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在前面我们分析了nginx进程间通信机制的共享内存和套接字。这次我们分析剩下一种进程间通信机制---信号。
首先要区分信号和信号量：信号是用于进程间通信的机制，而信号量是用于保证共享资源不被并发访问的机制，如可使用信号量作为互斥锁实现多进程下对共享资源的同步。
1.nginx中什么地方用到了信号？
比如我们输入命令：./nginx -s reload或./nginx -s stop，nginx则重新加载配置文件或终止。这些地方都是使用了信号机制的。再比如：worker进程挂了，master进程是如何得知的呢？答案就是靠信号，当worker进程挂掉时，nginx能够检测到SIGCHLD信号，从而进行后续处理。
2.nginx的信号如何实现呢？
nginx中将信号相关信息都放在了结构体ngx_signal_t中，如下：
.../os/unix/ngx_process.c:
/* nginx的信号 */
typedef struct {
int signo; /* 信号编号 */
char *signame; /* 信号名 */
char *name; /* 信号对应的nginx命令 */
void (*handler)(int signo); /* 处理信号的回调方法 */
} ngx_signal_t;
每个信号对应一个结构体，所有的信号都放在信号数组signals中，如下：
.../os/unix/ngx_process.c:
/* 进程将会定义的所有信号 */
ngx_signal_t signals[] = {
{ ngx_signal_value(NGX_RECONFIGURE_SIGNAL),
"SIG" ngx_value(NGX_RECONFIGURE_SIGNAL),
"reload",
ngx_signal_handler },
{ ngx_signal_value(NGX_REOPEN_SIGNAL),
"SIG" ngx_value(NGX_REOPEN_SIGNAL),
"reopen",
ngx_signal_handler },
{ ngx_signal_value(NGX_NOACCEPT_SIGNAL),
"SIG" ngx_value(NGX_NOACCEPT_SIGNAL),
"",
ngx_signal_handler },
{ ngx_signal_value(NGX_TERMINATE_SIGNAL),
"SIG" ngx_value(NGX_TERMINATE_SIGNAL),
"stop",
ngx_signal_handler },
{ ngx_signal_value(NGX_SHUTDOWN_SIGNAL),
"SIG" ngx_value(NGX_SHUTDOWN_SIGNAL),
"quit",
ngx_signal_handler },
{ ngx_signal_value(NGX_CHANGEBIN_SIGNAL),
"SIG" ngx_value(NGX_CHANGEBIN_SIGNAL),
"",
ngx_signal_handler },
{ SIGALRM, "SIGALRM", "", ngx_signal_handler },
{ SIGINT, "SIGINT", "", ngx_signal_handler },
{ SIGIO, "SIGIO", "", ngx_signal_handler },
{ SIGCHLD, "SIGCHLD", "", ngx_signal_handler },
{ SIGSYS, "SIGSYS, SIG_IGN", "", SIG_IGN },
{ SIGPIPE, "SIGPIPE, SIG_IGN", "", SIG_IGN },
{ 0, NULL, "", NULL }
};
ngx_signal_t中信号名如下：
...core/ngx_config.h:/* TODO: #ifndef */
#define NGX_SHUTDOWN_SIGNAL QUIT
#define NGX_TERMINATE_SIGNAL TERM
#define NGX_NOACCEPT_SIGNAL WINCH
#define NGX_RECONFIGURE_SIGNAL HUP
#if (NGX_LINUXTHREADS)
#define NGX_REOPEN_SIGNAL INFO
#define NGX_CHANGEBIN_SIGNAL XCPU
#else
#define NGX_REOPEN_SIGNAL USR1
#define NGX_CHANGEBIN_SIGNAL USR2
#endif
3.注册信号及其回调方法
注册信号是在函数ngx_init_signals中完成的，信号的回调函数统一为ngx_signal_handler，在该函数内部再判断是什么信号回调了该方法，从而执行设置相应的位。再继续后续的处理。
源码：
ngx_int_t
ngx_init_signals(ngx_log_t *log)
{
ngx_signal_t *sig;
struct sigaction sa;
/* 添加signals数组中的信号 */
for (sig = signals; sig->signo != 0; sig++) {
ngx_memzero(&sa, sizeof(struct sigaction));
sa.sa_handler = sig->handler; /* 信号的回调方法 */
sigemptyset(&sa.sa_mask);
if (sigaction(sig->signo, &sa, NULL) == -1) { /* 注册信号 */
#if (NGX_VALGRIND)
ngx_log_error(NGX_LOG_ALERT, log, ngx_errno,
"sigaction(%s) failed, ignored", sig->signame);
#else
ngx_log_error(NGX_LOG_EMERG, log, ngx_errno,
"sigaction(%s) failed", sig->signame);
return NGX_ERROR;
#endif
}
}
return NGX_OK;
}
void
ngx_signal_handler(int signo)
{
char *action;
ngx_int_t ignore;
ngx_err_t err;
ngx_signal_t *sig;
ignore = 0;
err = ngx_errno;
for (sig = signals; sig->signo != 0; sig++) {
if (sig->signo == signo) { /* 循环遍历信号 */
break;
}
}
ngx_time_sigsafe_update();
action = "";
switch (ngx_process) {
case NGX_PROCESS_MASTER:
case NGX_PROCESS_SINGLE:
switch (signo) {
case ngx_signal_value(NGX_SHUTDOWN_SIGNAL):
ngx_quit = 1;
action = ", shutting down";
break;
case ngx_signal_value(NGX_TERMINATE_SIGNAL):
case SIGINT:
ngx_terminate = 1;
action = ", exiting";
break;
case ngx_signal_value(NGX_NOACCEPT_SIGNAL):
if (ngx_daemonized) {
ngx_noaccept = 1;
action = ", stop accepting connections";
}
break;
case ngx_signal_value(NGX_RECONFIGURE_SIGNAL):
ngx_reconfigure = 1;
action = ", reconfiguring";
break;
case ngx_signal_value(NGX_REOPEN_SIGNAL):
ngx_reopen = 1;
action = ", reopening logs";
break;
case ngx_signal_value(NGX_CHANGEBIN_SIGNAL):
if (getppid() > 1 || ngx_new_binary > 0) {
/*
* Ignore the signal in the new binary if its parent is
* not the init process, i.e. the old binary's process
* is still running. Or ignore the signal in the old binary's
* process if the new binary's process is already running.
*/
action = ", ignoring";
ignore = 1;
break;
}
ngx_change_binary = 1;
action = ", changing binary";
break;
case SIGALRM:
ngx_sigalrm = 1;
break;
case SIGIO:
ngx_sigio = 1;
break;
case SIGCHLD:
ngx_reap = 1;
break;
}
break;
case NGX_PROCESS_WORKER:
case NGX_PROCESS_HELPER:
switch (signo) {
case ngx_signal_value(NGX_NOACCEPT_SIGNAL):
if (!ngx_daemonized) {
break;
}
ngx_debug_quit = 1;
case ngx_signal_value(NGX_SHUTDOWN_SIGNAL):
ngx_quit = 1;
action = ", shutting down";
break;
case ngx_signal_value(NGX_TERMINATE_SIGNAL):
case SIGINT:
ngx_terminate = 1;
action = ", exiting";
break;
case ngx_signal_value(NGX_REOPEN_SIGNAL):
ngx_reopen = 1;
action = ", reopening logs";
break;
case ngx_signal_value(NGX_RECONFIGURE_SIGNAL):
case ngx_signal_value(NGX_CHANGEBIN_SIGNAL):
case SIGIO:
action = ", ignoring";
break;
}
break;
}
ngx_log_error(NGX_LOG_NOTICE, ngx_cycle->log, 0,
"signal %d (%s) received%s", signo, sig->signame, action);
if (ignore) {
ngx_log_error(NGX_LOG_CRIT, ngx_cycle->log, 0,
"the changing binary signal is ignored: "
"you should shutdown or terminate "
"before either old or new binary's process");
}
if (signo == SIGCHLD) {
ngx_process_get_status();
}
ngx_set_errno(err);
}