quic: fixup set_final_size

Ignore subsequent calls to set_final_size unless the new size
is more than the previous, in which case, we have us a bug.

src/quic/node_quic_stream-inl.h

@@ -35,7 +35,12 @@ void QuicStream::set_flag(int32_t flag, bool on) {
 }
 
 void QuicStream::set_final_size(uint64_t final_size) {
-  CHECK_EQ(GetStat(&QuicStreamStats::final_size), 0);
+  // Only set the final size once.
+  if (is_flag_set(QUICSTREAM_FLAG_FIN)) {
+    CHECK_LE(final_size, GetStat(&QuicStreamStats::final_size));
+    return;
+  }
+  set_flag(QUICSTREAM_FLAG_FIN);
   SetStat(&QuicStreamStats::final_size, final_size);
 }
 

src/quic/node_quic_stream.cc

@@ -355,7 +355,6 @@ void QuicStream::ReceiveData(
   // When fin != 0, we've received that last chunk of data for this
   // stream, indicating that the stream will no longer be readable.
   if (flags & NGTCP2_STREAM_DATA_FLAG_FIN) {
-    set_flag(QUICSTREAM_FLAG_FIN);
     set_final_size(offset + datalen);
     EmitRead(UV_EOF);
   }

src/quic/node_quic_stream.h

@@ -256,7 +256,11 @@ class QuicStream : public AsyncWrap,
   // Specifies the kind of headers currently being processed.
   inline void set_headers_kind(QuicStreamHeadersKind kind);
 
-  // Set the final size for the QuicStream
+  // Set the final size for the QuicStream. This only works
+  // the first time it is called. Subsequent calls will be
+  // ignored unless the subsequent size is greater than the
+  // prior set size, in which case we have a bug and we'll
+  // assert.
   inline void set_final_size(uint64_t final_size);
 
   // The final size is the maximum amount of data that has been