Lingering close for connections with pipelined requests. This is expected to help with clients using pipelining with some constant depth, such as apt[1][2]. When downloading many resources, apt uses pipelining with some constant depth, a number of requests in flight. This essentially means that after receiving a response it sends an additional request to the server, and this can result in requests arriving to the server at any time. Further, additional requests are sent one-by-one, and can be easily seen as such (neither as pipelined, nor followed by pipelined requests). The only safe approach to close such connections (for example, when keepalive_requests is reached) is with lingering. To do so, now nginx monitors if pipelining was used on the connection, and if it was, closes the connection with lingering. [1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=973861#10 [2] https://mailman.nginx.org/pipermail/nginx-devel/2023-January/ZA2SP5SJU55LHEBCJMFDB2AZVELRLTHI.html

/*
 * Copyright (C) Igor Sysoev
 * Copyright (C) Nginx, Inc.
 */


#ifndef _NGX_MAIL_SMTP_MODULE_H_INCLUDED_
#define _NGX_MAIL_SMTP_MODULE_H_INCLUDED_


#include <ngx_config.h>
#include <ngx_core.h>
#include <ngx_mail.h>
#include <ngx_mail_smtp_module.h>


typedef struct {
    ngx_msec_t   greeting_delay;

    size_t       client_buffer_size;

    ngx_str_t    capability;
    ngx_str_t    starttls_capability;
    ngx_str_t    starttls_only_capability;

    ngx_str_t    server_name;
    ngx_str_t    greeting;

    ngx_uint_t   auth_methods;

    ngx_array_t  capabilities;
} ngx_mail_smtp_srv_conf_t;


void ngx_mail_smtp_init_session(ngx_mail_session_t *s, ngx_connection_t *c);
void ngx_mail_smtp_init_protocol(ngx_event_t *rev);
void ngx_mail_smtp_auth_state(ngx_event_t *rev);
ngx_int_t ngx_mail_smtp_parse_command(ngx_mail_session_t *s);


extern ngx_module_t  ngx_mail_smtp_module;


#endif /* _NGX_MAIL_SMTP_MODULE_H_INCLUDED_ */