Fix T93560: crash with image paint undo and cycles preview render

Cycles preview rendering could free the image buffers being used by drawing in
another thread due to a race condition. This race condition was unlikely before,
but now that preview renders are started right before we draw the image in the
image editor or load it as a texture in the 3D viewport, it's likely to happen.

As we are close to release this is too risky to fix properly, just avoid freeing
the cache for preview renders instead and accept increased memory usage in some
cases.

3 files changed, 14 insertions, 6 deletions

diff --git a/intern/cycles/blender/image.cpp b/intern/cycles/blender/image.cpp
index 3ea3a47c1f4..4f5744d86a8 100644
--- a/intern/cycles/blender/image.cpp
+++ b/intern/cycles/blender/image.cpp
@@ -24,8 +24,14 @@ CCL_NAMESPACE_BEGIN
 
 /* Packed Images */
 
-BlenderImageLoader::BlenderImageLoader(BL::Image b_image, int frame)
-    : b_image(b_image), frame(frame), free_cache(!b_image.has_data())
+BlenderImageLoader::BlenderImageLoader(BL::Image b_image,
+                                       const int frame,
+                                       const bool is_preview_render)
+    : b_image(b_image),
+      frame(frame),
+      /* Don't free cache for preview render to avoid race condition from T93560, to be fixed
+         properly later as we are close to release. */
+      free_cache(!is_preview_render && !b_image.has_data())
 {
 }
 
diff --git a/intern/cycles/blender/image.h b/intern/cycles/blender/image.h
index 6f1e72c21af..b4aa4299ed1 100644
--- a/intern/cycles/blender/image.h
+++ b/intern/cycles/blender/image.h
@@ -25,7 +25,7 @@ CCL_NAMESPACE_BEGIN
 
 class BlenderImageLoader : public ImageLoader {
  public:
-  BlenderImageLoader(BL::Image b_image, int frame);
+  BlenderImageLoader(BL::Image b_image, const int frame, const bool is_preview_render);
 
   bool load_metadata(const ImageDeviceFeatures &features, ImageMetaData &metadata) override;
   bool load_pixels(const ImageMetaData &metadata,
diff --git a/intern/cycles/blender/shader.cpp b/intern/cycles/blender/shader.cpp
index 0cd9052b47a..0f17834906d 100644
--- a/intern/cycles/blender/shader.cpp
+++ b/intern/cycles/blender/shader.cpp
@@ -762,7 +762,8 @@ static ShaderNode *add_node(Scene *scene,
         int scene_frame = b_scene.frame_current();
         int image_frame = image_user_frame_number(b_image_user, b_image, scene_frame);
         image->handle = scene->image_manager->add_image(
-            new BlenderImageLoader(b_image, image_frame), image->image_params());
+            new BlenderImageLoader(b_image, image_frame, b_engine.is_preview()),
+            image->image_params());
       }
       else {
         ustring filename = ustring(
@@ -797,8 +798,9 @@ static ShaderNode *add_node(Scene *scene,
       if (is_builtin) {
         int scene_frame = b_scene.frame_current();
         int image_frame = image_user_frame_number(b_image_user, b_image, scene_frame);
-        env->handle = scene->image_manager->add_image(new BlenderImageLoader(b_image, image_frame),
-                                                      env->image_params());
+        env->handle = scene->image_manager->add_image(
+            new BlenderImageLoader(b_image, image_frame, b_engine.is_preview()),
+            env->image_params());
       }
       else {
         env->set_filename(