HTTP/3: postponed session creation to init() callback. Now the session object is assigned to c->data while ngx_http_connection_t object is referenced by its http_connection field, similar to ngx_http_v2_connection_t and ngx_http_request_t. The change allows to eliminate v3_session field from ngx_http_connection_t. The field was under NGX_HTTP_V3 macro, which was a source of binary compatibility problems when nginx/module is build with/without HTTP/3 support. Postponing is essential since c->data should retain the reference to ngx_http_connection_t object throughout QUIC handshake, because SSL callbacks ngx_http_ssl_servername() and ngx_http_ssl_alpn_select() rely on this.

--- a/src/http/ngx_http_request.h
+++ b/src/http/ngx_http_request.h
@@ -324,10 +324,6 @@ typedef struct {
 #endif
 #endif
 
-#if (NGX_HTTP_V3 || NGX_COMPAT)
-    ngx_http_v3_session_t            *v3_session;
-#endif
-
     ngx_chain_t                      *busy;
     ngx_int_t                         nbusy;