Replicate AWS behavior of lambda exposed through API Gateway WebSockets

[VenomAV]

Description

I changed the implementation of WebSocketClients._processEvent in order to use the result of lambda invocation.
If the result contains a body and the websocketClient is still OPEN, the body of the lambda result is sent to the client.

Motivation and Context

The current implementation doesn't behave like an AWS API Gateway with WebSockets. This change replicates the behavior exposed by AWS API Gateway.

How Has This Been Tested?

serverless.yml

service:
  name: test

plugins:
  - serverless-plugin-typescript
  - serverless-offline

provider:
  name: aws
  runtime: nodejs12.x

functions:
  actionHandler:
    handler: src/echo-handler.handler
    events:
      - websocket:
          route: echo
          routeResponseSelectionExpression: $default

src/echo-handler.ts

import { APIGatewayProxyEvent, APIGatewayProxyResult, Callback, Context } from "aws-lambda"

export const handler = async (
    event: APIGatewayProxyEvent,
    context: Context,
    callback: Callback<APIGatewayProxyResult>,
): Promise<APIGatewayProxyResult> => {
    const body = JSON.parse(event.body)
    return {
        statusCode: 200,
        body: JSON.stringify({ message: `echo ${body.message}` }),
    }
}

Run serverless with serverless offline
Use wscat to interact with serverless:

$ wscat -c "ws://localhost:3001"
Connected (press CTRL+C to quit)
> {"action": "echo", "message": "ooooooooooooooooo"}
< {"message":"echo ooooooooooooooooo"}

Second to last line is the message sent to the API Gateway.
Last line is the lambda response which is missing in the current implementation

[kevbot-git]

Hey @VenomAV, I've had the same issue and opened #1064 a while ago... I'm wondering—would you see merit in me marking mine as duplicate, and you perhaps including the error response I added in that PR, in this one? We're keen to see it merged! 😄

[dherault]

1 yo PR with conflicts, please feel free to reopen.

[dnalborczyk]

superseded by: #1301