为什么多层异步函数不捕获节点中最低级别抛出的错误？

Question

我正在尝试测试某些邮件代码的故障模式，这些代码在最低级别可能会引发错误。测试和抛出函数之间的所有层都是异步的，并在它们下面的函数上使用 await 。在顶层（也在异步函数中，我有一个 try catch 块。但是节点在错误传播到此级别之前抛出未处理的 promise 异常。

我的测试代码是这样的

beforeEach(function() {
  //set default values - tests can change them
  this.reasons = '';
  this.reschedules = 0;
  this.params.cid = 35124;

  this.startTest = async () => {
    /*  this.confirmation is an async function under test, 
        this.mailer is a mock mailer with an async "send" method
        which will throw an error in the correct test */
    const doner = this.confirmation(this.mailer);  
    // ..other actions related to mocking database access made by confirmation
    await doner;
    return this.mailer.maildata; //provide info on parameters passed to this.mailer
  };
});
it('Failure to send is reported', async function() {
  this.mailer.sendResolve = false; //tell mock mailer to fail send request
  try {
    await this.startTest();
    expect(true).to.be.false;
  } catch(err) {
    expect(err).to.be.instanceOf(Error);
  }
});

mock mailer有点像这样

class Mailer {
    constructor(user,params){
...
    }
    ...
    async send(subject, to, cc, bcc) {
      this.maildata.subject = subject;
      if (to !== undefined) this.maildata.to = to;
      if (cc !== undefined) this.maildata.cc = cc;
      if (bcc !== undefined) this.maildata.bcc = bcc;
      if (!this.sendResolve) throw new Error('Test Error');
    }
    ...
 }

和被测代码摘要

 module.exports = async function(mailer) {
    //get confirm data from database
    const cData = await confirm(mailer.params.cid, mailer.db);
    if (cData.count > 0) {
       // ... format the email message and build it into maildata
       await mailer.send(
        subject,
        emailAddress,
        null,
        process.env.PAS_MAIL_FROM,
        {
          pid:cData.pid,
          type: 'confirmation',
          extra: `Calendar ID ${mailer.params.cid} with procedure ${cData.procedure}`
        }
      );
      debug('message sent, update the database');
      await mailer.db.exec(async connection => {
 ...
       });
      debug('success');
    } else {
      debug('invalid calendarid');
      throw new Error('Invalid Calendar ID');
    }
  };

可以看出，从async send函数返回堆栈到try {}catch(){}的调用路径都是异步函数。但是当我运行这个测试节点输出一个未处理的承诺拒绝时。

我已经尝试使用 visual studio 代码调试器来单步执行此操作，我有点迷失在包装异步函数以将它们变成承诺提供者的机制中。据我所知，一层错误处理正确，然后在下一层失败。

这是否意味着每个异步函数都必须有一个 try catch 块来捕获并重新抛出任何错误？我找不到任何说明我必须这样做的解释。

Answer 1

回答你的问题：

Does this mean that every async function must have a try catch block to catch and rethrow any error?

如您所料，错误会通过 await-ed 调用向上传播：

const assert = require('assert');

const outer = async () => {
  await middle();
}

const middle = async () => {
  await inner();
}

const inner = async () => {
  throw new Error('something bad happened');
}

it('should catch the error', async () => {
  let errorMessage;
  try {
    await outer();
  }
  catch (err) {
    errorMessage = err.message;
  }
  assert(errorMessage === 'something bad happened');  // Success!
});

...所以不，你不需要在每个级别都有一个 try / catch 块。

追踪未处理的 Promise 拒绝

我无法确切地看到 await 链在您的示例代码中可能被破坏的位置，但为了帮助追踪未处理的 Promise 拒绝，您可以添加 process handler for the unhandledRejection event并查看记录的 Promise 以查看拒绝从何处开始并从那里向后跟踪调用堆栈：

const assert = require('assert');

const outer = async () => {
  await middle();
}

const middle = async () => {
  inner();  // <= this will cause an Unhandled Rejection
}

const inner = async () => {
  throw new Error('something bad happened');
}

it('should catch the error', async () => {
  let errorMessage;
  try {
    await outer();
  }
  catch (err) {
    errorMessage = err.message;
  }
  assert(errorMessage === undefined);  // Success!  (broken await chain)
})

process.on('unhandledRejection', (reason, p) => {
  console.log('Unhandled Rejection at:', p);
  console.log('reason:', reason);
});

...在这种情况下记录：

Unhandled Rejection at: Promise {
  <rejected> Error: something bad happened
      at inner (.../code.test.js:12:9)
      at inner (.../code.test.js:8:3)
      at middle (.../code.test.js:4:9)  // <= this is the broken link
      at Context.outer (.../code.test.js:18:11)
      at callFn (...\node_modules\mocha\lib\runnable.js:387:21)
      ...

...这将我们指向 inner 中抛出的 Error，通过追踪链，我们发现 middle 是损坏的 link。

为什么多层异步函数不捕获节点中最低级别抛出的错误？

Why doesn't multilayered async functions catch error thrown at lowest level in node?

asynchronous

mocha.js

node.js

async-await