caddyhttp: close quic connections when server closes (#6202)

* close quic connections when server closes

* fix lint

* add comment about CloseGracefully

modules/caddyhttp/server.go

@@ -568,12 +568,30 @@ func (s *Server) serveHTTP3(addr caddy.NetworkAddress, tlsCfg *tls.Config) error
 	// create HTTP/3 server if not done already
 	if s.h3server == nil {
 		s.h3server = &http3.Server{
-			Handler:        s,
+			// Currently when closing a http3.Server, only listeners are closed. But caddy reuses these listeners
+			// if possible, requests are still read and handled by the old handler. Close these connections manually.
+			// see issue: https://github.com/caddyserver/caddy/issues/6195
+			// Will interrupt ongoing requests.
+			// TODO: remove the handler wrap after http3.Server.CloseGracefully is implemented, see App.Stop
+			Handler: http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
+				select {
+				case <-s.ctx.Done():
+					if quicConn, ok := request.Context().Value(quicConnCtxKey).(quic.Connection); ok {
+						//nolint:errcheck
+						quicConn.CloseWithError(quic.ApplicationErrorCode(http3.ErrCodeRequestRejected), "")
+					}
+				default:
+					s.ServeHTTP(writer, request)
+				}
+			}),
 			TLSConfig:      tlsCfg,
 			MaxHeaderBytes: s.MaxHeaderBytes,
 			// TODO: remove this config when draft versions are no longer supported (we have no need to support drafts)
 			QuicConfig: &quic.Config{
-				Versions: []quic.VersionNumber{quic.Version1, quic.Version2},
+				Versions: []quic.Version{quic.Version1, quic.Version2},
+			},
+			ConnContext: func(ctx context.Context, c quic.Connection) context.Context {
+				return context.WithValue(ctx, quicConnCtxKey, c)
 			},
 		}
 	}
@@ -992,6 +1010,10 @@ const (
 	// For referencing underlying net.Conn
 	ConnCtxKey caddy.CtxKey = "conn"
 
+	// For referencing underlying quic.Connection
+	// TODO: export if needed later
+	quicConnCtxKey caddy.CtxKey = "quic_conn"
+
 	// For tracking whether the client is a trusted proxy
 	TrustedProxyVarKey string = "trusted_proxy"