internal/graphicsdriver/metal: Do not retain MTLCommandBuffer for MTLBuffer

Before this change, a command buffer is retained indirectly by
a buffer, and this might extend the life of drawable unexpectedly.

This change stops using command buffers as a key of the buffers pool,
and use a counter increated by nextDrawable calls.

Updates #1196

internal/graphicsdriver/metal/graphics.go

@@ -317,8 +317,7 @@ type Graphics struct {
 
 	screenDrawable ca.MetalDrawable
 
-	buffers       map[mtl.CommandBuffer][]mtl.Buffer
-	unusedBuffers map[mtl.Buffer]struct{}
+	buffers map[mtl.Buffer]int
 
 	lastDstTexture  mtl.Texture
 	lastStencilMode stencilMode
@@ -388,54 +387,40 @@ func pow2(x uintptr) uintptr {
 	return p2
 }
 
-func (g *Graphics) availableBuffer(length uintptr) mtl.Buffer {
-	if g.cb == (mtl.CommandBuffer{}) {
-		g.cb = g.cq.MakeCommandBuffer()
+func (g *Graphics) ageBuffers() {
+	for b, age := range g.buffers {
+		if age <= maximumDrawableCount {
+			g.buffers[b]++
+		}
+	}
 }
 
+func (g *Graphics) availableBuffer(length uintptr) mtl.Buffer {
 	var newBuf mtl.Buffer
 
-	for cb, bs := range g.buffers {
-		// If the command buffer still lives, the buffer must not be updated.
-		// TODO: Handle an error?
-		if cb.Status() != mtl.CommandBufferStatusCompleted {
+	var availBufs []mtl.Buffer
+	for b, age := range g.buffers {
+		// If the buffer is too young, its command buffer might still live.
+		// The buffer must not be updated in this case.
+		if age <= maximumDrawableCount {
 			continue
 		}
 
-		for _, b := range bs {
 		if newBuf == (mtl.Buffer{}) && b.Length() >= length {
 			newBuf = b
 			continue
 		}
-			if g.unusedBuffers == nil {
-				g.unusedBuffers = map[mtl.Buffer]struct{}{}
-			}
-			g.unusedBuffers[b] = struct{}{}
-		}
-		delete(g.buffers, cb)
-		cb.Release()
-	}
-
-	for b := range g.unusedBuffers {
-		if b.Length() >= length {
-			newBuf = b
-			delete(g.unusedBuffers, b)
-			break
-		}
+		availBufs = append(availBufs, b)
 	}
 
 	// GC unused buffers.
 	const maxUnusedBuffers = 10
-	if len(g.unusedBuffers) > maxUnusedBuffers {
-		bufs := make([]mtl.Buffer, 0, len(g.unusedBuffers))
-		for b := range g.unusedBuffers {
-			bufs = append(bufs, b)
-		}
-		sort.Slice(bufs, func(a, b int) bool {
-			return bufs[a].Length() > bufs[b].Length()
+	if len(availBufs) > maxUnusedBuffers {
+		sort.Slice(availBufs, func(a, b int) bool {
+			return availBufs[a].Length() > availBufs[b].Length()
 		})
-		for _, b := range bufs[maxUnusedBuffers:] {
-			delete(g.unusedBuffers, b)
+		for _, b := range availBufs[maxUnusedBuffers:] {
+			delete(g.buffers, b)
 			b.Release()
 		}
 	}
@@ -443,14 +428,10 @@ func (g *Graphics) availableBuffer(length uintptr) mtl.Buffer {
 	if newBuf == (mtl.Buffer{}) {
 		newBuf = g.view.getMTLDevice().MakeBufferWithLength(pow2(length), resourceStorageMode)
 	}
-
 	if g.buffers == nil {
-		g.buffers = map[mtl.CommandBuffer][]mtl.Buffer{}
+		g.buffers = map[mtl.Buffer]int{}
 	}
-	if _, ok := g.buffers[g.cb]; !ok {
-		g.cb.Retain()
-	}
-	g.buffers[g.cb] = append(g.buffers[g.cb], newBuf)
+	g.buffers[newBuf] = 0
 	return newBuf
 }
 
@@ -1229,6 +1210,10 @@ func (i *Image) mtlTexture() mtl.Texture {
 				return mtl.Texture{}
 			}
 			g.screenDrawable = drawable
+
+			// nextDrawable blocks until the new drawable is available.
+			// If nextDrawable returns a valid drawable, this means that at least one drawable is already processed.
+			g.ageBuffers()
 		}
 		return g.screenDrawable.Texture()
 	}

internal/graphicsdriver/metal/view.go

@@ -24,6 +24,8 @@ import (
 	"github.com/hajimehoshi/ebiten/v2/internal/graphicsdriver/metal/mtl"
 )
 
+const maximumDrawableCount = 3
+
 type view struct {
 	window uintptr
 	uiview uintptr
@@ -69,7 +71,7 @@ func (v *view) reset() error {
 	// MTLPixelFormatBGRA8Unorm_sRGB, MTLPixelFormatRGBA16Float, MTLPixelFormatBGRA10_XR, or
 	// MTLPixelFormatBGRA10_XR_sRGB.
 	v.ml.SetPixelFormat(mtl.PixelFormatBGRA8UNorm)
-	v.ml.SetMaximumDrawableCount(3)
+	v.ml.SetMaximumDrawableCount(maximumDrawableCount)
 
 	// The vsync state might be reset. Set the state again (#1364).
 	v.ml.SetDisplaySyncEnabled(v.vsync)