Promise写入文件完成后无法读取文件内容的问题排查

我写了一段代码从S3下载文件到本地目录，之后读取文件内容并打印到控制台。我把文件复制逻辑封装进Promise，想用await确保文件写入完成后再执行读取操作，但运行代码后，文件能正确复制，读取部分却只打印空行；即使不删除已复制的文件再次运行，结果仍是空行。不过若将读取打印代码移至Promise上方，却能正常打印内容。刚接触Promise和异步编程，不清楚问题出在哪里，代码如下：

const AWS = require('aws-sdk');
const s3 = new AWS.S3();
const fs = require('fs');

(async function copyAndPrintFile() {
    const S3Bucket = "ExampleBucketName";
    const S3File = "ExampleFileName"

    // Copy file from S3 to local directory
    const readParams = {
        Bucket: S3Bucket,
        Key: S3File,
        ResponseContentType: "text/plain"
    };
    const copyFilePromise = new Promise((resolve, reject) =>{
        const file = fs.createWriteStream(S3File);
        s3.getObject(readParams).createReadStream().pipe(file);
        resolve(console.log("File has been written"));
    })
    await copyFilePromise;

    // Read contents of new file and print it to the console
    let data = fs.readFileSync(S3File, 'utf-8');
    console.log(data);
})();

你的Promise里resolve调用得太早了——pipe是异步操作，S3的数据流还在往本地文件写的时候，你就直接执行了resolve，导致await提前结束，此时文件其实还没写完，所以后续用fs.readFileSync读到的是空内容。

而把读取代码移到Promise上方时，你读的是之前运行已经完整写入的文件，所以能正常打印内容。

需要监听本地写流的finish事件，在这个事件触发时（表示文件已经完全写入）再调用resolve；同时还要监听error事件处理传输或写入中的错误，避免Promise一直处于pending状态。修改后的代码如下：

const AWS = require('aws-sdk');
const s3 = new AWS.S3();
const fs = require('fs');

(async function copyAndPrintFile() {
    const S3Bucket = "ExampleBucketName";
    const S3File = "ExampleFileName"

    const readParams = {
        Bucket: S3Bucket,
        Key: S3File,
        ResponseContentType: "text/plain"
    };
    
    const copyFilePromise = new Promise((resolve, reject) =>{
        const file = fs.createWriteStream(S3File);
        const readStream = s3.getObject(readParams).createReadStream();
        
        // 监听写入完成事件
        file.on('finish', () => {
            console.log("File has been written");
            resolve();
        });
        
        // 监听错误事件，处理读写过程中的异常
        readStream.on('error', reject);
        file.on('error', reject);
        
        readStream.pipe(file);
    })
    
    await copyFilePromise;

    let data = fs.readFileSync(S3File, 'utf-8');
    console.log(data);
})();

关键修改点： #

• 给写流添加finish事件监听，确保文件完全写入后才resolve
• 给读流和写流都添加error事件监听，出现错误时调用reject，避免Promise挂起
• 把resolve的调用移到finish事件回调里，不再提前触发

内容的提问来源于stack exchange，提问作者Thorium