Fix T61759: Dynamic paint smudge crash

The issue was caused by mesh needed for dynamic paint brush being
stored in the modifier. That make it to be freed and set to NULL
when running copy-on-write.

Solved by moving the mesh to be stored in modifier's runtime data.

source/blender/blenkernel/BKE_dynamicpaint.h

@@ -22,6 +22,7 @@
  */
 
 struct Depsgraph;
+struct DynamicPaintRuntime;
 struct DynamicPaintCanvasSettings;
 struct DynamicPaintModifierData;
 struct Main;
@@ -63,6 +64,7 @@ struct Mesh *dynamicPaint_Modifier_do(
         struct DynamicPaintModifierData *pmd, struct Depsgraph *depsgraph, struct Scene *scene,
         struct Object *ob, struct Mesh *me);
 void dynamicPaint_Modifier_free(struct DynamicPaintModifierData *pmd);
+void dynamicPaint_Modifier_free_runtime(struct DynamicPaintRuntime *runtime);
 void dynamicPaint_Modifier_copy(const struct DynamicPaintModifierData *pmd,
                                 struct DynamicPaintModifierData *tsmd,
                                 int flag);

source/blender/blenkernel/intern/dynamicpaint.c

@@ -231,6 +231,50 @@ typedef struct PaintAdjData {
 	int total_border; /* size of border */
 } PaintAdjData;
 
+/************************* Runtime evaluation store ***************************/
+
+void dynamicPaint_Modifier_free_runtime(DynamicPaintRuntime *runtime_data)
+{
+	if (runtime_data == NULL) {
+		return;
+	}
+	if (runtime_data->canvas_mesh) {
+		BKE_id_free(NULL, runtime_data->canvas_mesh);
+	}
+	if (runtime_data->brush_mesh) {
+		BKE_id_free(NULL, runtime_data->brush_mesh);
+	}
+	MEM_freeN(runtime_data);
+}
+
+static DynamicPaintRuntime *dynamicPaint_Modifier_runtime_ensure(
+        DynamicPaintModifierData *pmd)
+{
+	if (pmd->modifier.runtime == NULL) {
+		pmd->modifier.runtime = MEM_callocN(
+		        sizeof(DynamicPaintRuntime), "dynamic paint runtime");
+	}
+	return (DynamicPaintRuntime *)pmd->modifier.runtime;
+}
+
+static Mesh *dynamicPaint_canvas_mesh_get(DynamicPaintCanvasSettings *canvas)
+{
+	if (canvas->pmd->modifier.runtime == NULL) {
+		return NULL;
+	}
+	DynamicPaintRuntime *runtime_data = (DynamicPaintRuntime *)canvas->pmd->modifier.runtime;
+	return runtime_data->canvas_mesh;
+}
+
+static Mesh *dynamicPaint_brush_mesh_get(DynamicPaintBrushSettings *brush)
+{
+	if (brush->pmd->modifier.runtime == NULL) {
+		return NULL;
+	}
+	DynamicPaintRuntime *runtime_data = (DynamicPaintRuntime *)brush->pmd->modifier.runtime;
+	return runtime_data->brush_mesh;
+}
+
 /***************************** General Utils ******************************/
 
 /* Set canvas error string to display at the bake report */
@@ -249,7 +293,8 @@ static int dynamicPaint_surfaceNumOfPoints(DynamicPaintSurface *surface)
 		return 0; /* not supported atm */
 	}
 	else if (surface->format == MOD_DPAINT_SURFACE_F_VERTEX) {
-		return (surface->canvas->mesh) ? surface->canvas->mesh->totvert : 0;
+		const Mesh *canvas_mesh = dynamicPaint_canvas_mesh_get(surface->canvas);
+		return (canvas_mesh) ? canvas_mesh->totvert : 0;
 	}
 
 	return 0;
@@ -812,11 +857,6 @@ static void surfaceGenerateGrid(struct DynamicPaintSurface *surface)
 void dynamicPaint_freeBrush(struct DynamicPaintModifierData *pmd)
 {
 	if (pmd->brush) {
-		if (pmd->brush->mesh) {
-			BKE_id_free(NULL, pmd->brush->mesh);
-		}
-		pmd->brush->mesh = NULL;
-
 		if (pmd->brush->paint_ramp)
 			MEM_freeN(pmd->brush->paint_ramp);
 		if (pmd->brush->vel_ramp)
@@ -945,27 +985,22 @@ void dynamicPaint_freeCanvas(DynamicPaintModifierData *pmd)
 			surface = next_surface;
 		}
 
-		/* free mesh copy */
-		if (pmd->canvas->mesh) {
-			BKE_id_free(NULL, pmd->canvas->mesh);
-		}
-		pmd->canvas->mesh = NULL;
-
 		MEM_freeN(pmd->canvas);
 		pmd->canvas = NULL;
 	}
 }
 
 /* Free whole dp modifier */
-void dynamicPaint_Modifier_free(struct DynamicPaintModifierData *pmd)
+void dynamicPaint_Modifier_free(DynamicPaintModifierData *pmd)
 {
-	if (pmd) {
-		dynamicPaint_freeCanvas(pmd);
-		dynamicPaint_freeBrush(pmd);
+	if (pmd == NULL) {
+		return;
 	}
+	dynamicPaint_freeCanvas(pmd);
+	dynamicPaint_freeBrush(pmd);
+	dynamicPaint_Modifier_free_runtime(pmd->modifier.runtime);
 }
 
-
 /***************************** Initialize and reset ******************************/
 
 /*
@@ -1061,7 +1096,6 @@ bool dynamicPaint_createType(struct DynamicPaintModifierData *pmd, int type, str
 			if (!canvas)
 				return false;
 			canvas->pmd = pmd;
-			canvas->mesh = NULL;
 
 			/* Create one surface */
 			if (!dynamicPaint_createNewSurface(canvas, scene))
@@ -1101,8 +1135,6 @@ bool dynamicPaint_createType(struct DynamicPaintModifierData *pmd, int type, str
 			brush->smudge_strength = 0.3f;
 			brush->max_velocity = 1.0f;
 
-			brush->mesh = NULL;
-
 			/* Paint proximity falloff colorramp. */
 			{
 				CBData *ramp;
@@ -1299,7 +1331,7 @@ static bool surface_usesAdjData(DynamicPaintSurface *surface)
 static void dynamicPaint_initAdjacencyData(DynamicPaintSurface *surface, const bool force_init)
 {
 	PaintSurfaceData *sData = surface->data;
-	Mesh *mesh = surface->canvas->mesh;
+	Mesh *mesh = dynamicPaint_canvas_mesh_get(surface->canvas);
 	PaintAdjData *ad;
 	int *temp_data;
 	int neigh_points = 0;
@@ -1533,7 +1565,7 @@ static void dynamicPaint_setInitialColor(const Scene *scene, DynamicPaintSurface
 {
 	PaintSurfaceData *sData = surface->data;
 	PaintPoint *pPoint = (PaintPoint *)sData->type_data;
-	Mesh *mesh = surface->canvas->mesh;
+	Mesh *mesh = dynamicPaint_canvas_mesh_get(surface->canvas);
 	int i;
 	const bool scene_color_manage = BKE_scene_check_color_management_enabled(scene);
 
@@ -2032,10 +2064,11 @@ static Mesh *dynamicPaint_Modifier_apply(
 	}
 	/* make a copy of mesh to use as brush data */
 	if (pmd->brush) {
-		if (pmd->brush->mesh) {
-			BKE_id_free(NULL, pmd->brush->mesh);
+		DynamicPaintRuntime *runtime_data = dynamicPaint_Modifier_runtime_ensure(pmd);
+		if (runtime_data->brush_mesh != NULL) {
+			BKE_id_free(NULL, runtime_data->brush_mesh);
 		}
-		pmd->brush->mesh = BKE_mesh_copy_for_eval(result, false);
+		runtime_data->brush_mesh = BKE_mesh_copy_for_eval(result, false);
 	}
 
 	return result;
@@ -2052,11 +2085,12 @@ void dynamicPaint_cacheUpdateFrames(DynamicPaintSurface *surface)
 
 static void canvas_copyMesh(DynamicPaintCanvasSettings *canvas, Mesh *mesh)
 {
-	if (canvas->mesh) {
-		BKE_id_free(NULL, canvas->mesh);
+	DynamicPaintRuntime *runtime = dynamicPaint_Modifier_runtime_ensure(canvas->pmd);
+	if (runtime->canvas_mesh != NULL) {
+		BKE_id_free(NULL, runtime->canvas_mesh);
 	}
 
-	canvas->mesh = BKE_mesh_copy_for_eval(mesh, false);
+	runtime->canvas_mesh = BKE_mesh_copy_for_eval(mesh, false);
 }
 
 /*
@@ -2769,7 +2803,7 @@ int dynamicPaint_createUVSurface(Scene *scene, DynamicPaintSurface *surface, flo
 
 	PaintSurfaceData *sData;
 	DynamicPaintCanvasSettings *canvas = surface->canvas;
-	Mesh *mesh = canvas->mesh;
+	Mesh *mesh = dynamicPaint_canvas_mesh_get(canvas);
 
 	PaintUVPoint *tempPoints = NULL;
 	Vec3f *tempWeights = NULL;
@@ -3678,7 +3712,7 @@ static void dynamicPaint_brushMeshCalculateVelocity(
 
 	BKE_object_modifier_update_subframe(
 	            depsgraph, scene, ob, true, SUBFRAME_RECURSION, BKE_scene_frame_get(scene), eModifierType_DynamicPaint);
-	mesh_p = BKE_mesh_copy_for_eval(brush->mesh, false);
+	mesh_p = BKE_mesh_copy_for_eval(dynamicPaint_brush_mesh_get(brush), false);
 	numOfVerts_p = mesh_p->totvert;
 	mvert_p = mesh_p->mvert;
 	copy_m4_m4(prev_obmat, ob->obmat);
@@ -3689,7 +3723,7 @@ static void dynamicPaint_brushMeshCalculateVelocity(
 
 	BKE_object_modifier_update_subframe(
 	            depsgraph, scene, ob, true, SUBFRAME_RECURSION, BKE_scene_frame_get(scene), eModifierType_DynamicPaint);
-	mesh_c = brush->mesh;
+	mesh_c = dynamicPaint_brush_mesh_get(brush);
 	numOfVerts_c = mesh_c->totvert;
 	mvert_c = mesh_c->mvert;
 
@@ -4123,8 +4157,10 @@ static int dynamicPaint_paintMesh(Depsgraph *depsgraph, DynamicPaintSurface *sur
 	if (brush->flags & MOD_DPAINT_USES_VELOCITY)
 		dynamicPaint_brushMeshCalculateVelocity(depsgraph, scene, brushOb, brush, &brushVelocity, timescale);
 
-	if (!brush->mesh)
+	Mesh *brush_mesh = dynamicPaint_brush_mesh_get(brush);
+	if (brush_mesh == NULL) {
 		return 0;
+	}
 
 	{
 		BVHTreeFromMesh treeData = {NULL};
@@ -4135,7 +4171,7 @@ static int dynamicPaint_paintMesh(Depsgraph *depsgraph, DynamicPaintSurface *sur
 		Bounds3D mesh_bb = {{0}};
 		VolumeGrid *grid = bData->grid;
 
-		mesh = BKE_mesh_copy_for_eval(brush->mesh, false);
+		mesh = BKE_mesh_copy_for_eval(brush_mesh, false);
 		mvert = mesh->mvert;
 		mlooptri = BKE_mesh_runtime_looptri_ensure(mesh);
 		mloop = mesh->mloop;
@@ -4604,7 +4640,8 @@ static int dynamicPaint_paintSinglePoint(
 	if (brush->flags & MOD_DPAINT_USES_VELOCITY)
 		dynamicPaint_brushObjectCalculateVelocity(depsgraph, scene, brushOb, &brushVel, timescale);
 
-	const MVert *mvert = brush->mesh->mvert;
+	const Mesh *brush_mesh = dynamicPaint_brush_mesh_get(brush);
+	const MVert *mvert = brush_mesh->mvert;
 
 	/*
 	 * Loop through every surface point
@@ -5623,7 +5660,7 @@ static bool dynamicPaint_surfaceHasMoved(DynamicPaintSurface *surface, Object *o
 {
 	PaintSurfaceData *sData = surface->data;
 	PaintBakeData *bData = sData->bData;
-	Mesh *mesh = surface->canvas->mesh;
+	Mesh *mesh = dynamicPaint_canvas_mesh_get(surface->canvas);
 	MVert *mvert = mesh->mvert;
 
 	int numOfVerts = mesh->totvert;
@@ -5771,7 +5808,7 @@ static int dynamicPaint_generateBakeData(DynamicPaintSurface *surface, Depsgraph
 {
 	PaintSurfaceData *sData = surface->data;
 	PaintBakeData *bData = sData->bData;
-	Mesh *mesh = surface->canvas->mesh;
+	Mesh *mesh = dynamicPaint_canvas_mesh_get(surface->canvas);
 	int index;
 	bool new_bdata = false;
 	const bool do_velocity_data = ((surface->effect & MOD_DPAINT_EFFECT_DO_DRIP) ||
@@ -6045,8 +6082,9 @@ int dynamicPaint_calculateFrame(
 	float timescale = 1.0f;
 
 	/* apply previous displace on derivedmesh if incremental surface */
-	if (surface->flags & MOD_DPAINT_DISP_INCREMENTAL)
-		dynamicPaint_applySurfaceDisplace(surface, surface->canvas->mesh);
+	if (surface->flags & MOD_DPAINT_DISP_INCREMENTAL) {
+		dynamicPaint_applySurfaceDisplace(surface, dynamicPaint_canvas_mesh_get(surface->canvas));
+	}
 
 	/* update bake data */
 	dynamicPaint_generateBakeData(surface, depsgraph, cObject);

source/blender/blenloader/intern/readfile.c

@@ -5590,7 +5590,6 @@ static void direct_link_modifiers(FileData *fd, ListBase *lb)
 			if (pmd->canvas) {
 				pmd->canvas = newdataadr(fd, pmd->canvas);
 				pmd->canvas->pmd = pmd;
-				pmd->canvas->mesh = NULL;
 				pmd->canvas->flags &= ~MOD_DPAINT_BAKING; /* just in case */
 
 				if (pmd->canvas->surfaces.first) {
@@ -5613,7 +5612,6 @@ static void direct_link_modifiers(FileData *fd, ListBase *lb)
 				pmd->brush->psys = newdataadr(fd, pmd->brush->psys);
 				pmd->brush->paint_ramp = newdataadr(fd, pmd->brush->paint_ramp);
 				pmd->brush->vel_ramp = newdataadr(fd, pmd->brush->vel_ramp);
-				pmd->brush->mesh = NULL;
 			}
 		}
 		else if (md->type == eModifierType_Collision) {

source/blender/makesdna/DNA_dynamicpaint_types.h

@@ -91,6 +91,14 @@ enum {
 	MOD_DPAINT_INITIAL_VERTEXCOLOR = 3,
 };
 
+/* Is stored in ModifierData.runtime. */
+#
+#
+typedef struct DynamicPaintRuntime {
+	struct Mesh *canvas_mesh;
+	struct Mesh *brush_mesh;
+} DynamicPaintRuntime;
+
 typedef struct DynamicPaintSurface {
 
 	struct DynamicPaintSurface *next, *prev;
@@ -169,7 +177,6 @@ enum {
 typedef struct DynamicPaintCanvasSettings {
 	/** For fast RNA access. */
 	struct DynamicPaintModifierData *pmd;
-	struct Mesh *mesh;
 
 	struct ListBase surfaces;
 	short active_sur, flags;
@@ -248,7 +255,6 @@ enum {
 typedef struct DynamicPaintBrushSettings {
 	/** For fast RNA access. */
 	struct DynamicPaintModifierData *pmd;
-	struct Mesh *mesh;
 	struct ParticleSystem *psys;
 
 	int flags;

source/blender/modifiers/intern/MOD_dynamicpaint.c

@@ -58,6 +58,15 @@ static void copyData(const ModifierData *md, ModifierData *target, const int fla
 	dynamicPaint_Modifier_copy(pmd, tpmd, flag);
 }
 
+static void freeRuntimeData(void *runtime_data_v)
+{
+	if (runtime_data_v == NULL) {
+		return;
+	}
+	DynamicPaintRuntime *runtime_data = (DynamicPaintRuntime *)runtime_data_v;
+	dynamicPaint_Modifier_free_runtime(runtime_data);
+}
+
 static void freeData(ModifierData *md)
 {
 	DynamicPaintModifierData *pmd = (DynamicPaintModifierData *) md;
@@ -192,5 +201,5 @@ ModifierTypeInfo modifierType_DynamicPaint = {
 	/* foreachObjectLink */ NULL,
 	/* foreachIDLink */     foreachIDLink,
 	/* foreachTexLink */    foreachTexLink,
-	/* freeRuntimeData */   NULL,
+	/* freeRuntimeData */   freeRuntimeData,
 };