Fix for [#26712] Particle group instance 'Use Count' value gets reset on file-load.

* New object pointers can't be loaded properly for library linked groups, so the weight groups now store an index to the group objects at save time. This index is used at load time to set the objects without relying on the old pointers.
* If the library linked group is modified the indices can be wrong, but this can't really be avoided easily as there's no way to relate objects in a linked group between loads.

source/blender/blenloader/intern/readfile.c

@@ -75,6 +75,7 @@
 #include "DNA_node_types.h"
 #include "DNA_object_fluidsim.h" // NT
 #include "DNA_packedFile_types.h"
+#include "DNA_particle_types.h"
 #include "DNA_property_types.h"
 #include "DNA_text_types.h"
 #include "DNA_view3d_types.h"
@@ -3161,9 +3162,37 @@ static void lib_link_particlesettings(FileData *fd, Main *main)
 			if(part->effector_weights)
 				part->effector_weights->group = newlibadr(fd, part->id.lib, part->effector_weights->group);
 
-			dw = part->dupliweights.first;
-			for(; dw; dw=dw->next)
-				dw->ob = newlibadr(fd, part->id.lib, dw->ob);
+			if(part->dupliweights.first) {
+				int index_ok = 0;
+				/* check for old files without indices (all indexes 0) */
+				dw = part->dupliweights.first;
+				if(part->dupliweights.first == part->dupliweights.last) {
+					/* special case for only one object in the group */
+					index_ok = 1;
+				}
+				else { 
+					for(; dw; dw=dw->next) {
+						if(dw->index > 0) {
+							index_ok = 1;
+							break;
+						}
+					}
+				}
+
+				if(index_ok) {
+					/* if we have indexes, let's use them */
+					dw = part->dupliweights.first;
+					for(; dw; dw=dw->next) {
+						GroupObject *go = (GroupObject *)BLI_findlink(&part->dup_group->gobject, dw->index);
+						dw->ob = go ? go->ob : NULL;
+					}
+				}
+				else {
+					/* otherwise try to get objects from own library (won't work on library linked groups) */
+					for(; dw; dw=dw->next)
+						dw->ob = newlibadr(fd, part->id.lib, dw->ob);
+				}
+			}
 
 			if(part->boids) {
 				BoidState *state = part->boids->states.first;

source/blender/blenloader/intern/writefile.c

@@ -837,6 +837,7 @@ static void write_particlesettings(WriteData *wd, ListBase *idbase)
 {
 	ParticleSettings *part;
 	ParticleDupliWeight *dw;
+	GroupObject *go;
 	int a;
 
 	part= idbase->first;
@@ -851,8 +852,16 @@ static void write_particlesettings(WriteData *wd, ListBase *idbase)
 			writestruct(wd, DATA, "EffectorWeights", 1, part->effector_weights);
 
 			dw = part->dupliweights.first;
-			for(; dw; dw=dw->next)
+			for(; dw; dw=dw->next) {
+				/* update indices */
+				dw->index = 0;
+				go = part->dup_group->gobject.first;
+				while(go && go->ob != dw->ob) {
+					go=go->next;
+					dw->index++;
+				}
 				writestruct(wd, DATA, "ParticleDupliWeight", 1, dw);
+			}
 
 			if(part->boids && part->phystype == PART_PHYS_BOIDS) {
 				BoidState *state = part->boids->states.first;

source/blender/makesdna/DNA_particle_types.h

@@ -89,7 +89,8 @@ typedef struct ParticleDupliWeight {
 	struct ParticleDupliWeight *next, *prev;
 	struct Object *ob;
 	short count;
-	short flag, rt[2];
+	short flag;
+	short index, rt; /* only updated on file save and used on file load */
 } ParticleDupliWeight;
 
 typedef struct ParticleData {