[Bug] Request body lost when Upgrade: h2c + Transfer-Encoding: chunked is used

Overview

When sending a POST request from a Java RestClient (Spring Boot 3.2+, Java 21) to a FastAPI backend running on Uvicorn + httptools, we encountered a strange issue where the request body was missing.

The request looked like this:

POST /endpoint HTTP/1.1
Host: my-api.com
Upgrade: h2c
Connection: Upgrade, HTTP2-Settings
Transfer-Encoding: chunked
Content-Type: application/json

3\r\nabc\r\n0\r\n\r\n

On the server side, Uvicorn logs showed:

• Unsupported upgrade request
• No request body
• Invalid HTTP request received

But when we routed the same request through ngrok or used RestTemplate instead of RestClient, it worked fine.

---

🔍 Root Cause

After analyzing Uvicorn’s httptools_impl.py and httptools parser behavior, we found this:

• Upgrade: h2c is ignored by Uvicorn (as expected).
• But internally, httptools still enters the upgrade state.
• Since the upgrade is ignored and the parser is not reset, no body is parsed.
• This violates RFC 7230 §6.7, which allows the server to ignore upgrades and proceed normally.

---

Proposed Fix

Patch parser.pyx to resume HTTP/1.1 parsing after upgrade is ignored:

cdef int cb_on_headers_complete(cparser.llhttp_t* parser) except -1:
    cdef HttpParser pyparser = <HttpParser>parser.data
    try:
        if parser.upgrade and not pyparser._should_upgrade():
            cparser.llhttp_resume_after_upgrade(parser)
        pyparser._on_headers_complete()
    except BaseException as ex:
        pyparser._last_error = ex
        return -1
    return 0

Also expose this from Python:

def resume_after_upgrade(self):
    httptools.llhttp_resume_after_upgrade(self.cparser)

Then frameworks like Uvicorn can call it in:

def on_headers_complete(self):
    if self.upgrade and self.upgrade.lower() != b"websocket":
        self.parser.resume_after_upgrade()

Reproducible Test

def test_chunked_body_with_ignored_upgrade():
    headers = {
        "Upgrade": "h2c",
        "Connection": "Upgrade",
        "Transfer-Encoding": "chunked"
    }
    body = b"4\r\ntest\r\n0\r\n\r\n"
    request = b"POST / HTTP/1.1\r\n" + headers_to_bytes(headers) + b"\r\n" + body

    parser = HttpRequestParser(TestProtocol())
    parser.feed_data(request)

    assert protocol.body == b"test"

Why it matters

This is RFC-compliant behavior that should be supported.

RestClient in Java 21+ sends Upgrade: h2c by default.

Any server not resetting its parser state will lose the body.

This breaks many interop scenarios between Spring Boot and Python ASGI apps.

I'm happy to submit a PR if maintainers are open to it. Thanks for your time and for maintaining this great project!

Callback của parser nằm trong parser.pyx; trước tiên hãy lần theo cb_on_headers_complete và phần xử lý upgrade hiện có, sau đó kiểm tra wrapper parser Python và bài kiểm thử parser có thể tái hiện. Tái hiện yêu cầu h2c được chia thành các chunk và xác minh rằng body vẫn được giữ lại sau một upgrade bị bỏ qua, với các bài kiểm thử parser liên quan đều vượt qua.