restorable: Bug fix: Guard images by mutex

The map must be guarded by mutex since this can be accessed by
multiple goroutines, including finalizer goroutine.

Fixes #820

internal/restorable/images.go

@@ -16,6 +16,7 @@ package restorable
 
 import (
 	"image"
+	"sync"
 
 	"github.com/hajimehoshi/ebiten/internal/graphicscommand"
 )
@@ -42,6 +43,8 @@ func EnableRestoringForTesting() {
 type images struct {
 	images     map[*Image]struct{}
 	lastTarget *Image
+
+	m sync.Mutex
 }
 
 // theImages represents the images for the current process.
@@ -72,8 +75,15 @@ func Restore() error {
 }
 
 func Images() []image.Image {
+	return theImages.images()
+}
+
+func (i *images) images() []image.Image {
+	i.m.Lock()
+	defer i.m.Unlock()
+
 	var imgs []image.Image
-	for img := range theImages.images {
+	for img := range i.images {
 		if img.volatile {
 			continue
 		}
@@ -103,21 +113,27 @@ func Images() []image.Image {
 
 // add adds img to the images.
 func (i *images) add(img *Image) {
+	i.m.Lock()
 	i.images[img] = struct{}{}
+	i.m.Unlock()
 }
 
 // remove removes img from the images.
 func (i *images) remove(img *Image) {
+	i.m.Lock()
 	i.makeStaleIfDependingOnImpl(img)
 	delete(i.images, img)
+	i.m.Unlock()
 }
 
 // resolveStaleImages resolves stale images.
 func (i *images) resolveStaleImages() {
+	i.m.Lock()
 	i.lastTarget = nil
 	for img := range i.images {
 		img.resolveStale()
 	}
+	i.m.Unlock()
 }
 
 // makeStaleIfDependingOn makes all the images stale that depend on target.
@@ -125,8 +141,10 @@ func (i *images) resolveStaleImages() {
 // When target is changed, all images depending on target can't be restored with target.
 // makeStaleIfDependingOn is called in such situation.
 func (i *images) makeStaleIfDependingOn(target *Image) {
+	i.m.Lock()
 	// Avoid defer for performance
 	i.makeStaleIfDependingOnImpl(target)
+	i.m.Unlock()
 }
 
 func (i *images) makeStaleIfDependingOnImpl(target *Image) {
@@ -150,6 +168,9 @@ func (i *images) restore() error {
 		panic("restorable: restore cannot be called when restoring is disabled")
 	}
 
+	i.m.Lock()
+	defer i.m.Unlock()
+
 	// Dispose image explicitly
 	for img := range i.images {
 		img.image.Dispose()