clip: check Path ops are not mixed with others

Multiple operations Op, such as clip.Path, cannot
be interleaved with other ops. This patch adds a
mechanism to ensure that is the case by starting
multi ops with ops.BeginMulti and ending them with
ops.EndMulti while operations are written to op.Ops
with ops.WriteMulti.
This mechanism is applied to clip.Path.

Fixes: https://todo.sr.ht/~eliasnaur/gio/336
Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

gpu/internal/rendertest/clip_test.go

@@ -227,3 +227,27 @@ func TestPathReuse(t *testing.T) {
 	}, func(r result) {
 	})
 }
+
+func TestPathInterleave(t *testing.T) {
+	t.Run("interleave op in clip.Path", func(t *testing.T) {
+		defer func() {
+			if err := recover(); err == nil {
+				t.Error("expected panic did not occur")
+			}
+		}()
+		ops := new(op.Ops)
+		var path clip.Path
+		path.Begin(ops)
+		path.LineTo(f32.Point{X: 123, Y: 456})
+		paint.ColorOp{}.Add(ops)
+		path.End()
+	})
+	t.Run("use ops after clip.Path", func(t *testing.T) {
+		ops := new(op.Ops)
+		var path clip.Path
+		path.Begin(ops)
+		path.LineTo(f32.Point{X: 123, Y: 456})
+		path.End()
+		paint.ColorOp{}.Add(ops)
+	})
+}

internal/ops/ops.go

@@ -22,6 +22,8 @@ type Ops struct {
 	// nextStateID is the id allocated for the next
 	// StateOp.
 	nextStateID int
+	// multipOp indicates a multi-op such as clip.Path is being added.
+	multipOp bool
 
 	macroStack stack
 	stacks     [5]stack
@@ -192,6 +194,31 @@ func Reset(o *Ops) {
 }
 
 func Write(o *Ops, n int) []byte {
+	if o.multipOp {
+		panic("cannot mix multi ops with single ones")
+	}
+	o.data = append(o.data, make([]byte, n)...)
+	return o.data[len(o.data)-n:]
+}
+
+func BeginMulti(o *Ops) {
+	if o.multipOp {
+		panic("cannot interleave multi ops")
+	}
+	o.multipOp = true
+}
+
+func EndMulti(o *Ops) {
+	if !o.multipOp {
+		panic("cannot end non multi ops")
+	}
+	o.multipOp = false
+}
+
+func WriteMulti(o *Ops, n int) []byte {
+	if !o.multipOp {
+		panic("cannot use multi ops in single ops")
+	}
 	o.data = append(o.data, make([]byte, n)...)
 	return o.data[len(o.data)-n:]
 }

op/clip/clip.go

@@ -126,7 +126,8 @@ func (p *Path) Begin(o *op.Ops) {
 		contour: 1,
 	}
 	p.hash.SetSeed(pathSeed)
-	data := ops.Write(p.ops, ops.TypeAuxLen)
+	ops.BeginMulti(p.ops)
+	data := ops.WriteMulti(p.ops, ops.TypeAuxLen)
 	data[0] = byte(ops.TypeAux)
 }
 
@@ -134,6 +135,7 @@ func (p *Path) Begin(o *op.Ops) {
 func (p *Path) End() PathSpec {
 	p.gap()
 	c := p.macro.Stop()
+	ops.EndMulti(p.ops)
 	return PathSpec{
 		spec:        c,
 		hasSegments: p.hasSegments,
@@ -163,7 +165,7 @@ func (p *Path) gap() {
 	if p.pen != p.start {
 		// A closed contour starts and ends in the same point.
 		// This move creates a gap in the contour, register it.
-		data := ops.Write(p.ops, scene.CommandSize+4)
+		data := ops.WriteMulti(p.ops, scene.CommandSize+4)
 		bo := binary.LittleEndian
 		bo.PutUint32(data[0:], uint32(p.contour))
 		p.cmd(data[4:], scene.Gap(p.pen, p.start))
@@ -183,7 +185,7 @@ func (p *Path) Line(delta f32.Point) {
 
 // LineTo moves the pen to the absolute point specified, recording a line.
 func (p *Path) LineTo(to f32.Point) {
-	data := ops.Write(p.ops, scene.CommandSize+4)
+	data := ops.WriteMulti(p.ops, scene.CommandSize+4)
 	bo := binary.LittleEndian
 	bo.PutUint32(data[0:], uint32(p.contour))
 	p.cmd(data[4:], scene.Line(p.pen, to))
@@ -251,7 +253,7 @@ func (p *Path) Quad(ctrl, to f32.Point) {
 // QuadTo records a quadratic Bézier from the pen to end
 // with the control point ctrl, with absolute coordinates.
 func (p *Path) QuadTo(ctrl, to f32.Point) {
-	data := ops.Write(p.ops, scene.CommandSize+4)
+	data := ops.WriteMulti(p.ops, scene.CommandSize+4)
 	bo := binary.LittleEndian
 	bo.PutUint32(data[0:], uint32(p.contour))
 	p.cmd(data[4:], scene.Quad(p.pen, ctrl, to))
@@ -297,7 +299,7 @@ func (p *Path) CubeTo(ctrl0, ctrl1, to f32.Point) {
 	if ctrl0 == p.pen && ctrl1 == p.pen && to == p.pen {
 		return
 	}
-	data := ops.Write(p.ops, scene.CommandSize+4)
+	data := ops.WriteMulti(p.ops, scene.CommandSize+4)
 	bo := binary.LittleEndian
 	bo.PutUint32(data[0:], uint32(p.contour))
 	p.cmd(data[4:], scene.Cubic(p.pen, ctrl0, ctrl1, to))