Bug: compressed routes do not send Vary: Accept-Encoding #8710

Expected Behaviour

When compression is enabled for a route (@app.get(..., compress=True) or Response(compress=True)), the response body depends on the request's Accept-Encoding, so the response should carry Vary: Accept-Encoding. That tells API Gateway caching, CloudFront and other shared caches to keep gzipped and plain responses apart. This is the same reasoning as the Vary: Origin that CORS responses got in #8638.

Current Behaviour

ResponseBuilder gzips the body and sets Content-Encoding: gzip, but never adds Accept-Encoding to Vary, neither on the compressed response nor on the plain one served to a client without gzip. A cache keyed only on the URL can store the gzipped body and serve it to a client that did not ask for gzip, or keep serving the plain body to everyone.

Code snippet

from aws_lambda_powertools.event_handler import ApiGatewayResolver, Response, content_types

app = ApiGatewayResolver()

@app.get("/report", compress=True)
def report():
    return Response(200, content_types.TEXT_PLAIN, "Foo")

result = app({"path": "/report", "httpMethod": "GET", "headers": {"Accept-Encoding": "gzip"}}, None)
result["multiValueHeaders"]
# {'Content-Type': ['text/plain'], 'Content-Encoding': ['gzip']}   no Vary

Steps to Reproduce

1. Define a route with compress=True.
2. Call it with and without Accept-Encoding: gzip.
3. Neither response has a Vary header.

Possible Solution

When compression is enabled for the route or the response, add Accept-Encoding to Vary, keeping any Vary value the route already set (as the CORS code does for Origin). I can open a PR.

Powertools for AWS Lambda (Python) version

latest (develop, 3.38.0)

AWS Lambda function runtime

3.12

Packaging format used

PyPi

Debugging logs

No response