Add ping/pong functionality for websockets

[mrwillis]

It is often the case that Polygon Edge will sit behind a load balancer, reverse proxy, or other similar upstream infrastructure including NATs and firewalls. In this case, to protect the server, upstream infrastructure will usually have some sort of idle or network timeouts. This helps prevent the downstream server from hitting connection limits and ultimately protects the origin.

For WebSockets, since connections are long lived, they can often become stale if they are just using eth_subscribe for example. The solution to this without messing with the network level / proxy level parameters for idle timeouts and such is usually to have a client-initiated ping-pong mechanism. At the very simplest level, the server would respond with "pong" every time the client sends "ping"

Edge currently doesn't have this functionality, but it would be useful to help edge nodes stay safe in case they are handling large amounts of WebSocket connections.

Examples

server-side managed (more advanced): https://docs.deribit.com/#public-set_heartbeat
simple: Pure client side:

import { Networkish } from "@ethersproject/providers";
import { WebSocketLike } from "@ethersproject/providers/lib/websocket-provider";
import { WebSocketProvider } from "@ethersproject/providers";

const KEEP_ALIVE_CHECK_INTERVAL = 10000;

export class PingWebSocketProvider extends WebSocketProvider {
  public keepAliveInterval: number | null = null;

  constructor(url: string | WebSocketLike, network?: Networkish) {
    super(url, network);
    this.websocket.onopen = () => {
      this.keepAliveInterval = setInterval(() => {
        this.websocket.send(JSON.stringify({
          jsonrpc: "2.0",
          method: "eth_chainId",
          params: [],
          id: 1
        })); // supposed to be the canonical "ping" message
      }, KEEP_ALIVE_CHECK_INTERVAL) as any;
    };
    // Add functionality to support pong 
    this._websocket.onclose = () => {
      if (this.keepAliveInterval) {
        clearInterval(this.keepAliveInterval);
        this.keepAliveInterval = null;
      }
    };
  }
}