Fix 48831, Step I: Mismatch issues bewteen ID icon and preview system.

- icon_id from ID and PreviewImage were not guaranteed to be in sync.
- PreviewImage one was not reset on file read.
- Through RNA e.g., it was possible to ensure an ID icon via its preview image,
  which was running code designed for custom previews/icons system, instead
  of generating correct 'auto ID icon'.

source/blender/blenkernel/BKE_icons.h

@@ -55,7 +55,7 @@ void BKE_icons_init(int first_dyn_id);
 /* return icon id for library object or create new icon if not found */
 int BKE_icon_id_ensure(struct ID *id);
 
-int BKE_icon_preview_ensure(struct PreviewImage *preview);
+int BKE_icon_preview_ensure(struct ID *id, struct PreviewImage *preview);
 
 /* retrieve icon for id */
 struct Icon *BKE_icon_get(int icon_id);

source/blender/blenkernel/intern/icons.c

@@ -423,10 +423,26 @@ void BKE_icon_changed(int id)
 	}
 }
 
-int BKE_icon_id_ensure(struct ID *id)
+static int icon_id_ensure_create_icon(struct ID *id)
 {
 	Icon *new_icon = NULL;
 
+	new_icon = MEM_mallocN(sizeof(Icon), __func__);
+
+	new_icon->obj = id;
+	new_icon->type = GS(id->name);
+
+	/* next two lines make sure image gets created */
+	new_icon->drawinfo = NULL;
+	new_icon->drawinfo_free = NULL;
+
+	BLI_ghash_insert(gIcons, SET_INT_IN_POINTER(id->icon_id), new_icon);
+
+	return id->icon_id;
+}
+
+int BKE_icon_id_ensure(struct ID *id)
+{
 	if (!id || G.background)
 		return 0;
 
@@ -440,32 +456,39 @@ int BKE_icon_id_ensure(struct ID *id)
 		return 0;
 	}
 
-	new_icon = MEM_mallocN(sizeof(Icon), __func__);
+	/* Ensure we synchronize ID icon_id with its previewimage if it has one. */
+	PreviewImage **p_prv = BKE_previewimg_id_get_p(id);
+	if (p_prv && *p_prv) {
+		BLI_assert(ELEM((*p_prv)->icon_id, 0, id->icon_id));
+		(*p_prv)->icon_id = id->icon_id;
+	}
 
-	new_icon->obj = id;
-	new_icon->type = GS(id->name);
-	
-	/* next two lines make sure image gets created */
-	new_icon->drawinfo = NULL;
-	new_icon->drawinfo_free = NULL;
-
-	BLI_ghash_insert(gIcons, SET_INT_IN_POINTER(id->icon_id), new_icon);
-	
-	return id->icon_id;
+	return icon_id_ensure_create_icon(id);
 }
 
 /**
  * Return icon id of given preview, or create new icon if not found.
  */
-int BKE_icon_preview_ensure(PreviewImage *preview)
+int BKE_icon_preview_ensure(ID *id, PreviewImage *preview)
 {
 	Icon *new_icon = NULL;
 
 	if (!preview || G.background)
 		return 0;
 
-	if (preview->icon_id)
+	if (id) {
+		BLI_assert(BKE_previewimg_id_ensure(id) == preview);
+	}
+
+	if (preview->icon_id) {
+		BLI_assert(!id || !id->icon_id || id->icon_id == preview->icon_id);
 		return preview->icon_id;
+	}
+
+	if (id && id->icon_id) {
+		preview->icon_id = id->icon_id;
+		return preview->icon_id;
+	}
 
 	preview->icon_id = get_next_free_id();
 
@@ -474,6 +497,12 @@ int BKE_icon_preview_ensure(PreviewImage *preview)
 		return 0;
 	}
 
+	/* Ensure we synchronize ID icon_id with its previewimage if available, and generate suitable 'ID' icon. */
+	if (id) {
+		id->icon_id = preview->icon_id;
+		return icon_id_ensure_create_icon(id);
+	}
+
 	new_icon = MEM_mallocN(sizeof(Icon), __func__);
 
 	new_icon->obj = preview;

source/blender/blenloader/intern/readfile.c

@@ -2144,6 +2144,7 @@ static PreviewImage *direct_link_preview_image(FileData *fd, PreviewImage *old_p
 			}
 			prv->gputexture[i] = NULL;
 		}
+		prv->icon_id = 0;
 	}
 	
 	return prv;

source/blender/makesrna/intern/rna_ID.c

@@ -698,7 +698,7 @@ static void rna_ImagePreview_icon_pixels_float_set(PointerRNA *ptr, const float
 static int rna_ImagePreview_icon_id_get(PointerRNA *ptr)
 {
 	/* Using a callback here allows us to only generate icon matching that preview when icon_id is requested. */
-	return BKE_icon_preview_ensure((PreviewImage *)(ptr->data));
+	return BKE_icon_preview_ensure(ptr->id.data, (PreviewImage *)(ptr->data));
 }
 static void rna_ImagePreview_icon_reload(PreviewImage *prv)
 {