Fix T49829: Removal of custom icon previews during add-on unregister crashes Blender.

Issue was happening when removal of custom icons was done while they
were still being rendered by preview job.

Now add a 'deffered deletion' system, to prevent main thread to delete
preview image until loading thread is done with them.

Note that ideally, calling `ED_preview_kill_jobs()` on custom icon
removal would have been simpler, but we don't have easy access to
context here...

2 files changed, 16 insertions, 5 deletions

diff --git a/source/blender/blenkernel/BKE_icons.h b/source/blender/blenkernel/BKE_icons.h
index efef8d4be78..6944c5ccd28 100644
--- a/source/blender/blenkernel/BKE_icons.h
+++ b/source/blender/blenkernel/BKE_icons.h
@@ -114,6 +114,7 @@ struct PreviewImage *BKE_previewimg_cached_thumbnail_read(
         const char *name, const char *path, const int source, bool force_update);
 
 void BKE_previewimg_cached_release(const char *name);
+void BKE_previewimg_cached_release_pointer(struct PreviewImage *prv);
 
 #define ICON_RENDER_DEFAULT_HEIGHT 32
 
diff --git a/source/blender/blenkernel/intern/icons.c b/source/blender/blenkernel/intern/icons.c
index 2d5b15c8f9d..7669c4ba112 100644
--- a/source/blender/blenkernel/intern/icons.c
+++ b/source/blender/blenkernel/intern/icons.c
@@ -143,7 +143,7 @@ static PreviewImage *previewimg_create_ex(size_t deferred_data_size)
 	memset(prv_img, 0, sizeof(*prv_img));  /* leave deferred data dirty */
 
 	if (deferred_data_size) {
-		prv_img->use_deferred = true;
+		prv_img->tag |= PRV_TAG_DEFFERED;
 	}
 
 	for (i = 0; i < NUM_ICON_SIZES; ++i) {
@@ -355,11 +355,14 @@ PreviewImage *BKE_previewimg_cached_thumbnail_read(
 	return prv;
 }
 
-void BKE_previewimg_cached_release(const char *name)
+void BKE_previewimg_cached_release_pointer(PreviewImage *prv)
 {
-	PreviewImage *prv = BLI_ghash_popkey(gCachedPreviews, name, MEM_freeN);
-
 	if (prv) {
+		if (prv->tag & PRV_TAG_DEFFERED_RENDERING) {
+			/* We cannot delete the preview while it is being loaded in another thread... */
+			prv->tag |= PRV_TAG_DEFFERED_DELETE;
+			return;
+		}
 		if (prv->icon_id) {
 			BKE_icon_delete(prv->icon_id);
 		}
@@ -367,11 +370,18 @@ void BKE_previewimg_cached_release(const char *name)
 	}
 }
 
+void BKE_previewimg_cached_release(const char *name)
+{
+	PreviewImage *prv = BLI_ghash_popkey(gCachedPreviews, name, MEM_freeN);
+
+	BKE_previewimg_cached_release_pointer(prv);
+}
+
 /** Handle deferred (lazy) loading/generation of preview image, if needed.
  * For now, only used with file thumbnails. */
 void BKE_previewimg_ensure(PreviewImage *prv, const int size)
 {
-	if (prv->use_deferred) {
+	if ((prv->tag & PRV_TAG_DEFFERED) != 0) {
 		const bool do_icon = ((size == ICON_SIZE_ICON) && !prv->rect[ICON_SIZE_ICON]);
 		const bool do_preview = ((size == ICON_SIZE_PREVIEW) && !prv->rect[ICON_SIZE_PREVIEW]);