app_server: fix race condition in alpha mask cloning

* Add a lock which is acquired when reattaching/regenerating masks,
  and also acquired for a cached mask before making a clone of it,
  to prevent the clone from having an inconsistent state in
  concurrent edge cases.

* Maybe fixes #12469

src/servers/app/drawing/AlphaMask.cpp

@@ -22,6 +22,8 @@
 #include "Shape.h"
 #include "ShapePrivate.h"
 
+#include <AutoLocker.h>
+
 
 // #pragma mark - AlphaMask
 
@@ -103,6 +105,8 @@ AlphaMask::~AlphaMask()
 IntPoint
 AlphaMask::SetCanvasGeometry(IntPoint origin, IntRect bounds)
 {
+	AutoLocker<BLocker> locker(fLock);
+
 	if (origin == fCanvasOrigin && bounds.Width() == fCanvasBounds.Width()
 		&& bounds.Height() == fCanvasBounds.Height())
 		return fCanvasOrigin;
@@ -159,6 +163,8 @@ AlphaMask::_CreateTemporaryBitmap(BRect bounds) const
 void
 AlphaMask::_Generate()
 {
+	AutoLocker<BLocker> locker(fLock);
+
 	ServerBitmap* const bitmap = _RenderSource(fCanvasBounds);
 	BReference<ServerBitmap> bitmapRef(bitmap, true);
 	if (bitmap == NULL) {
@@ -484,6 +490,7 @@ ShapeAlphaMask::Create(AlphaMask* previousMask, const shape_data& shape,
 		// TODO: don't make a new mask if the cache entry has no drawstate
 		// using it anymore, because then we ca just immediately reuse it
 		AlphaMask* cachedMask = mask;
+		AutoLocker<BLocker> locker(mask->fLock);
 		mask = new(std::nothrow) ShapeAlphaMask(previousMask, mask);
 		cachedMask->ReleaseReference();
 	}

src/servers/app/drawing/AlphaMask.h

@@ -15,6 +15,8 @@
 #include "drawing/Painter/defines.h"
 #include "IntRect.h"
 
+#include <Locker.h>
+
 
 class BShape;
 class ServerBitmap;
@@ -63,6 +65,7 @@ protected:
 			BReference<AlphaMask> fPreviousMask;
 			IntRect				fBounds;
 			bool				fClippedToCanvas;
+			BLocker				fLock;
 
 private:
 	friend class AlphaMaskCache;