internal/ui: bug fix: setRunning(true) must be called after the main thread is set

Some functions like ebiten.SetCursorMode use `running` to detect
whether the game starts or not. If the game starts, the main thread
must exist, but there was a timing when `running` was true but the
main thread didn't exist.

This change fixes this issue by changing the timing to call
`setRunning(true)` after the main thread initialization and before
`initOnMainThread`. `initOnMainThread` assumes that `running` is
true.

Closes #2742

internal/processtest/testdata/issue2742.go

@@ -0,0 +1,57 @@
+// Copyright 2023 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build ignore
+
+package main
+
+import (
+	"time"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+type Game struct {
+	ch <-chan struct{}
+}
+
+func (g *Game) Update() error {
+	select {
+	case <-g.ch:
+		return ebiten.Termination
+	default:
+	}
+	return nil
+}
+
+func (g *Game) Draw(screen *ebiten.Image) {
+}
+
+func (g *Game) Layout(outsideWidth, outsideHeight int) (screenWidth, screenHeight int) {
+	return outsideWidth, outsideHeight
+}
+
+func main() {
+	ch := make(chan struct{})
+	go func() {
+		for i := 0; i < 100; i++ {
+			ebiten.SetCursorMode(ebiten.CursorModeHidden)
+			time.Sleep(time.Millisecond)
+		}
+		close(ch)
+	}()
+	if err := ebiten.RunGame(&Game{ch: ch}); err != nil {
+		panic(err)
+	}
+}

internal/ui/run_glfw_notsinglethread.go

@@ -28,6 +28,13 @@ import (
 func (u *userInterfaceImpl) Run(game Game, options *RunOptions) error {
 	u.context = newContext(game)
 
+	u.mainThread = thread.NewOSThread()
+	u.renderThread = thread.NewOSThread()
+	graphicscommand.SetRenderThread(u.renderThread)
+
+	// Set the running state true after the main thread is set, and before initOnMainThread is called (#2742).
+	// TODO: As the existance of the main thread is the same as the value of `running`, this is redundant.
+	// Make `mainThread` atomic and remove `running` if possible.
 	u.setRunning(true)
 	defer u.setRunning(false)
 
@@ -35,10 +42,6 @@ func (u *userInterfaceImpl) Run(game Game, options *RunOptions) error {
 		return err
 	}
 
-	u.mainThread = thread.NewOSThread()
-	u.renderThread = thread.NewOSThread()
-	graphicscommand.SetRenderThread(u.renderThread)
-
 	ctx, cancel := stdcontext.WithCancel(stdcontext.Background())
 	defer cancel()