add optional nginx keepalive_timeout env to prevent race condition wi…

…th gorouter

the current default of 30 causes the same inconsistent 502's as
https://community.pivotal.io/s/article/5004y00001buMQz1626802995951?language=en_US

and we need to set it to >90s so the gorouter is in charge of closing
the connections
https://docs.cloudfoundry.org/adminguide/routing-keepalive.html

buildpack/core/nginx.py

@@ -104,6 +104,7 @@ def update_config():
         samesite_cookie_workaround_enabled=samesite_cookie_workaround_enabled,
         locations=_get_locations(),
         default_headers=_get_http_headers(),
+        nginx_keepalive_timeout=_get_nginx_keepalive_timeout(),
         nginx_port=str(util.get_nginx_port()),
         runtime_port=str(util.get_runtime_port()),
         admin_port=str(util.get_admin_port()),
@@ -135,6 +136,8 @@ def update_config():
 
     _generate_password_file({"MxAdmin": security.get_m2ee_password()})
 
+def _get_nginx_keepalive_timeout():
+    return os.environ.get("NGINX_KEEPALIVE_TIMEOUT", None)
 
 def _get_proxy_buffer_size():
     return os.environ.get("NGINX_PROXY_BUFFER_SIZE", None)

etc/nginx/conf/nginx.conf.j2

@@ -40,7 +40,7 @@ http {
     gzip_types text/plain text/css text/js text/xml text/javascript application/javascript application/x-javascript application/json application/xml application/xml+rss;
 
     tcp_nopush on;
-    keepalive_timeout 30;
+    keepalive_timeout {% if nginx_keepalive_timeout %}{{ nginx_keepalive_timeout }}{% else %}30{% endif %};
     absolute_redirect off;
     server_tokens off;