lazy-mirror.ts

// Lazy multipurpose mirror handler for AWS Lambda@Edge or API Gateway

import { S3Client, HeadObjectCommand, PutObjectCommand } from '@aws-sdk/client-s3';
import fetch from 'node-fetch';

const s3 = new S3Client({ region: 'your-region' });
const BUCKET = 'qt-mirror-cache';

// Prefix -> upstream URL
const routeTable: Record<string, string> = {
  '/qt': 'https://download.qt.io',
  '/npm': 'https://registry.npmjs.org',
  '/foo': 'https://example.com/foo',
};

export async function handler(event: any) {
  const path = decodeURIComponent(event.path);
  const prefix = Object.keys(routeTable).find(p => path.startsWith(p));
  if (!prefix) {
    console.warn(`[404] Unknown prefix for path: ${path}`);
    return { statusCode: 404, body: 'Unknown mirror path' };
  }

  const upstream = routeTable[prefix];
  const upstreamPath = path.slice(prefix.length);
  const upstreamUrl = `${upstream}${upstreamPath}`;
  const s3Key = path.startsWith('/') ? path.slice(1) : path;

  try {
    await s3.send(new HeadObjectCommand({ Bucket: BUCKET, Key: s3Key }));
    console.info(`[HIT] Cache hit: s3://${BUCKET}/${s3Key}`);
    return redirectToS3(s3Key);
  } catch {
    console.info(`[MISS] Cache miss: s3://${BUCKET}/${s3Key}`);
  }

  try {
    const res = await fetch(upstreamUrl);
    if (!res.ok) {
      console.error(`[UPSTREAM FAIL] ${res.status} ${res.statusText} - ${upstreamUrl}`);
      return { statusCode: res.status, body: 'Upstream error' };
    }

    const body = await res.buffer();
    await s3.send(new PutObjectCommand({
      Bucket: BUCKET,
      Key: s3Key,
      Body: body,
      ContentType: res.headers.get('content-type') || 'application/octet-stream',
    }));
    console.info(`[FETCHED] ${upstreamUrl} → s3://${BUCKET}/${s3Key}`);

    return redirectToS3(s3Key);
  } catch (err) {
    console.error(`[ERROR] Failed to fetch and store ${upstreamUrl}:`, err);
    return { statusCode: 500, body: 'Mirror error' };
  }
}

function redirectToS3(key: string) {
  return {
    statusCode: 302,
    headers: {
      Location: `https://${BUCKET}.s3.amazonaws.com/${key}`,
    },
  };
}