Querystrings that have percent encodings get double encoded for Lambda ALB Events in lambda-http

[heathprovost]

This was an issue originally raised here: awslabs/aws-lambda-web-adapter#99. Since the root issue is not in that project but this one I'm moving discussion here.

Problem: Whenever a url that contains a querystring with percent encoded values is passed through lambda-http, it gets double encoded in the newly generated http request if the original event source was an ALB. This is because the Lambda Event for ALBs does not automatically decode querystrings, whereas the same event for API Gateways do:

https://serverless-training.com/articles/api-gateway-vs-application-load-balancer-technical-details/ (about half way down the page)

Note that the keys and values of evt.queryStringParameters will be:

• already URL decoded for you on API Gateway. e.g. ?foo%5Ba%5D=foo%5B will be { 'foo[a]': [ 'foo[' ] }
• but not URL decoded on ALB. e.g. ?foo%5Ba%5D=foo%5B will be { 'foo%5Ba%5D': [ 'foo%5B' ] }

Discussion on the original issue lead to the thought that this difference should be handled here (but currently is not):

aws-lambda-rust-runtime/lambda-http/src/request.rs

Line 209 in fd2ea23

Some((&alb.multi_value_query_string_parameters, &alb.query_string_parameters)),

Here is an example of a url that triggers this issue:

https://test.com/?originalQueryString=%3FshowAll%3Dtrue

When passed from an API Gateway the url passes through correctly:

https://test.com/?originalQueryString=%3FshowAll%3Dtrue

But if the original source was an ALB you get this instead:

https://test.com/?originalQueryString=%253FshowAll%253Dtrue

Thanks in advance.

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