fix(quic): refuse a FIN below received stream data

quest/m1/quic/README.md

@@ -70,7 +70,6 @@ cache limits remain separate; bitrate pacing composes with this memory cap.
   tail grace
 - [Schedule datagrams by subscription priority](/quest/m1/quic/datagram-priority.md) -
   datagrams join their subscription's send group instead of preempting every stream
-- [A FIN below received data is a final size error](/quest/m1/quic/final-size-below-received.md) - a shrunken final size closes with FINAL_SIZE_ERROR instead of blocking the reader forever
 - [A stateless reset keeps a closed connection's reason](/quest/m1/quic/close-reason-after-reset.md) - a reset after a local transport error neither replaces the reason nor reports a second loss
 - [0-RTT streams adopt a raised stream window](/quest/m1/quic/zero-rtt-window-raise.md) - a server's larger stream window reaches the client's 0-RTT streams, so they no longer stall
 

rs/moq-quic/README.md

@@ -54,7 +54,7 @@ Changes on top of the upstream commit, besides the renames:
 - **BBR3 packet retirement**: per-space packet lookup and round-indexed expiry retire completed snapshots incrementally, preserving the extra ACK batch of metadata for late loss callbacks. The bounded indexes handle sparse packet numbers without walking the live flight. Upstream submission is deferred while quinn#2481 remains open: this fork also carries the packet-space and loss callback adaptations, so a port needs to target that implementation after its shape settles.
 - **Classic ECN for BBR3**, ported from [moq-dev/noq#12](https://github.com/moq-dev/noq/pull/12): CE exits Startup, stops a bandwidth probe, or lowers the short-term model, once per recovery episode, instead of counting as a loss.
 - **Pacer rounding**: a wait shorter than a nanosecond rounds up instead of re-arming the pacing timer at the current instant.
-- **Malformed control input**: `RETIRE_CONNECTION_ID` for a sequence that was never issued (`issued` is a count, so `sequence >= issued`) and a repeated `grease_quic_bit` or `min_ack_delay` transport parameter close the connection. RFC 9000 section 7.4 only says SHOULD for a duplicate parameter; this fork refuses it, as it does every other known parameter.
+- **Malformed control input**: `RETIRE_CONNECTION_ID` for a sequence that was never issued (`issued` is a count, so `sequence >= issued`) and a repeated `grease_quic_bit` or `min_ack_delay` transport parameter close the connection. RFC 9000 section 7.4 only says SHOULD for a duplicate parameter; this fork refuses it, as it does every other known parameter. A STREAM frame whose FIN sets a final size below data already received closes with `FINAL_SIZE_ERROR`, as RFC 9000 section 4.5 requires, where quinn accepts it and leaves the reader blocked forever. Not offered upstream yet.
 - **qlog pacing rate** in bits per second, as qlog defines it.
 - moq-dev/noq's congestion-callback regressions, reworked onto quinn's test harness.
 - [quinn#2724](https://github.com/quinn-rs/quinn/pull/2724) (`moq_sock::udp`): when the kernel rejects a GSO batch with `EIO` or `EINVAL`, the socket halts GSO and resends the batch as individual datagrams instead of dropping it.

rs/moq-quic/src/connection/streams/recv.rs

@@ -64,6 +64,12 @@ impl Recv {
             debug!(end, final_offset, "final size error");
             return Err(TransportError::FINAL_SIZE_ERROR(""));
         }
+        // A first FIN can't end the stream below data already received (RFC 9000 section 4.5)
+        if frame.fin && end < self.end {
+            return Err(TransportError::FINAL_SIZE_ERROR(
+                "lower than high water mark",
+            ));
+        }
 
         let new_bytes = self.credit_consumed_by(end, received, max_data)?;
 

rs/moq-quic/src/tests/characterize/frames.rs

@@ -367,10 +367,7 @@ fn stream_final_size_changed() {
     );
 }
 
-/// Today the reader gets all five bytes and then blocks forever: the shrunken final size never
-/// matches what was read, so the stream neither finishes nor errors.
 #[test]
-#[ignore = "bug: /quest/m1/quic/final-size-below-received.md"]
 fn stream_final_size_below_received() {
     let s = id(Side::Client, Dir::Uni, 0);
     let frames = Frames::default().stream(s, 0, b"abcde", false);