Wrong `queryStringParameters` after serializing `AlbTargetGroupRequest`.

[DiscreteTom]

Currently AlbTargetGroupRequest::query_string_parameters is serialized with the default QueryMap serializer, which will serialize the value as a list instead of a string.

aws-lambda-rust-runtime/lambda-events/src/event/alb/mod.rs

Lines 19 to 20 in f69280e

	#[serde(default)]
	pub query_string_parameters: QueryMap,

It should be serialized like in APIGW event, using query_map::serde::aws_api_gateway_v1::serialize_query_string_parameters.

aws-lambda-rust-runtime/lambda-events/src/event/apigw/mod.rs

Lines 34 to 35 in f69280e

	#[serde(serialize_with = "query_map::serde::aws_api_gateway_v1::serialize_query_string_parameters")]
	pub query_string_parameters: QueryMap,

Minimal repro:

#[cfg(test)]
mod tests {
    use aws_lambda_events::alb::{AlbTargetGroupRequest, AlbTargetGroupRequestContext, ElbContext};
    use axum::http::{request::Builder, Method};
    use std::collections::HashMap;

    #[test]
    fn test() {
        let (parts, body) = Builder::new()
            .method(Method::GET)
            .uri("https://example.com/")
            .header("key", "value")
            .body("")
            .unwrap()
            .into_parts();

        let query_string_parameters = HashMap::from([("k".to_string(), "v".to_string())]).into();

        let json = serde_json::to_string(&AlbTargetGroupRequest {
            http_method: parts.method,
            headers: parts.headers,
            path: "/".to_string().into(),
            query_string_parameters,
            body: body.to_string().into(),
            is_base64_encoded: false,
            request_context: AlbTargetGroupRequestContext {
                elb: ElbContext { target_group_arn: None },
            },
            multi_value_headers: Default::default(),
            multi_value_query_string_parameters: Default::default(),
        })
        .unwrap();

        println!("{}", json);
    }
}

Got output:

{"httpMethod":"GET","path":"/","queryStringParameters":{"k":["v"]},"multiValueQueryStringParameters":{},"headers":{"key":"value"},"multiValueHeaders":{},"requestContext":{"elb":{"targetGroupArn":null}},"isBase64Encoded":false,"body":""}

Expected: the value of "k" in queryStringParameters should be a string "v" instead of a list ["v"].

[bnusunny]

This is actually the correct behavior. You can verify this by using a real alb to trigger a Lambda function.

[DiscreteTom]

Tested with a real ALB (multi-value-headers disabled) and a nodejs lambda function to print the event using console.log(JSON.stringify(event));. Here are the results:

Case 1:

curl 1234567890.us-east-1.elb.amazonaws.com/lambda?k=v

Output:

{
    "requestContext": {
        "elb": {
            "targetGroupArn": "arn:aws:elasticloadbalancing:us-east-1:1234567890:targetgroup/lambda-print-request/1234567890"
        }
    },
    "httpMethod": "GET",
    "path": "/lambda",
    "queryStringParameters": {
        "k": "v"
    },
    "headers": {
        "accept": "*/*",
        "host": "1234567890.us-east-1.elb.amazonaws.com",
        "user-agent": "curl/8.9.1",
        "x-amzn-trace-id": "Root=1-1234567890-1234567890",
        "x-forwarded-for": "1.1.1.1",
        "x-forwarded-port": "80",
        "x-forwarded-proto": "http"
    },
    "body": "",
    "isBase64Encoded": false
}

Case 2:

curl "1234567890.us-east-1.elb.amazonaws.com/lambda?k=v&k=v2"

Output:

{
    "requestContext": {
        "elb": {
            "targetGroupArn": "arn:aws:elasticloadbalancing:us-east-1:1234567890:targetgroup/lambda-print-request/1234567890"
        }
    },
    "httpMethod": "GET",
    "path": "/lambda",
    "queryStringParameters": {
        "k": "v2"
    },
    "headers": {
        "accept": "*/*",
        "host": "1234567890.us-east-1.elb.amazonaws.com",
        "user-agent": "curl/8.9.1",
        "x-amzn-trace-id": "Root=1-1234567890-1234567890",
        "x-forwarded-for": "1.1.1.1",
        "x-forwarded-port": "80",
        "x-forwarded-proto": "http"
    },
    "body": "",
    "isBase64Encoded": false
}

The behavior follows the documentation:

With the default format, the load balancer uses the last value sent by the client and sends you an event that includes query string parameters using queryStringParameters.