ledgerflow-backend/lambdas/auth/index.js

// lambdas/auth/index.js
// Public (unauthenticated) endpoints:
//   GET  /auth/me     — returns user profile from Google token (client calls after login)
//   GET  /auth/config — returns Google Client ID so the frontend can init Google Sign-In
//   POST /auth/logout — clears server-side session record

const { OAuth2Client } = require("google-auth-library");
const { DynamoDBDocumentClient, DeleteCommand } = require("@aws-sdk/lib-dynamodb");
const { DynamoDBClient } = require("@aws-sdk/client-dynamodb");
const { ok, unauthorized, serverError, CORS_HEADERS, parseBody } = require("@ledgerflow/shared");

const GOOGLE_CLIENT_ID = process.env.GOOGLE_CLIENT_ID;
const SESSIONS_TABLE   = process.env.SESSIONS_TABLE || "ledgerflow-sessions";

const googleClient = new OAuth2Client(GOOGLE_CLIENT_ID);
const dynamo = DynamoDBDocumentClient.from(
  new DynamoDBClient({ region: process.env.AWS_REGION || "us-east-1" })
);

exports.handler = async (event) => {
  // CORS preflight
  if (event.requestContext?.http?.method === "OPTIONS") {
    return { statusCode: 200, headers: CORS_HEADERS, body: "" };
  }

  const method = event.requestContext?.http?.method || event.httpMethod;
  const path   = event.rawPath || event.path || "";

  try {
    // ── GET /auth/config ────────────────────────────────────────────────────
    // Returns public config the frontend needs to initialize Google Sign-In
    if (method === "GET" && path.endsWith("/config")) {
      return ok({
        googleClientId: GOOGLE_CLIENT_ID,
        // The frontend redirects here after Google login
        // Using Google's newer Identity Services (one-tap / button flow)
        // No redirect URI needed — token is returned directly to the JS callback
      });
    }

    // ── GET /auth/me ────────────────────────────────────────────────────────
    // Validates Bearer token and returns user profile
    // Called immediately after Google Sign-In succeeds on the frontend
    if (method === "GET" && path.endsWith("/me")) {
      const authHeader = event.headers?.authorization || event.headers?.Authorization || "";
      if (!authHeader.startsWith("Bearer ")) return unauthorized("Missing token");

      const token = authHeader.slice(7);
      const ticket = await googleClient.verifyIdToken({
        idToken: token,
        audience: GOOGLE_CLIENT_ID,
      });
      const payload = ticket.getPayload();
      if (!payload) return unauthorized("Invalid token");

      return ok({
        userId:  payload.sub,
        email:   payload.email,
        name:    payload.name,
        picture: payload.picture,
        domain:  payload.hd || null,   // Google Workspace hosted domain
      });
    }

    // ── POST /auth/logout ───────────────────────────────────────────────────
    // Removes server-side session (best-effort)
    if (method === "POST" && path.endsWith("/logout")) {
      const authHeader = event.headers?.authorization || event.headers?.Authorization || "";
      if (authHeader.startsWith("Bearer ")) {
        const token = authHeader.slice(7);
        try {
          const ticket = await googleClient.verifyIdToken({ idToken: token, audience: GOOGLE_CLIENT_ID });
          const payload = ticket.getPayload();
          if (payload?.sub) {
            await dynamo.send(new DeleteCommand({ TableName: SESSIONS_TABLE, Key: { userId: payload.sub } }));
          }
        } catch {
          // Best-effort; token may already be expired
        }
      }
      return ok({ message: "Logged out" });
    }

    return { statusCode: 404, headers: CORS_HEADERS, body: JSON.stringify({ error: "Not Found" }) };

  } catch (err) {
    console.error("[AUTH]", err);
    return serverError("Auth error", err);
  }
};