Fix premature abandoment of STOP_SENDING transmission

quinn-proto/src/connection/streams.rs

@@ -533,13 +533,13 @@ impl Streams {
                 Some(x) => x,
                 None => break,
             };
-            let stream = match self.recv.get_mut(&frame.id) {
-                Some(x) => x,
-                None => continue,
-            };
-            if !stream.is_receiving() {
-                continue;
-            }
+            // We may need to transmit STOP_SENDING even for streams whose state we have discarded,
+            // because we are able to discard local state for stopped streams immediately upon
+            // receiving FIN, even if the peer still has arbitrarily large amounts of data to
+            // (re)transmit due to loss or unconventional sending strategy. We could fine-tune this
+            // a little by dropping the frame if we specifically know the stream's been reset by the
+            // peer, but we discard that information as soon as the application consumes it, so it
+            // can't be relied upon regardless.
             trace!(stream = %frame.id, "STOP_SENDING");
             frame.encode(buf);
             retransmits.get_or_create().stop_sending.push(frame);

quinn-proto/src/connection/streams/recv.rs

@@ -121,6 +121,10 @@ impl Recv {
         self.assembler.clear();
         // Issue flow control credit for unread data
         let read_credits = self.end - self.assembler.bytes_read();
+        // This may send a spurious STOP_SENDING if we've already received all data, but it's a bit
+        // fiddly to distinguish that from the case where we've received a FIN but are missing some
+        // data that the peer might still be trying to retransmit, in which case a STOP_SENDING is
+        // still useful.
         Ok((read_credits, ShouldTransmit(self.is_receiving())))
     }