Assets: Sanitize threaded preview creation with undo

Basically, this fixes disappearing previews when editing asset metadata
or performing undo/redo actions.

The preview generation in a background job will eventually modify ID
data, but the undo push was done prior to that. So obviously, an undo
then would mean the preview is lost.

This patch makes it so undo/redo will regenerate the preview, if the preview
rendering was invoked but not finished in the undone/redone state.

The preview flag PRV_UNFINISHED wasn't entirely what we needed. So I had to
change it to a slightly different flag, with different semantics.

1 files changed, 2 insertions, 1 deletions

diff --git a/source/blender/editors/interface/interface_icons.c b/source/blender/editors/interface/interface_icons.c
index 6f119d55d3c..311965ac502 100644
--- a/source/blender/editors/interface/interface_icons.c
+++ b/source/blender/editors/interface/interface_icons.c
@@ -1270,7 +1270,7 @@ static void icon_create_rect(struct PreviewImage *prv_img, enum eIconSizes size)
   else if (!prv_img->rect[size]) {
     prv_img->w[size] = render_size;
     prv_img->h[size] = render_size;
-    prv_img->flag[size] |= (PRV_CHANGED | PRV_UNFINISHED);
+    prv_img->flag[size] |= PRV_CHANGED;
     prv_img->changed_timestamp[size] = 0;
     prv_img->rect[size] = MEM_callocN(render_size * render_size * sizeof(uint), "prv_rect");
   }
@@ -1419,6 +1419,7 @@ static void icon_set_image(const bContext *C,
 
   const bool delay = prv_img->rect[size] != NULL;
   icon_create_rect(prv_img, size);
+  prv_img->flag[size] |= PRV_RENDERING;
 
   if (use_job && (!id || BKE_previewimg_id_supports_jobs(id))) {
     /* Job (background) version */