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gh-150107: Fix asyncio sendfile fallback ignoring non-zero offset #150270

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gh-150107: Fix asyncio sendfile fallback ignoring non-zero offset #150270

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5 changes: 2 additions & 3 deletions Lib/asyncio/base_events.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
Expand Down Expand Up @@ -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)

Expand All @@ -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)
Expand Down
3 changes: 3 additions & 0 deletions Lib/asyncio/windows_events.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down
26 changes: 26 additions & 0 deletions Lib/test/test_asyncio/test_sendfile.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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()
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-05-22-17-09-28.gh-issue-150107.GD72-D.rst
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Original file line number Diff line number Diff line change
@@ -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).
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