HTTP/3: allowed QUIC stream connection reuse. A QUIC stream connection is treated as reusable until first bytes of request arrive, which is also when the request object is now allocated. A connection closed as a result of draining, is reset with the error code H3_REQUEST_REJECTED. Such behavior is allowed by quic-http-34: Once a request stream has been opened, the request MAY be cancelled by either endpoint. Clients cancel requests if the response is no longer of interest; servers cancel requests if they are unable to or choose not to respond. When the server cancels a request without performing any application processing, the request is considered "rejected." The server SHOULD abort its response stream with the error code H3_REQUEST_REJECTED. The client can treat requests rejected by the server as though they had never been sent at all, thereby allowing them to be retried later.

--- a/src/http/v3/ngx_http_v3.h
+++ b/src/http/v3/ngx_http_v3.h
@@ -90,6 +90,9 @@
 #define ngx_http_v3_shutdown_connection(c, code, reason)                      \
     ngx_quic_shutdown_connection(c->quic->parent, code, reason)
 
+#define ngx_http_v3_connection(c)                                             \
+    ((c)->quic ? ngx_http_quic_get_connection(c)->addr_conf->http3 : 0)
+
 
 typedef struct {
     size_t                        max_table_capacity;
@@ -138,6 +141,7 @@ struct ngx_http_v3_session_s {
 
 
 void ngx_http_v3_init(ngx_connection_t *c);
+void ngx_http_v3_reset_connection(ngx_connection_t *c);
 ngx_int_t ngx_http_v3_init_session(ngx_connection_t *c);
 ngx_int_t ngx_http_v3_check_flood(ngx_connection_t *c);