Node.js ChildProcess 关闭、退出事件之间的区别

在 node 之前.js 0.7.7中，子进程上只有一个"退出"事件(没有"关闭"事件).当子进程退出且所有流(stdin、stdout、stdout)关闭时，将触发此事件.

In Node 0.7.7，引入了"关闭"事件(see commit).

当子进程的stdio流关闭时，会发出"close"事件.这与"退出"事件不同，因为多个进程可能共享相同的stdio流.

如果你只是生成了一个程序，没有对stdio做任何特殊的操作，那么"退出"之后就会触发"关闭"事件.

因此，如果您只对进程终止感兴趣(例如，因为进程拥有独占资源)，那么监听"退出"就足够了.

实验:在杀死子元素之前摧毁stdio

通过实验(在Node.js v7.2.0中)，我发现如果子进程不使用stdio流，那么"close"事件只有在程序退出后才会触发:

// The "sleep" command takes no input and gives no output.
cp = require('child_process').spawn('sleep', ['100']);
cp.on('exit', console.log.bind(console, 'exited'));
cp.on('close', console.log.bind(console, 'closed'));
cp.stdin.end();
cp.stdout.destroy();
cp.stderr.destroy();
console.log('Closed all stdio');
setTimeout(function() { 
    console.log('Going to kill');
    cp.kill();
}, 500);

上述产生"睡眠"输出的程序:

Closed all stdio
Going to kill
exited null SIGTERM
closed null SIGTERM

当我把第一行改成只输出，

// The "yes" command continuously outputs lines with "y"
cp = require('child_process').spawn('yes');

... 那么输出是:

Closed all stdio
exited 1 null
closed 1 null
Going to kill

类似地，当我修改生成一个只从stdin读取的程序时，

// Keeps reading from stdin.
cp = require('child_process').spawn('node', ['-e', 'process.stdin.resume()']);

或者当我从stdin读取数据并输出到stdout时，

// "cat" without arguments reads from stdin, and outputs to stdout
cp = require('child_process').spawn('cat');

实验:将程序转移到另一个，杀死第一个程序

之前的实验是相当人工的.下一个实验更现实一些:将一个程序传输到另一个程序，然后杀死第一个程序.

// Reads from stdin, output the input to stdout, repeat.
cp = require('child_process').spawn('bash', ['-c', 'while read x ; do echo "$x" ; done']);
cp.on('exit', console.log.bind(console, 'exited'));
cp.on('close', console.log.bind(console, 'closed'));

cpNext = require('child_process').spawn('cat');
cp.stdout.pipe(cpNext.stdin);

setTimeout(function() {
    // Let's assume that it has started. Now kill it.
    cp.kill();
    console.log('Called kill()');
}, 500);

输出:

Called kill()
exited null SIGTERM
closed null SIGTERM

类似地，当第一个程序只读取输入而从不输出时:

// Keeps reading from stdin, never outputs.
cp = require('child_process').spawn('bash', ['-c', 'while read ; do : ; done']);

当第一个程序在不等待stdin的情况下继续输出时，行为就不同了，下一个实验显示了这一点.

实验:将大量输出的程序输送到另一个程序，杀死第一个程序

// Equivalent to "yes | cat".
cp = require('child_process').spawn('yes');
cp.on('exit', console.log.bind(console, 'exited'));
cp.on('close', console.log.bind(console, 'closed'));

cpNext = require('child_process').spawn('cat');
cp.stdout.pipe(cpNext.stdin);

setTimeout(function() {
    // Let's assume that it has started. Now kill it.
    cp.kill();
    console.log('Called kill()');
    setTimeout(function() {
        console.log('Expecting "exit" to have fired, and not "close"');
        // cpNext.kill();
        // ^ Triggers 'error' event, errno ECONNRESET.
        // ^ and does not fire the 'close' event!

        // cp.stdout.unpipe(cpNext.stdin);
        // ^ Does not appear to have any effect.
        // ^ calling cpNext.kill() throws ECONNRESET.
        // ^ and does not fire the 'close' event!

        cp.stdout.destroy(); // <-- triggers 'close'
        cpNext.stdin.destroy();
        // ^ Without this, cpNext.kill() throws ECONNRESET.

        cpNext.kill();
    }, 500);
}, 500);

上述程序输出以下内容，然后退出:

Called kill()
exited null SIGTERM
Expecting "exit" to have fired, and not "close"
closed null SIGTERM