Lambda Edge 502 with custom response from viewer response - json

Question

I'm using a URL query string to debug my viewer-request and viewer-response lambda#edge functions by returning the event as JSON to the frontend (FYI so I can check for the presence/absence of certain things via an external monitoring tool).
This works fine with the viewer-request: if I go to https://example.org/?debug_viewer_request_event I get a JSON of the viewer-request event:
import json
def lambda_handler(event, context):
request = event["Records"][0]["cf"]["request"]
if "debug_viewer_request_event" in request["querystring"]:
response = {
"status": "200",
"statusDescription": "OK",
"headers": {
"cache-control": [
{
"key": "Cache-Control",
"value": "no-cache"
}
],
"content-type": [
{
"key": "Content-Type",
"value": "application/json"
}
]
},
"body": json.dumps(event)
}
return response
# rest of viewer-request logic...
Testing with cURL:
curl -i https://example.org/?debug_viewer_request_event
HTTP/2 200
content-type: application/json
content-length: 854
server: CloudFront
date: Mon, 26 Apr 2021 06:05:28 GMT
cache-control: no-cache
x-cache: LambdaGeneratedResponse from cloudfront
via: 1.1 xxxxxxxxxxx.cloudfront.net (CloudFront)
x-amz-cf-pop: AMS50-C1
x-amz-cf-id: pU0ItvQA1-r5v3yR1Dl6Z3VpPW_EuuUCHhnOD60uLhng...
{"Records": [{"cf": {"config": {"distributionDomainName": "xxxxxxx.cloudfront.net", "distributionId": "xxxxxxx", "eventType": "viewer-request", "requestId": "pU0ItvQA1-r5v3yR1Dl6Z3VpPW_EuuUCHhnOD60uLhng...
However when I do the same with the viewer-response I get a 502 error:
the code is the same except debug_viewer_request_event is debug_viewer_response_event
if I don't include the debug query string, the response is 200 OK so I know overall both lambdas are working properly (with the exception of the debug for the viewer-response)
Here is the cURL output:
curl -i https://example.org/?debug_viewer_response_event
HTTP/2 502
content-type: text/html
content-length: 1013
server: CloudFront
date: Mon, 26 Apr 2021 06:07:39 GMT
x-cache: LambdaValidationError from cloudfront
via: 1.1 xxxxxxxxx.cloudfront.net (CloudFront)
x-amz-cf-pop: AMS50-C1
x-amz-cf-id: NqXQ-FFEsIX-fEt8IvlHFTYoQdrZSGPScq1H-KNwVWR0-xxxxxx
The Lambda function result failed validation: The function tried to add, delete, or change a read-only header
If I look at the docs, the list of "Read-only Headers for CloudFront Viewer Response Events" is:
Content-Encoding
Content-Length
Transfer-Encoding
Warning
Via
As far as I can see I'm not directly changing any of these headers, but I'm guessing because I'm modifying the response, headers such as Content-Length are modified
Q: Is there a way to return the viewer-response event as JSON to the frontend for debugging or is it simply not possible due to not being able to change Content-Length?

Answer 1

As far as I can see I'm not directly changing any of these headers,
but I'm guessing because I'm modifying the response, headers such as
Content-Length are modified
I agree, I think your issue is that you are returning the response instead of calling
callback(null, response);
where callback should be the third argument to your lambda handler func:
def lambda_handler(event, context, callback):
Since content-length is not mutable, we should assume (and I checked this is true in practice at least for viewer request functions), cloudfront will generate it for you when you generate a response in the edge function.