如何在AWS Lambda函数中访问文件？本地正常Lambda报错ENOENT

最近切换到Serverless架构后，Lambda函数无法访问文件，本地运行正常的代码部署后报错：

Error: ENOENT: no such file or directory, open 'tmp/file 1-1692962065917.pdf'

原代码及说明 #

用户上传PDF文件后，将其保存到本地tmp目录，根据用户选择调用处理函数完成操作，之后删除原文件。原代码如下：

const serverless = require("serverless-http");
const express = require("express");
const multer = require('multer');
const fs = require('fs');
const { PDFDocument } = require('pdf-lib');
const http = require('http');
const cors = require('cors');

const os = require('os');
const cluster = require('cluster');
const { default: e } = require('express');
const path = require('path');

// count total number of cpu
const cpuNums = os.cpus().length;
console.log("Total Number of Cpus(CORS):"+cpuNums);

var date = new Date();

const app = express();
var check=0;

app.use(cors({
  origin: '*',
}));

var fname;

// Set up multer to handle file uploads
const storage = multer.diskStorage({
  destination: (req, file, cb) => {
         cb(null, './tmp/');
  },
  filename: (req, file, cb) => {
    check++;
         fname = `${file.fieldname} ${check}-${Date.now()}.pdf`;
         console.log("Filename : "+fname);
         cb(null, fname);
  },
});

const upload = multer({ storage });

app.post('/', upload.single('file'), (req, res,next) => { 
console.log("=================================================");
console.log("API Last Build Date " + date.toLocaleDateString());
console.log("API Last Build  Time" + date.toLocaleTimeString());

console.log(date.getFullYear()+'-'+(date.getMonth()+1)+'-'+date.getDate());
console.log("===========================");

console.log("LAST API CALL TIME:--" + date.toLocaleString('en-US', { timeZone: 'Asia/Kolkata' }));

console.log("===========================");
console.log("Your IP Addresss is: " + req.socket.localAddress);
console.log("=================================================");
    // Read the input PDF file
    const file = req.file;
    if (!file) {
           const error = new Error("Please upload a file");
           error.httpStatusCode = 400;
           return next(error);
    }
    // crop uploaded file

    async function cropPDF(inputPath, outputPath, x, y, width, height) {
           // Read the input PDF file
           let pdfDoc = await PDFDocument.load(fs.readFileSync(inputPath));

           //   Loop through all pages in the PDF
           for (let i = 0; i < pdfDoc.getPages().length; i++) {
                  // Get the page that you want to crop
                  let page = pdfDoc.getPage(i);

                  // Set the crop box for the page
                  page.setCropBox(x, y, width, height);
           }
           // Save the output PDF file
           fs.writeFileSync(outputPath, await pdfDoc.save());
           fs.unlink( './tmp/'+fname, (err) => {
                  if (err) {
                    console.error(err);
                  } else {
                    console.log('File deleted successfully');
                  }
                });

                res.download(outputPath);
    }

     if(req.body.Ecommerce==2)
    {
    cropPDF('./tmp/' + fname,'outputfiledownload.pdf', 170, 467, 255, 353)
           .then(() => {
                  console.log("PDF is cropped");
           })
           .catch((error) => {
                  console.log(error);
           });
    }
    });

module.exports.handler = serverless(app);

问题原因及解决方案 #

• Lambda临时目录路径错误
  Lambda运行环境中，唯一可写的临时目录是/tmp/（绝对路径），而你使用的相对路径./tmp/会指向代码包内的只读目录，且该目录默认不存在，导致文件保存失败。

• 关键代码修改

  1. 修改multer的存储目录为绝对路径：

  const storage = multer.diskStorage({
    destination: (req, file, cb) => {
           cb(null, '/tmp/'); // 改用绝对路径
    },
    // 其余代码不变
  });
  

  2. 调整所有文件路径为绝对路径：

  async function cropPDF(inputPath, outputPath, x, y, width, height) {
         let pdfDoc = await PDFDocument.load(fs.readFileSync(inputPath));
         for (let i = 0; i < pdfDoc.getPages().length; i++) {
                let page = pdfDoc.getPage(i);
                page.setCropBox(x, y, width, height);
         }
         // 输出文件也存到/tmp目录
         const tempOutputPath = `/tmp/${outputPath}`;
         fs.writeFileSync(tempOutputPath, await pdfDoc.save());
         // 删除原文件时用绝对路径
         fs.unlink(`/tmp/${fname}`, (err) => {
                if (err) console.error(err);
                else console.log('File deleted successfully');
              });
         res.download(tempOutputPath);
  }
  

  3. 调用cropPDF时传入正确路径：

  cropPDF(`/tmp/${fname}`, 'outputfiledownload.pdf', 170, 467, 255, 353)
  

• 移除无用代码
  Lambda是单进程运行环境，cluster和os.cpus()相关代码无意义，建议删除以减少资源消耗。

内容的提问来源于stack exchange，提问作者Hasti Ghelani