Optimistic session creation (#3352)

* Optimistically creating a session when a window is created to improve launch time

* Fix linting issues

app/ui/window.js

@@ -87,6 +87,34 @@ module.exports = class Window {
         }
       });
 
+      function createSession(options) {
+        const uid = uuid.v4();
+        const session = new Session(Object.assign({}, options, {uid}));
+        sessions.set(uid, session);
+        return {uid, session};
+      }
+
+      // Optimistically create the initial session so that when the window sends
+      // the first "new" IPC message, there's a session already warmed up.
+      function createInitialSession() {
+        let {session, uid} = createSession({});
+        const initialEvents = [];
+        const handleData = data => initialEvents.push(['session data', uid + data]);
+        const handleExit = () => initialEvents.push(['session exit']);
+        session.on('data', handleData);
+        session.on('exit', handleExit);
+
+        function flushEvents() {
+          for (let args of initialEvents) {
+            rpc.emit(...args);
+          }
+          session.removeListener('data', handleData);
+          session.removeListener('exit', handleExit);
+        }
+        return {session, uid, flushEvents};
+      }
+      let initialSession = createInitialSession();
+
       rpc.on('new', options => {
         let cwd = null;
         if (workingDirectory) {
@@ -108,29 +136,32 @@ module.exports = class Window {
           options
         );
 
-        const initSession = (opts, fn_) => {
-          fn_(uuid.v4(), new Session(opts));
-        };
+        const {uid, session} = initialSession || createSession();
 
-        initSession(sessionOpts, (uid, session) => {
-          sessions.set(uid, session);
-          rpc.emit('session add', {
-            rows: sessionOpts.rows,
-            cols: sessionOpts.cols,
-            uid,
-            splitDirection: sessionOpts.splitDirection,
-            shell: session.shell,
-            pid: session.pty.pid
-          });
+        sessions.set(uid, session);
+        rpc.emit('session add', {
+          rows: sessionOpts.rows,
+          cols: sessionOpts.cols,
+          uid,
+          splitDirection: sessionOpts.splitDirection,
+          shell: session.shell,
+          pid: session.pty.pid
+        });
 
-          session.on('data', data => {
-            rpc.emit('session data', uid + data);
-          });
+        // If this is the initial session, flush any events that might have
+        // occurred while the window was initializing
+        if (initialSession) {
+          initialSession.flushEvents();
+          initialSession = null;
+        }
 
-          session.on('exit', () => {
-            rpc.emit('session exit', {uid});
-            sessions.delete(uid);
-          });
+        session.on('data', data => {
+          rpc.emit('session data', uid + data);
+        });
+
+        session.on('exit', () => {
+          rpc.emit('session exit', {uid});
+          sessions.delete(uid);
         });
       });
       rpc.on('exit', ({uid}) => {