Fix T38403: Laplacian smooth on instanced objects leads to crash.

The laplacian modifiers (smooth and deform) use the OpenNL library,
which is not threadsafe due to the use of a global context variable.
Ideally this would be changed so that an explicit context can be
created for every caller of the OpenNL functions, but since OpenNL's
most recent version is from 2010 this is unlikely to happen.

As a workaround for now just use a mutex to prevent conflicting OpenNL
calls. Eventually OpenNL can be replaced by eigen or ceres.

source/blender/modifiers/intern/MOD_laplaciandeform.c

@@ -452,6 +452,10 @@ static void laplacianDeformPreview(LaplacianSystem *sys, float (*vertexCos)[3])
 	n = sys->total_verts;
 	na = sys->total_anchors;
 
+#ifdef OPENNL_THREADING_HACK
+	modifier_opennl_lock();
+#endif
+
 	if (!sys->is_matrix_computed) {
 		nlNewContext();
 		sys->context = nlGetCurrent();
@@ -530,12 +534,9 @@ static void laplacianDeformPreview(LaplacianSystem *sys, float (*vertexCos)[3])
 			sys->has_solution = false;
 		}
 		sys->is_matrix_computed = true;
-	}
-	else {
-		if (!sys->has_solution) {
-			return;
-		}
 
+	}
+	else if (sys->has_solution) {
 		nlBegin(NL_SYSTEM);
 		nlBegin(NL_MATRIX);
 
@@ -589,6 +590,10 @@ static void laplacianDeformPreview(LaplacianSystem *sys, float (*vertexCos)[3])
 			sys->has_solution = false;
 		}
 	}
+
+#ifdef OPENNL_THREADING_HACK
+	modifier_opennl_unlock();
+#endif
 }
 
 static bool isValidVertexGroup(LaplacianDeformModifierData *lmd, Object *ob, DerivedMesh *dm)

source/blender/modifiers/intern/MOD_laplaciansmooth.c

@@ -534,6 +534,11 @@ static void laplaciansmoothModifier_do(
 	sys->vert_centroid[1] = 0.0f;
 	sys->vert_centroid[2] = 0.0f;
 	memset_laplacian_system(sys, 0);
+
+#ifdef OPENNL_THREADING_HACK
+	modifier_opennl_lock();
+#endif
+
 	nlNewContext();
 	sys->context = nlGetCurrent();
 	nlSolverParameteri(NL_NB_VARIABLES, numVerts);
@@ -618,6 +623,11 @@ static void laplaciansmoothModifier_do(
 	}
 	nlDeleteContext(sys->context);
 	sys->context = NULL;
+
+#ifdef OPENNL_THREADING_HACK
+	modifier_opennl_unlock();
+#endif
+
 	delete_laplacian_system(sys);
 }
 

source/blender/modifiers/intern/MOD_util.c

@@ -61,6 +61,10 @@
 
 #include "RE_shader_ext.h"
 
+#ifdef OPENNL_THREADING_HACK
+#include "BLI_threads.h"
+#endif
+
 void modifier_init_texture(Scene *scene, Tex *tex)
 {
 	if (!tex)
@@ -235,6 +239,24 @@ void modifier_get_vgroup(Object *ob, DerivedMesh *dm, const char *name, MDeformV
 	}
 }
 
+
+#ifdef OPENNL_THREADING_HACK
+
+static ThreadMutex opennl_context_mutex = BLI_MUTEX_INITIALIZER;
+
+void modifier_opennl_lock(void)
+{
+	BLI_mutex_lock(&opennl_context_mutex);
+}
+
+void modifier_opennl_unlock(void)
+{
+	BLI_mutex_unlock(&opennl_context_mutex);
+}
+
+#endif
+
+
 /* only called by BKE_modifier.h/modifier.c */
 void modifier_type_init(ModifierTypeInfo *types[])
 {

source/blender/modifiers/intern/MOD_util.h

@@ -52,4 +52,21 @@ struct DerivedMesh *get_dm_for_modifier(struct Object *ob, ModifierApplyFlag fla
 void modifier_get_vgroup(struct Object *ob, struct DerivedMesh *dm,
                          const char *name, struct MDeformVert **dvert, int *defgrp_index);
 
+/* XXX workaround for non-threadsafe context in OpenNL (T38403)
+ * OpenNL uses global pointer for "current context", which causes
+ * conflict when multiple modifiers get evaluated in threaded depgraph.
+ * This is just a stupid hack to prevent assert failure / crash,
+ * otherwise we'd have to modify OpenNL on a large scale.
+ * OpenNL should be replaced eventually, there are other options (eigen, ceres).
+ * - lukas_t
+ */
+#ifdef WITH_OPENNL
+#define OPENNL_THREADING_HACK
+#endif
+
+#ifdef OPENNL_THREADING_HACK
+void modifier_opennl_lock(void);
+void modifier_opennl_unlock(void);
+#endif
+
 #endif /* __MOD_UTIL_H__ */