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


#ifndef _NGX_PROCESS_CYCLE_H_INCLUDED_
#define _NGX_PROCESS_CYCLE_H_INCLUDED_


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


#define NGX_PROCESS_SINGLE     0
#define NGX_PROCESS_MASTER     1
#define NGX_PROCESS_SIGNALLER  2
#define NGX_PROCESS_WORKER     3


void ngx_master_process_cycle(ngx_cycle_t *cycle);
void ngx_single_process_cycle(ngx_cycle_t *cycle);
void ngx_close_handle(HANDLE h);


extern ngx_uint_t      ngx_process;
extern ngx_uint_t      ngx_worker;
extern ngx_pid_t       ngx_pid;
extern ngx_uint_t      ngx_exiting;

extern sig_atomic_t    ngx_quit;
extern sig_atomic_t    ngx_terminate;
extern sig_atomic_t    ngx_reopen;

extern ngx_uint_t      ngx_inherited;
extern ngx_pid_t       ngx_new_binary;


extern HANDLE          ngx_master_process_event;
extern char            ngx_master_process_event_name[];


#endif /* _NGX_PROCESS_CYCLE_H_INCLUDED_ */