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, 12 insertions, 17 deletions

diff --git a/source/blender/blenkernel/intern/icons.cc b/source/blender/blenkernel/intern/icons.cc
index c2250dd04f9..b9ccbedfa81 100644
--- a/source/blender/blenkernel/intern/icons.cc
+++ b/source/blender/blenkernel/intern/icons.cc
@@ -251,7 +251,7 @@ static PreviewImage *previewimg_create_ex(size_t deferred_data_size)
   }
 
   for (int i = 0; i < NUM_ICON_SIZES; i++) {
-    prv_img->flag[i] |= (PRV_CHANGED | PRV_UNFINISHED);
+    prv_img->flag[i] |= PRV_CHANGED;
     prv_img->changed_timestamp[i] = 0;
   }
   return prv_img;
@@ -308,7 +308,7 @@ void BKE_previewimg_clear_single(struct PreviewImage *prv, enum eIconSizes size)
     GPU_texture_free(prv->gputexture[size]);
   }
   prv->h[size] = prv->w[size] = 0;
-  prv->flag[size] |= (PRV_CHANGED | PRV_UNFINISHED);
+  prv->flag[size] |= PRV_CHANGED;
   prv->flag[size] &= ~PRV_USER_EDITED;
   prv->changed_timestamp[size] = 0;
 }
@@ -565,7 +565,7 @@ void BKE_previewimg_ensure(PreviewImage *prv, const int size)
           prv->w[ICON_SIZE_PREVIEW] = thumb->x;
           prv->h[ICON_SIZE_PREVIEW] = thumb->y;
           prv->rect[ICON_SIZE_PREVIEW] = (uint *)MEM_dupallocN(thumb->rect);
-          prv->flag[ICON_SIZE_PREVIEW] &= ~(PRV_CHANGED | PRV_USER_EDITED | PRV_UNFINISHED);
+          prv->flag[ICON_SIZE_PREVIEW] &= ~(PRV_CHANGED | PRV_USER_EDITED | PRV_RENDERING);
         }
         if (do_icon) {
           if (thumb->x > thumb->y) {
@@ -584,7 +584,7 @@ void BKE_previewimg_ensure(PreviewImage *prv, const int size)
           prv->w[ICON_SIZE_ICON] = icon_w;
           prv->h[ICON_SIZE_ICON] = icon_h;
           prv->rect[ICON_SIZE_ICON] = (uint *)MEM_dupallocN(thumb->rect);
-          prv->flag[ICON_SIZE_ICON] &= ~(PRV_CHANGED | PRV_USER_EDITED | PRV_UNFINISHED);
+          prv->flag[ICON_SIZE_ICON] &= ~(PRV_CHANGED | PRV_USER_EDITED | PRV_RENDERING);
         }
         IMB_freeImBuf(thumb);
       }
@@ -616,12 +616,12 @@ ImBuf *BKE_previewimg_to_imbuf(PreviewImage *prv, const int size)
 void BKE_previewimg_finish(PreviewImage *prv, const int size)
 {
   /* Previews may be calculated on a thread. */
-  atomic_fetch_and_and_int16(&prv->flag[size], ~PRV_UNFINISHED);
+  atomic_fetch_and_and_int16(&prv->flag[size], ~PRV_RENDERING);
 }
 
 bool BKE_previewimg_is_finished(const PreviewImage *prv, const int size)
 {
-  return (prv->flag[size] & PRV_UNFINISHED) == 0;
+  return (prv->flag[size] & PRV_RENDERING) == 0;
 }
 
 void BKE_previewimg_blend_write(BlendWriter *writer, const PreviewImage *prv)
@@ -655,16 +655,11 @@ void BKE_previewimg_blend_read(BlendDataReader *reader, PreviewImage *prv)
       BLO_read_data_address(reader, &prv->rect[i]);
     }
     prv->gputexture[i] = nullptr;
-    /* For now consider previews read from file as finished to not confuse File Browser preview
-     * loading. That could be smarter and check if there's a preview job running instead.
-     * If the preview is tagged as changed, it needs to be updated anyway, so don't remove the tag.
-     */
-    if ((prv->flag[i] & PRV_CHANGED) == 0) {
-      BKE_previewimg_finish(prv, i);
-    }
-    else {
-      /* Only for old files that didn't write the flag. */
-      prv->flag[i] |= PRV_UNFINISHED;
+
+    /* PRV_RENDERING is a runtime only flag currently, but don't mess with it on undo! It gets
+     * special handling in #memfile_undosys_restart_unfinished_id_previews() then. */
+    if (!BLO_read_data_is_undo(reader)) {
+      prv->flag[i] &= ~PRV_RENDERING;
     }
   }
   prv->icon_id = 0;
@@ -694,7 +689,7 @@ void BKE_icon_changed(const int icon_id)
     /* If we have previews, they all are now invalid changed. */
     if (p_prv && *p_prv) {
       for (int i = 0; i < NUM_ICON_SIZES; i++) {
-        (*p_prv)->flag[i] |= (PRV_CHANGED | PRV_UNFINISHED);
+        (*p_prv)->flag[i] |= PRV_CHANGED;
         (*p_prv)->changed_timestamp[i]++;
       }
     }