Fix T83280: Crash when deleting hair collision collection.

Root of the issue was missing management of ID pointers in the cloth
modifier data stored in ParticleSystem for hair physics, in the
'foreach_id' particle system code.

Using modifier's 'foreach_id' code in psys one unfortunately requires
some ugly conversion gymnastics, but this is still better than having
dedicated code for that case.

Note that this is actually a fairly critical issue, fix should be
backported to 2.91.1 should we do it, and to 2.83 LTS as well I think.

1 files changed, 28 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/particle_system.c b/source/blender/blenkernel/intern/particle_system.c
index 35265cf8b68..79ce6769160 100644
--- a/source/blender/blenkernel/intern/particle_system.c
+++ b/source/blender/blenkernel/intern/particle_system.c
@@ -4978,6 +4978,24 @@ void particle_system_update(struct Depsgraph *depsgraph,
 
 /* ID looper */
 
+/* unfortunately PSys and modifier ID loopers are not directly compatible, so we need this struct
+ * and the callback below to map the former to the latter (thanks to psys embedding a Cloth
+ * modifier data struct now, for Hair physics simulations). */
+typedef struct ParticleSystemIDLoopForModifier {
+  ParticleSystem *psys;
+  ParticleSystemIDFunc func;
+  void *userdata;
+} ParticleSystemIDLoopForModifier;
+
+static void particlesystem_modifiersForeachIDLink(void *user_data,
+                                                  Object *UNUSED(object),
+                                                  ID **id_pointer,
+                                                  int cb_flag)
+{
+  ParticleSystemIDLoopForModifier *data = (ParticleSystemIDLoopForModifier *)user_data;
+  data->func(data->psys, id_pointer, data->userdata, cb_flag);
+}
+
 void BKE_particlesystem_id_loop(ParticleSystem *psys, ParticleSystemIDFunc func, void *userdata)
 {
   ParticleTarget *pt;
@@ -4986,6 +5004,16 @@ void BKE_particlesystem_id_loop(ParticleSystem *psys, ParticleSystemIDFunc func,
   func(psys, (ID **)&psys->target_ob, userdata, IDWALK_CB_NOP);
   func(psys, (ID **)&psys->parent, userdata, IDWALK_CB_NOP);
 
+  if (psys->clmd != NULL) {
+    const ModifierTypeInfo *mti = BKE_modifier_get_info(psys->clmd->modifier.type);
+
+    if (mti->foreachIDLink != NULL) {
+      ParticleSystemIDLoopForModifier data = {.psys = psys, .func = func, .userdata = userdata};
+      mti->foreachIDLink(
+          &psys->clmd->modifier, NULL, particlesystem_modifiersForeachIDLink, &data);
+    }
+  }
+
   for (pt = psys->targets.first; pt; pt = pt->next) {
     func(psys, (ID **)&pt->ob, userdata, IDWALK_CB_NOP);
   }