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, 43 insertions, 0 deletions

diff --git a/source/blender/editors/render/render_preview.c b/source/blender/editors/render/render_preview.c
index 0cc944436b2..25353dc30fe 100644
--- a/source/blender/editors/render/render_preview.c
+++ b/source/blender/editors/render/render_preview.c
@@ -103,6 +103,8 @@
 #include "ED_view3d.h"
 #include "ED_view3d_offscreen.h"
 
+#include "UI_interface_icons.h"
+
 #ifndef NDEBUG
 /* Used for database init assert(). */
 #  include "BLI_threads.h"
@@ -1948,4 +1950,45 @@ void ED_preview_kill_jobs(wmWindowManager *wm, Main *UNUSED(bmain))
   }
 }
 
+typedef struct PreviewRestartQueueEntry {
+  struct PreviewRestartQueueEntry *next, *prev;
+
+  enum eIconSizes size;
+  ID *id;
+} PreviewRestartQueueEntry;
+
+static ListBase /* #PreviewRestartQueueEntry */ G_restart_previews_queue;
+
+void ED_preview_restart_queue_free(void)
+{
+  BLI_freelistN(&G_restart_previews_queue);
+}
+
+void ED_preview_restart_queue_add(ID *id, enum eIconSizes size)
+{
+  PreviewRestartQueueEntry *queue_entry = MEM_mallocN(sizeof(*queue_entry), __func__);
+  queue_entry->size = size;
+  queue_entry->id = id;
+  BLI_addtail(&G_restart_previews_queue, queue_entry);
+}
+
+void ED_preview_restart_queue_work(const bContext *C)
+{
+  LISTBASE_FOREACH_MUTABLE (PreviewRestartQueueEntry *, queue_entry, &G_restart_previews_queue) {
+    PreviewImage *preview = BKE_previewimg_id_get(queue_entry->id);
+    if (!preview) {
+      continue;
+    }
+    if (preview->flag[queue_entry->size] & PRV_USER_EDITED) {
+      /* Don't touch custom previews. */
+      continue;
+    }
+
+    BKE_previewimg_clear_single(preview, queue_entry->size);
+    UI_icon_render_id(C, NULL, queue_entry->id, queue_entry->size, true);
+
+    BLI_freelinkN(&G_restart_previews_queue, queue_entry);
+  }
+}
+
 /** \} */