gh-150107: Fix asyncio sendfile fallback ignoring non-zero offset

The fallback paths in BaseEventLoop._sock_sendfile_fallback and
_sendfile_fallback only seeked the file when offset was truthy, so an
offset of 0 was respected but later non-zero offsets were dropped when
the file lacked seek tracking. Seek whenever the file supports seek().
Also seek the CRT file pointer on Windows TransmitFile, which ignores
OVERLAPPED.Offset for handles not opened with FILE_FLAG_OVERLAPPED.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.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()