add debug logging

src/http/ngx_http_upstream.c

@@ -1945,6 +1945,9 @@ ngx_http_upstream_send_response(ngx_http_request_t *r, ngx_http_upstream_t *u)
         }
     }
 
+    ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
+                   "http cacheable: %d", u->cacheable);
+
 #endif
 
     p = u->pipe;