[react-packager] Fix races

Summary:
A few potential races to fix:

1. Multiple clients maybe racing to delete a zombie socket
2. Servers who should die because other servers are already listening are taking the socket with them (move `process.on('exit'` code to after the server is listening
3. Servers which are redundant should immediatly die

packager/react-packager/src/SocketInterface/SocketServer.js

@@ -32,6 +32,7 @@ class SocketServer {
           options
         );
         resolve(this);
+        process.on('exit', () => fs.unlinkSync(sockPath));
       });
     });
     this._server.on('connection', (sock) => this._handleConnection(sock));
@@ -41,8 +42,6 @@ class SocketServer {
     this._packagerServer = new Server(options);
     this._jobs = 0;
     this._dieEventually();
-
-    process.on('exit', () => fs.unlinkSync(sockPath));
   }
 
   onReady() {
@@ -138,12 +137,17 @@ class SocketServer {
           process.send({ type: 'createdServer' });
         },
         error => {
-          debug('error creating server', error.code);
           if (error.code === 'EADDRINUSE') {
             // Server already listening, this may happen if multiple
             // clients where started in quick succussion (buck).
             process.send({ type: 'createdServer' });
+
+            // Kill this server because some other server with the same
+            // config and socket already started.
+            debug('server already started');
+            setImmediate(() => process.exit());
           } else {
+            debug('error creating server', error.code);
             throw error;
           }
         }

packager/react-packager/src/SocketInterface/index.js

@@ -13,6 +13,7 @@ const SocketClient = require('./SocketClient');
 const SocketServer = require('./SocketServer');
 const _ = require('underscore');
 const crypto = require('crypto');
+const debug = require('debug')('ReactPackager:SocketInterface');
 const fs = require('fs');
 const net = require('net');
 const path = require('path');
@@ -45,7 +46,12 @@ const SocketInterface = {
           resolve(SocketClient.create(sockPath));
         });
         sock.on('error', (e) => {
-          fs.unlinkSync(sockPath);
+          try {
+            debug('deleting socket for not responding', sockPath);
+            fs.unlinkSync(sockPath);
+          } catch (err) {
+            // Another client might have deleted it first.
+          }
           createServer(resolve, reject, options, sockPath);
         });
       } else {