🐛 QUIC Hijack() skips the status-written check that HTTP/2 enforces #1748

Describe the bug

Hijack() has different preconditions on the two transports.

HTTP/2, http2RespWriter.Hijack in connection/http2.go, refuses when no status was written yet:

if !rp.statusWritten {
	return nil, nil, fmt.Errorf("status not yet written before attempting to hijack connection")
}

QUIC, httpResponseAdapter.Hijack in connection/quic_connection.go, has no such check. It always returns a localProxyConnection, even when connectResponseSent is still false, so the caller can write raw bytes to the stream before any connect response went out.

Same caller, different outcome depending on the transport: on HTTP/2 it gets an error, on QUIC it gets a conn.

To Reproduce

Stock cloudflared writes the status before it hijacks, so the built-in ingress path does not hit this. I hit it in a fork that replaces the OriginProxy, with code that hijacks first. net/http/httputil.ReverseProxy does exactly that on a 101: handleUpgradeResponse calls Hijack() and then writes the status line onto the conn itself.

1. ProxyHTTP serves the request through httputil.ReverseProxy to a backend that answers 101.
2. With --protocol http2, Hijack fails and the client gets a 502.
3. With --protocol quic, Hijack succeeds and the caller can write to the stream before any connect response was sent.

Expected behavior

Both transports enforce the same precondition. The HTTP/2 check looks like the right one, so QUIC could refuse the same way while connectResponseSent is false.

Environment and versions

• OS: Linux
• Architecture: AMD64
• Version: 2026.9.1, and the Hijack bodies are unchanged on master as of 2026.9.3

Logs and errors

HTTP/2: status not yet written before attempting to hijack connection. QUIC: no error.

Additional context

I worked around it on my side, so it does not block me. Filing it because the two transports disagree on the ResponseWriter contract, and that is easy to miss.