Fix asyncio sendfile fallback ignoring non-zero offset

Signed-off-by: grantlouisherman <grantlouisherman041@gmail.com>

Author: grantlouisherman

Lib/asyncio/base_events.py

@@ -969,7 +969,7 @@ async def _sock_sendfile_native(self, sock, file, offset, count):
             f"and file {file!r} combination")
 
     async def _sock_sendfile_fallback(self, sock, file, offset, count):
-        if offset:
+        if hasattr(file, 'seek'):
             file.seek(offset)
         blocksize = (
             min(count, constants.SENDFILE_FALLBACK_READBUFFER_SIZE)
@@ -1286,7 +1286,6 @@ async def sendfile(self, transport, file, offset=0, count=None,
             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)
 
@@ -1295,7 +1294,7 @@ async def _sendfile_native(self, transp, file, offset, count):
             "sendfile syscall is not supported")
 
     async def _sendfile_fallback(self, transp, file, offset, count):
-        if offset:
+        if hasattr(file, 'seek'):
             file.seek(offset)
         blocksize = min(count, 16384) if count else 16384
         buf = bytearray(blocksize)

Lib/asyncio/windows_events.py

@@ -610,6 +610,9 @@ def sendfile(self, sock, file, offset, count):
         ov = _overlapped.Overlapped(NULL)
         offset_low = offset & 0xffff_ffff
         offset_high = (offset >> 32) & 0xffff_ffff
+        # TransmitFile ignores OVERLAPPED.Offset for handles not opened with
+        # FILE_FLAG_OVERLAPPED, so seek the CRT file pointer to match.
+        file.seek(offset)
         ov.TransmitFile(sock.fileno(),
                         msvcrt.get_osfhandle(file.fileno()),
                         offset_low, offset_high,

Lib/test/test_asyncio/test_sendfile.py

@@ -228,6 +228,32 @@ def test_sock_sendfile_zero_size(self):
         self.assertEqual(ret, 0)
         self.assertEqual(self.file.tell(), 0)
 
+    def check_sock_sendfile_offset(self, data, offset, force_fallback=False):
+        sock, proto = self.prepare_socksendfile()
+        with tempfile.TemporaryFile() as f:
+            f.write(data)
+            f.flush()
+            self.assertEqual(f.tell(), len(data))
+
+            if force_fallback:
+                async def _sock_sendfile_fail(sock, file, offset, count):
+                    raise asyncio.exceptions.SendfileNotAvailableError()
+                with support.swap_attr(self.loop, '_sock_sendfile_native', _sock_sendfile_fail):
+                    ret = self.run_loop(self.loop.sock_sendfile(sock, f, offset, None))
+            else:
+                ret = self.run_loop(self.loop.sock_sendfile(sock, f, offset, None))
+            self.assertEqual(f.tell(), len(data))
+            sock.close()
+            self.run_loop(proto.wait_closed())
+            self.assertEqual(ret, len(data) - offset)
+
+    def test_sock_sendfile_offset(self):
+        data = b'abcdef'
+        for offset in (0, len(data) // 2, len(data)):
+            for force_fallback in (False, True):
+                with self.subTest(offset=offset, force_fallback=force_fallback):
+                    self.check_sock_sendfile_offset(data, offset, force_fallback)
+
     def test_sock_sendfile_mix_with_regular_send(self):
         buf = b"mix_regular_send" * (4 * 1024)  # 64 KiB
         sock, proto = self.prepare_socksendfile()

Misc/NEWS.d/next/Core_and_Builtins/2026-05-22-17-09-28.gh-issue-150107.GD72-D.rst

@@ -0,0 +1,3 @@
+:mod:`asyncio`: ``sendfile()`` and ``sock_sendfile()`` event loop methods
+now call ``file.seek(offset)`` if *file* has a ``seek()`` method,
+even if *offset* is ``0`` (default value).