gh-152099: Raise SendfileNotAvailableError for fallback-only transports

Lib/asyncio/base_events.py

@@ -1283,7 +1283,7 @@ async def sendfile(self, transport, file, offset=0, count=None,
                     raise
 
         if not fallback:
-            raise RuntimeError(
+            raise exceptions.SendfileNotAvailableError(
                 f"fallback is disabled and native sendfile is not "
                 f"supported for transport {transport!r}")
         return await self._sendfile_fallback(transport, file,

Lib/test/test_asyncio/test_sendfile.py

@@ -354,7 +354,7 @@ def test_sendfile_not_supported(self):
         try:
             with self.assertRaisesRegex(RuntimeError, "not supported"):
                 self.run_loop(
-                    self.loop.sendfile(tr, self.file))
+                    self.loop.sendfile(tr, self.file, fallback=False))
             self.assertEqual(0, self.file.tell())
         finally:
             # don't use self.addCleanup because it produces resource warning
@@ -580,7 +580,8 @@ 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'):
+        with self.assertRaisesRegex(asyncio.SendfileNotAvailableError,
+                                    'fallback is disabled'):
             self.loop.run_until_complete(
                 self.loop.sendfile(transport, None, fallback=False))
 

Misc/NEWS.d/next/Library/2026-06-25-22-19-04.gh-issue-152099.L7fKq9.rst

@@ -0,0 +1,4 @@
+``asyncio``'s ``loop.sendfile(..., fallback=False)`` now consistently raises
+:exc:`asyncio.SendfileNotAvailableError` for fallback-only transports, such as
+SSL/TLS transports, when native sendfile cannot be used. Previously, this case
+raised :exc:`RuntimeError`.