clear r->keepalive to disable preventively calling
ngx_http_set_keepalive() while request cleanup

src/http/ngx_http_request.c

diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index 7efa282..6c42f38 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -2411,6 +2411,8 @@
         }
     }
 
+    r->keepalive = 0;
+
     ngx_http_free_request(r, 0);
 
     c->data = hc;