactor_events
@@ -2381,6 +2382,14 @@ class SendfileMixin:
         ret = self.run_loop(t)
         self.assertEqual(ret, len(self.DATA))
 
+    def test_sendfile_no_fallback_for_fallback_transport(self):
+        transport = mock.Mock()
+        transport.is_closing.side_effect = lambda: False
+        transport._sendfile_compatible = constants._SendfileMode.FALLBACK
+        with self.assertRaisesRegex(RuntimeError, 'fallback is disabled'):
+            self.loop.run_until_complete(
+                self.loop.sendfile(transport, None, fallback=False))
+
 
 if sys.platform == 'win32':