Empty response on Hasura auth hook using AWS Lambda - amazon-web-services

Question

I got some troubles configuring an Hasura auth hook using a Lambda. I need such a function as I am storing my JWT token in an HTTP-only cookie, for security reasons.
I'm using a serverless function which returns a correct response (either when testing a curl request directly, or even when logging lambda):
{
"statusCode":200,
"body":"{\"X-Hasura-User-Id\":\"74d3bfa9-0983-4f09-be02-6a36888b382e\",\"X-Hasura-Role\":\"user\"}"
}
Yet, Hasura hook doesn't seem to recognize the response:
{
"type": "webhook-log",
"timestamp": "2020-02-07T10:27:34.844+0000",
"level": "info",
"detail": {
"response": null,
"url": "http://serverless:3000/auth",
"method": "GET",
"http_error": null,
"status_code": 200
}
}
These two lines of logs are adjacent in my logs. I just reformatted them a little bit to ease reading.
My lambda code looks like:
export const handler = async (event) => {
const cookies = getCookiesFromHeader(event.headers);
const { access_token: accessToken } = cookies;
let decodedToken = null;
try {
const cert = fs.readFileSync("./src/pem/dev.pem");
decodedToken = jwt.verify(accessToken, cert);
} catch (err) {
console.error(err);
return {
statusCode: 401,
};
}
const hasuraClaims = decodedToken['https://hasura.io/jwt/claims'];
return {
statusCode: 200,
body: JSON.stringify({
"X-Hasura-User-Id": hasuraClaims['x-hasura-user-id'],
"X-Hasura-Role": hasuraClaims['x-hasura-default-role']
})
}
}
Any idea on what is going on? Note that I'm using serverless offline, in case of. :)

Answer 1

In AWS Lambda, the spec requires the response body to be stringified and the actual response will be a parsed JSON object which is what Hasura will receive from the auth webhook.
When you are using serverless-offline, the response body is returned as a String (since JSON.stringify is used) without getting parsed. A simple curl will give you the difference.
The above code will work on Lambda but not on local development using serverless-offline. You will have to use the event object to see if isOffline is true and return JSON directly and if not return the stringified version.
Example code:
if(event.isOffline) {
// make it work with serverless-offline
return { "x-hasura-role": "user" ....};
} else {
// make it work with lambda
return { statusCode: 200, body: JSON.stringify({"x-hasura-role": "user"}) };
}
Official example in the serverless-offline repo along with error handling.
Related issues:
https://github.com/dherault/serverless-offline/issues/530
https://github.com/dherault/serverless-offline/issues/488