readable.destroy（）不发出'close'和'error'事件Node.js

问题描述 投票：1回答：1

我写了一些恰好起作用的代码，但我认为readable.destroy()（例如，src.destroy()，在我的例子中）应该发出close和error事件......

这里有一个很小的例子来说明我的困惑：

const fs = require('fs');
const src = fs.createReadStream('streaming-example.js');
const dst = fs.createWriteStream('streaming-example.txt');
src.pipe(dst);
src.on('readable', () => {
  let chunk;
  while (null !== (chunk = src.read())) {
    /**
    * This should cause 'error' and 'close' events to emit
    * @see https://nodejs.org/dist/latest-v11.x/docs/api/stream.html#stream_readable_destroy_error
    */
    src.destroy();
  }
});
src.on('close', () => console.log(`'close' event emitted`));
src.on('end', () => console.log(`'end' event emitted`));
src.on('error', (err) => console.log(`'error' event emitted`));

这是该程序的示例运行：

$ node streaming-example.js 
'close' event emitted
$

（并且，它恰好也写完了一个名为streaming-example.txt的新文件）

如果不清楚，我预计会发出close和error事件，然后触发相应的回调。但是，似乎只发布了close事件。

error事件发生了怎么了？

javascript node.js events streaming dom-events
1个回答
1
投票

事实证明，在node.js代码库中占据一席之地可以解释这种混乱。

看看node/lib/_stream_readable.js，我们看到破坏函数在node/lib/internal/streams/destroy.js中定义。在这里，我们直接告诉（通过简单的javascript）error事件只在一个truthy值被传递到destroy函数时才被发出（在语义上，这将是在相同的客户端代码中生成的错误，destroy被调用）。

例如，如果我们只是通过更改来修改上面的示例代码

readable.destroy();

成为

readable.destroy(true); // or, more semantically correct, some Error value

我们得到以下输出：

$ node streaming-example.js 
'error' event emitted with err: true
$

但是，现在我们失去了close事件。那么......刚刚发生了什么？

再看看node/lib/internal/streams/destroy.js，我们注意到以下特殊情况逻辑：

const readableDestroyed = this._readableState &&
  this._readableState.destroyed;
const writableDestroyed = this._writableState &&
  this._writableState.destroyed;

if (readableDestroyed || writableDestroyed) {
  if (cb) {
    cb(err);
  } else if (err &&
              (!this._writableState || !this._writableState.errorEmitted)) {
    process.nextTick(emitErrorNT, this, err);
  }
  return this;
}

// We set destroyed to true before firing error callbacks in order
// to make it re-entrance safe in case destroy() is called within callbacks

if (this._readableState) {
  this._readableState.destroyed = true;
}

// If this is a duplex stream mark the writable part as destroyed as well
if (this._writableState) {
  this._writableState.destroyed = true;
}

error正确发射的事实，但它似乎close没有发出暗示我们正在处理双工流。我们可以查看它是什么，但让我们坚持使用计算机告诉我们的内容，以保持简单。用这个while循环替换原来的while循环

while (null !== (chunk = src.read())) {
  console.log('before-destroy, src:', JSON.stringify(src));
  src.destroy(true);  // should cause 'close' and 'error' events to emit
  console.log('after-destroy, src:', JSON.stringify(src));       
}

我们得到以下输出：

$ node streaming-example.js 
before-destroy, src: {"_readableState":{"objectMode":false,"highWaterMark":65536,"buffer":{"head":null,"tail":null,"length":0},"length":0,"pipes":{"_writableState":{"objectMode":false,"highWaterMark":16384,"finalCalled":false,"needDrain":false,"ending":false,"ended":false,"finished":false,"destroyed":false,"decodeStrings":true,"defaultEncoding":"utf8","length":633,"writing":true,"corked":0,"sync":false,"bufferProcessing":false,"writelen":633,"bufferedRequest":null,"lastBufferedRequest":null,"pendingcb":1,"prefinished":false,"errorEmitted":false,"emitClose":false,"autoDestroy":false,"bufferedRequestCount":0,"corkedRequestsFree":{"next":null,"entry":null}},"writable":true,"_events":{},"_eventsCount":5,"path":"streaming-example.txt","fd":24,"flags":"w","mode":438,"autoClose":true,"bytesWritten":0,"closed":false},"pipesCount":1,"flowing":false,"ended":false,"endEmitted":false,"reading":true,"sync":false,"needReadable":true,"emittedReadable":false,"readableListening":true,"resumeScheduled":false,"paused":false,"emitClose":false,"autoDestroy":false,"destroyed":false,"defaultEncoding":"utf8","awaitDrain":0,"readingMore":true,"decoder":null,"encoding":null},"readable":true,"_events":{"end":[null,null,null]},"_eventsCount":5,"path":"streaming-example.js","fd":23,"flags":"r","mode":438,"end":null,"autoClose":true,"bytesRead":633,"closed":false}
after-destroy, src: {"_readableState":{"objectMode":false,"highWaterMark":65536,"buffer":{"head":null,"tail":null,"length":0},"length":0,"pipes":{"_writableState":{"objectMode":false,"highWaterMark":16384,"finalCalled":false,"needDrain":false,"ending":false,"ended":false,"finished":false,"destroyed":false,"decodeStrings":true,"defaultEncoding":"utf8","length":633,"writing":true,"corked":0,"sync":false,"bufferProcessing":false,"writelen":633,"bufferedRequest":null,"lastBufferedRequest":null,"pendingcb":1,"prefinished":false,"errorEmitted":false,"emitClose":false,"autoDestroy":false,"bufferedRequestCount":0,"corkedRequestsFree":{"next":null,"entry":null}},"writable":true,"_events":{},"_eventsCount":5,"path":"streaming-example.txt","fd":24,"flags":"w","mode":438,"autoClose":true,"bytesWritten":0,"closed":false},"pipesCount":1,"flowing":false,"ended":false,"endEmitted":false,"reading":true,"sync":false,"needReadable":true,"emittedReadable":false,"readableListening":true,"resumeScheduled":false,"paused":false,"emitClose":false,"autoDestroy":false,"destroyed":true,"defaultEncoding":"utf8","awaitDrain":0,"readingMore":true,"decoder":null,"encoding":null},"readable":true,"_events":{"end":[null,null,null]},"_eventsCount":5,"path":"streaming-example.js","fd":null,"flags":"r","mode":438,"end":null,"autoClose":true,"bytesRead":633,"closed":false}
'error' event emitted
$

这告诉我们，我们可能正在处理双工流，或者至少这向我们解释了为什么只发出error事件（因为，特别是只看after-destroy输出，this._readableState和this._writeableState都是真的，因此destroy函数将局部变量readableDestroyed和writeableDestroyed设置为true，我们从console.log注意到this._writableState.errorEmitted是false，因此process.nextTick(emitErrorNT, this, err);在从destroy函数退出之前被执行）。

这个问题现在得到了充分的回答。

作为一个额外的，很高兴知道duplex流与其他类型的流之间的区别是什么。为此，this portion of the node.js documentation的快速参考是一个开始。

那么，当发出close和error事件时（例如，当我们不处理duplex流时）的示例怎么样？以下代码和执行执行此操作，如下所示：

const readable = process.stdin;
const writable = process.stdout;
readable.setEncoding('utf8');
readable.on('readable', () => {
  let chunk;
  while ((chunk = readable.read()) !== null) {
    writable.write(`data: ${chunk}`);
  }
  readable.destroy(true);
});
readable.on('close', () => console.log(`'close' event emitted`));
readable.on('error', (err) => console.log(`'error' event emitted with err:`, err));

执行此脚本以及一些I / O（键入asdf，然后按回车键/回车键），提供以下输出：

$ node streaming-example2.js 
asdf
data: asdf
'error' event emitted with err: true
'close' event emitted
$
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