Attach connection handler earlier

Summary:
This change attaches our event handlers as soon as the ws is created.

As a consequence, we need to wait until the server has created any necessary instances required to process incoming requests.

To achieve this, I created a type called `Lazy`.

This type wraps around a value and a promise to that value. Callers can check if the value is set. If not, callers can wait for it.

Ultimately, the value can be set outside of the promise itself.

Reviewed By: passy

Differential Revision: D37284939

fbshipit-source-id: 17dec548d7155a3d65440c9584cec07cbb826c37

desktop/flipper-server/src/index.tsx

@@ -16,11 +16,7 @@ import fs from 'fs-extra';
 import yargs from 'yargs';
 import open from 'open';
 import {initCompanionEnv} from 'flipper-server-companion';
-import {
-  attachSocketServer,
-  startFlipperServer,
-  startServer,
-} from 'flipper-server-core';
+import {startFlipperServer, startServer} from 'flipper-server-core';
 import {isTest} from 'flipper-common';
 
 const argv = yargs
@@ -97,7 +93,7 @@ async function start() {
     console.error('Failed to load keytar:', e);
   }
 
-  const {app, server, socket} = await startServer({
+  const {app, server, socket, readyForIncomingConnections} = await startServer({
     port: argv.port,
     staticDir,
     entry: 'index.web.dev.html',
@@ -123,7 +119,7 @@ async function start() {
   if (argv.bundler) {
     await attachDevServer(app, server, socket, rootDir);
   }
-  attachSocketServer(flipperServer, socket, companionEnv);
+  await readyForIncomingConnections(flipperServer, companionEnv);
 }
 
 start()