BootProbes: add a websocket probe

This self-test isn't perfect because we're running it from the backend
instead of the frontend. Conceivably the backend might have trouble
resolving its own url. Eventually we should move this test or
something like it to something that executes in the frontend.

app/server/lib/BootProbes.ts

@@ -4,6 +4,7 @@ import { removeTrailingSlash } from 'app/common/gutil';
 import { expressWrap, jsonErrorHandler } from 'app/server/lib/expressWrap';
 import { GristServer } from 'app/server/lib/GristServer';
 import * as express from 'express';
+import WS from 'ws';
 import fetch from 'node-fetch';
 
 /**
@@ -59,6 +60,7 @@ export class BootProbes {
     this._probes.push(_hostHeaderProbe);
     this._probes.push(_sandboxingProbe);
     this._probes.push(_authenticationProbe);
+    this._probes.push(_webSocketsProbe);
     this._probeById = new Map(this._probes.map(p => [p.id, p]));
   }
 }
@@ -105,6 +107,37 @@ const _homeUrlReachableProbe: Probe = {
   }
 };
 
+const _webSocketsProbe: Probe = {
+  id: 'websockets',
+  name: 'Can we open a websocket with the server',
+  apply: async (server, req) => {
+    return new Promise((resolve) => {
+      const url = new URL(server.getHomeUrl(req));
+      url.protocol = (url.protocol === 'https:') ? 'wss:' : 'ws:';
+      const ws = new WS.WebSocket(url.href);
+      const details: Record<string, any> = {
+        url,
+      };
+      ws.on('open', () => {
+        ws.send('Just nod if you can hear me.');
+        resolve({
+          success: true,
+          details,
+        });
+        ws.close();
+      });
+      ws.on('error', (ev) => {
+        details.error = ev.message;
+        resolve({
+          success: false,
+          details,
+        });
+        ws.close();
+      });
+    });
+  }
+};
+
 const _statusCheckProbe: Probe = {
   id: 'health-check',
   name: 'Is an internal health check passing',