[3.13] pythongh-131325: fix sendfile fallback implementation to drain data after writing to transport (pythonGH-131376) (python#131377)

pythongh-131325: fix sendfile fallback implementation to drain data after writing to transport (pythonGH-131376)
(cherry picked from commit 94f4d87)

Co-authored-by: Kumar Aditya <[email protected]>

Author: miss-islington

Lib/asyncio/base_events.py

@@ -1292,8 +1292,8 @@ async def _sendfile_fallback(self, transp, file, offset, count):
                 read = await self.run_in_executor(None, file.readinto, view)
                 if not read:
                     return total_sent  # EOF
-                await proto.drain()
                 transp.write(view[:read])
+                await proto.drain()
                 total_sent += read
         finally:
             if total_sent > 0 and hasattr(file, 'seek'):

Misc/NEWS.d/next/Library/2025-03-17-18-50-39.gh-issue-131325.wlasMF.rst

@@ -0,0 +1 @@
+Fix sendfile fallback implementation to drain data after writing to transport in :mod:`asyncio`.