[hoangchunghien]

Problem

A request header that appears more than once reaches the handler with only its last value when the event comes from API Gateway REST (v1), an ALB with multi-value headers enabled, or an API Gateway WebSocket.

These events carry a repeated header in both maps: headers holds the last value and multiValueHeaders holds all of them. lambda-http/src/request.rs builds the request headers like this:

let mut headers = ag.multi_value_headers;
headers.extend(ag.headers);

HeaderMap::extend replaces the values of keys that are already present, so the single-value copy overwrites the complete list. The same two lines appear in into_proxy_request, into_alb_request and into_websocket_request.

Fix

A small merge_headers helper keeps the multi-value entries, and takes a single-value entry only for a name the multi-value map does not have. Events that send only one of the two maps behave exactly as before.

Tests

Two fixtures copy the existing multi-value ones and add a header present in both maps ("X-Repeated": "second" and ["first", "second"]). Two tests assert that both values arrive:

• keeps_every_value_of_a_header_repeated_in_apigw_multi_value_headers
• keeps_every_value_of_a_header_repeated_in_alb_multi_value_headers

On main (7f89224) both fail with left: ["second"], right: ["first", "second"].

cargo test -p lambda_http --all-features            # all pass, including the 2 new tests
cargo clippy --workspace --all-features -- -D warnings
cargo +nightly fmt --all -- --check

(lambda-integration-tests needs TEST_ENDPOINT and SECRET_TOKEN, so I did not run it.)

I also checked it end to end. I found this through aws-lambda-web-adapter, which uses lambda_http, running in the Lambda Runtime Interface Emulator in front of an app that echoes its request headers. A REST API event with x-m: 1 and x-m: 2 reached the app as ['2'] with lambda_http 1.3.0, and as ['1', '2'] with this branch patched in. The same held for an ALB event.