URL query is not decoded when using ALB

[andrew0]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

all

Plugin version

2.0.2

Node.js version

14

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

not applicable

Description

When using Lambda as a target for an ALB, the queryStringParameters in the Lambda event is not decoded, unlike with API Gateway. This isn't really well documented, but there's a small section in the docs that mentions this (not really sure why they put it under the "multi-value headers" section, but it applies to the queryStringParameters and multiValueQueryStringParameters properties):

If the query parameters are URL-encoded, the load balancer does not decode them. You must decode them in your Lambda function.

Steps to Reproduce

This test fails:

test('GET with encoded query values', async (t) => {
  t.plan(1)

  const app = fastify()
  app.get('/test', async (request, reply) => {
    reply.send(request.query)
  })
  const proxy = awsLambdaFastify(app)

  const ret = await proxy({
    requestContext: {
      elb: {
        targetGroupArn: 'xxx'
      }
    },
    httpMethod: 'GET',
    path: '/test',
    queryStringParameters: {
      q: 'foo%3Fbar'
    }
  }))
  t.equal(ret.body, '{"q":"foo?bar"}')
})

Expected Behavior

When the Lambda is invoked by ALB (detectable by presence of requestContext.elb object on event), the values for queryStringParameter and multiValueQueryStringParameters should be decoded before being passed to fastify.

[adrai]

Would you like to send a Pull Request to address this? Remember to add unit tests.

[adrai]

//cc @danielsan