Resolver: make TCP write timer event cancelable. Similar to 70e65bf8dfd7, the change is made to ensure that the ability to cancel resolver tasks is fully controlled by the caller. As mentioned in the referenced commit, it is safe to make this timer cancelable because resolve tasks can have their own timeouts that are not cancelable. The scenario where this may become a problem is a periodic background resolve task (not tied to a specific request or a client connection), which receives a response with short TTL, large enough to warrant fallback to a TCP query. With each event loop wakeup, we either have a previously set write timer instance or schedule a new one. The non-cancelable write timer can delay or block graceful shutdown of a worker even if the ngx_resolver_ctx_t->cancelable flag is set by the API user, and there are no other tasks or connections. We use the resolver API in this way to maintain the list of upstream server addresses specified with the 'resolve' parameter, and there could be third-party modules implementing similar logic.

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


#ifndef _NGX_SYSLOG_H_INCLUDED_
#define _NGX_SYSLOG_H_INCLUDED_


typedef struct {
    ngx_uint_t        facility;
    ngx_uint_t        severity;
    ngx_str_t         tag;

    ngx_addr_t        server;
    ngx_connection_t  conn;
    unsigned          busy:1;
    unsigned          nohostname:1;
} ngx_syslog_peer_t;


char *ngx_syslog_process_conf(ngx_conf_t *cf, ngx_syslog_peer_t *peer);
u_char *ngx_syslog_add_header(ngx_syslog_peer_t *peer, u_char *buf);
void ngx_syslog_writer(ngx_log_t *log, ngx_uint_t level, u_char *buf,
    size_t len);
ssize_t ngx_syslog_send(ngx_syslog_peer_t *peer, u_char *buf, size_t len);


#endif /* _NGX_SYSLOG_H_INCLUDED_ */