René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Error in BaseEventLoop._sock_sendfile_fallback with offset 0","articleBody":"# Bug report\n\n### Bug description:\n\n`BaseEventLoop._sock_sendfile_fallback` (https://github.com/python/cpython/blob/main/Lib/asyncio/base_events.py#L971) erroneously doesn't seek when passed the offset 0.\n```python\n        if offset:\n            file.seek(offset)\n```\nThis check in the function doesn't call `file.seek` if offset is falsey. If the file in question has a file pointer that is not currently at the 0 offset then this will erroneously send from the current offset and not the start of the file as desired. This can happen when `sendfile` is called with the destination socket is an SSL socket.\n\nRepro code (generated by Claude), because of needing to create an SSL socket it's a bit involved - including making a self signed cert with the openssl command.\n```python\nimport asyncio\nimport os\nimport ssl\nimport subprocess\nimport sys\nimport tempfile\nfrom pathlib import Path\n\nCONTENT = b\"X\" * 1000\n\n\ndef make_throwaway_cert(tmp: Path) -\u003e tuple[Path, Path]:\n    cert = tmp / \"cert.pem\"\n    key = tmp / \"key.pem\"\n    subprocess.run(\n        [\n            \"openssl\", \"req\", \"-x509\", \"-newkey\", \"rsa:2048\", \"-nodes\",\n            \"-keyout\", str(key), \"-out\", str(cert),\n            \"-days\", \"1\", \"-subj\", \"/CN=localhost\",\n        ],\n        check=True,\n        capture_output=True,\n    )\n    return cert, key\n\n\nasync def main() -\u003e int:\n    with tempfile.TemporaryDirectory() as tmp_:\n        tmp = Path(tmp_)\n        cert, key = make_throwaway_cert(tmp)\n\n        data_path = tmp / \"data\"\n        data_path.write_bytes(CONTENT)\n\n        # SSL server that drains whatever it receives.\n        server_ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)\n        server_ctx.load_cert_chain(cert, key)\n        received = 0\n        done = asyncio.Event()\n\n        async def handle(reader: asyncio.StreamReader, writer: asyncio.StreamWriter) -\u003e None:\n            nonlocal received\n            try:\n                while chunk := await reader.read(4096):\n                    received += len(chunk)\n            finally:\n                writer.close()\n                done.set()\n\n        server = await asyncio.start_server(handle, \"127.0.0.1\", 0, ssl=server_ctx)\n        port = server.sockets[0].getsockname()[1]\n\n        # SSL client → transport's `_sendfile_compatible == UNSUPPORTED` → fallback path.\n        client_ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)\n        client_ctx.check_hostname = False\n        client_ctx.verify_mode = ssl.CERT_NONE\n        _, writer = await asyncio.open_connection(\n            \"127.0.0.1\", port, ssl=client_ctx, server_hostname=\"localhost\"\n        )\n\n        # Open the file and pre-advance its position. In the real-world bug\n        # this happens because the calling code previously read through the\n        # same fh (e.g. to parse an HTTP header).\n        f = open(data_path, \"rb\", buffering=0)\n        f.seek(len(CONTENT))  # file position is at EOF\n\n        # Send from offset=0. Expectation: 1000 bytes transferred.\n        loop = asyncio.get_running_loop()\n        sent = await loop.sendfile(writer.transport, f, offset=0)\n\n        writer.close()\n        await writer.wait_closed()\n        await done.wait()\n        server.close()\n        await server.wait_closed()\n        f.close()\n\n        print(f\"Python:       {sys.version}\")\n        print(f\"file size:    {len(CONTENT)}\")\n        print(f\"file.tell() before sendfile: {len(CONTENT)} (EOF)\")\n        print(f\"loop.sendfile(..., offset=0) returned: {sent}\")\n        print(f\"server received: {received} bytes\")\n        print()\n\n        if sent == 0 and received == 0:\n            print(\"BUG REPRODUCED: offset=0 was ignored; the fallback read from \"\n                  \"file.tell() instead of from the requested offset.\")\n            return 1\n        else:\n            print(\"Bug NOT reproduced on this Python build.\")\n            return 0\n\n\nsys.exit(asyncio.run(main()))\n```\nThe relevant lines are the line `f.seek(len(CONTENT))` and the line `sent = await loop.sendfile(writer.transport, f, offset=0)`. Most of the rest is setup and teardown.\n\n### CPython versions tested on:\n\n3.12, 3.13\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-150142\n* gh-150259\n* gh-150270\n* gh-150569\n* gh-150570\n* gh-150571\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/SteveJones","@type":"Person","name":"SteveJones"},"datePublished":"2026-05-19T19:42:39.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/150107/cpython/issues/150107"}