Nginx原始碼分析與實踐---程序間通訊機制（訊號）
阿新 • • 發佈：2019-01-10
在前面我們分析了nginx程序間通訊機制的共享記憶體和套接字。這次我們分析剩下一種程序間通訊機制---訊號。
首先要區分訊號和訊號量：訊號是用於程序間通訊的機制，而訊號量是用於保證共享資源不被併發訪問的機制，如可使用訊號量作為互斥鎖實現多程序下對共享資源的同步。
1.nginx中什麼地方用到了訊號？
比如我們輸入命令：./nginx -s reload或./nginx -s stop，nginx則重新載入配置檔案或終止。這些地方都是使用了訊號機制的。再比如：worker程序掛了，master程序是如何得知的呢？答案就是靠訊號，當worker程序掛掉時，nginx能夠檢測到SIGCHLD訊號， 從而進行後續處理。
2.nginx的訊號如何實現呢？
nginx中將訊號相關資訊都放在了結構體ngx_signal_t中，如下：
.../os/unix/ngx_process.c:
/* nginx的訊號 */
typedef struct {
int signo; /* 訊號編號 */
char *signame; /* 訊號名 */
char *name; /* 訊號對應的nginx命令 */
void (*handler)(int signo); /* 處理訊號的回撥方法 */
} ngx_signal_t;
每個訊號對應一個結構體，所有的訊號都放在訊號陣列signals中，如下：
.../os/unix/ngx_process.c:
在上面陣列中我們看到了很多熟悉的訊號，如reload,stop,SIGCHLD等等。如果我們想在ngixn中定義自己的訊號，就可以將訊號加入該陣列中即可。/* 程序將會定義的所有訊號 */ 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中訊號名如下：
...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.註冊訊號及其回撥方法
註冊訊號是在函式ngx_init_signals中完成的，訊號的回撥函式統一為ngx_signal_handler，在該函式內部再判斷是什麼訊號回調了該方法，從而執行設定相應的位。再繼續後續的處理。
原始碼：
ngx_int_t
ngx_init_signals(ngx_log_t *log)
{
ngx_signal_t *sig;
struct sigaction sa;
/* 新增signals陣列中的訊號 */
for (sig = signals; sig->signo != 0; sig++) {
ngx_memzero(&sa, sizeof(struct sigaction));
sa.sa_handler = sig->handler; /* 訊號的回撥方法 */
sigemptyset(&sa.sa_mask);
if (sigaction(sig->signo, &sa, NULL) == -1) { /* 註冊訊號 */
#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) { /* 迴圈遍歷訊號 */
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);
}