How to fix Error: EACCES: permission denied, write 'readonly' in Node.js

# Enable Node.js filesystem debugging
NODE_DEBUG=fs node app.js

# Or add logging to your application
const fs = require('fs');
const originalWriteFile = fs.writeFile;
fs.writeFile = function(...args) {
  console.log('Write attempt to:', args[0]);
  return originalWriteFile.apply(this, args);
};

services:
  app:
    image: myapp:latest
    security_opt:
      - no-new-privileges:true
    read_only: true
    volumes:
      - /tmp
      - ./logs:/app/logs
      - cache-volume:/app/.cache
    environment:
      - NODE_ENV=production

volumes:
  cache-volume:

apiVersion: apps/v1
kind: Deployment
spec:
  template:
    spec:
      securityContext:
        readOnlyRootFilesystem: true
        runAsNonRoot: true
      containers:
      - name: app
        image: myapp:latest
        volumeMounts:
        - name: tmp
          mountPath: /tmp
        - name: cache
          mountPath: /app/.cache
        - name: logs
          mountPath: /app/logs
      volumes:
      - name: tmp
        emptyDir: {}
      - name: cache
        emptyDir: {}
      - name: logs
        emptyDir: {}

# Dockerfile
FROM node:20-alpine

# Set writable directories for various tools
ENV NODE_OPTIONS="--max-old-space-size=2048"
ENV NPM_CONFIG_CACHE=/tmp/.npm
ENV PM2_HOME=/tmp/.pm2
ENV XDG_CACHE_HOME=/tmp/.cache
ENV TMPDIR=/tmp

WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production

COPY . .

# Ensure /tmp is writable at runtime
VOLUME /tmp

USER node
CMD ["node", "server.js"]

RUN npm run build  # Build .next during image creation

docker run --read-only \
  --tmpfs /tmp:rw,noexec,nosuid,size=100m \
  myapp:latest

services:
  app:
    image: myapp:latest
    read_only: true
    tmpfs:
      - /tmp:rw,noexec,nosuid,size=100m
      - /run:rw,noexec,nosuid,size=10m

volumeMounts:
- name: tmp
  mountPath: /tmp
volumes:
- name: tmp
  emptyDir:
    medium: Memory
    sizeLimit: 100Mi

const fs = require('fs');
const path = require('path');

// Helper to find writable directory
function getWritableDir() {
  const candidates = [
    process.env.TMPDIR,
    '/tmp',
    process.env.HOME,
    process.cwd()
  ];

  for (const dir of candidates) {
    if (dir) {
      try {
        const testFile = path.join(dir, '.write-test');
        fs.writeFileSync(testFile, 'test');
        fs.unlinkSync(testFile);
        return dir;
      } catch (err) {
        // Directory not writable, try next
      }
    }
  }

  throw new Error('No writable directory found');
}

const cacheDir = path.join(getWritableDir(), 'app-cache');

// Ensure cache directory exists
try {
  fs.mkdirSync(cacheDir, { recursive: true });
} catch (err) {
  if (err.code !== 'EEXIST' && err.code !== 'EROFS') {
    throw err;
  }
  console.warn('Cache directory unavailable, running without cache');
}