Error on unloaded ImageRenderTarget (#20503)

# Objective

When using `RenderTarget::Image` with an image set to use in the render
world only, the `camera_system` is unable to get its render target info
and fails silently, breaking things down the line. From what I saw the
view uniforms stopped updating, but there might be other things it broke
as well.

## Solution

At least make it fail loudly :)

Not sure if it should panic here, or return a better error enum that the
caller can decide what to do with. Should all failures to update the
render target info be hard errors?

## Testing

Error gets printed when running the project I found the bug with

Author: ecoskey

crates/bevy_render/src/camera.rs

@@ -20,14 +20,15 @@ use bevy_camera::{
     primitives::Frustum,
     visibility::{self, RenderLayers, VisibleEntities},
     Camera, Camera2d, Camera3d, CameraMainTextureUsages, CameraOutputMode, CameraUpdateSystems,
-    ClearColor, ClearColorConfig, Exposure, NormalizedRenderTarget, Projection, RenderTargetInfo,
-    Viewport,
+    ClearColor, ClearColorConfig, Exposure, ManualTextureViewHandle, NormalizedRenderTarget,
+    Projection, RenderTargetInfo, Viewport,
 };
 use bevy_derive::{Deref, DerefMut};
 use bevy_ecs::{
     change_detection::DetectChanges,
     component::Component,
     entity::{ContainsEntity, Entity},
+    error::BevyError,
     event::EventReader,
     lifecycle::HookContext,
     prelude::With,
@@ -167,7 +168,7 @@ pub trait NormalizedRenderTargetExt {
         resolutions: impl IntoIterator<Item = (Entity, &'a Window)>,
         images: &Assets<Image>,
         manual_texture_views: &ManualTextureViews,
-    ) -> Option<RenderTargetInfo>;
+    ) -> Result<RenderTargetInfo, MissingRenderTargetInfoError>;
 
     // Check if this render target is contained in the given changed windows or images.
     fn is_changed(
@@ -222,28 +223,34 @@ impl NormalizedRenderTargetExt for NormalizedRenderTarget {
         resolutions: impl IntoIterator<Item = (Entity, &'a Window)>,
         images: &Assets<Image>,
         manual_texture_views: &ManualTextureViews,
-    ) -> Option<RenderTargetInfo> {
+    ) -> Result<RenderTargetInfo, MissingRenderTargetInfoError> {
         match self {
             NormalizedRenderTarget::Window(window_ref) => resolutions
                 .into_iter()
                 .find(|(entity, _)| *entity == window_ref.entity())
                 .map(|(_, window)| RenderTargetInfo {
                     physical_size: window.physical_size(),
                     scale_factor: window.resolution.scale_factor(),
+                })
+                .ok_or(MissingRenderTargetInfoError::Window {
+                    window: window_ref.entity(),
                 }),
-            NormalizedRenderTarget::Image(image_target) => {
-                let image = images.get(&image_target.handle)?;
-                Some(RenderTargetInfo {
+            NormalizedRenderTarget::Image(image_target) => images
+                .get(&image_target.handle)
+                .map(|image| RenderTargetInfo {
                     physical_size: image.size(),
                     scale_factor: image_target.scale_factor.0,
                 })
-            }
-            NormalizedRenderTarget::TextureView(id) => {
-                manual_texture_views.get(id).map(|tex| RenderTargetInfo {
+                .ok_or(MissingRenderTargetInfoError::Image {
+                    image: image_target.handle.id(),
+                }),
+            NormalizedRenderTarget::TextureView(id) => manual_texture_views
+                .get(id)
+                .map(|tex| RenderTargetInfo {
                     physical_size: tex.size,
                     scale_factor: 1.0,
                 })
-            }
+                .ok_or(MissingRenderTargetInfoError::TextureView { texture_view: *id }),
         }
     }
 
@@ -265,6 +272,18 @@ impl NormalizedRenderTargetExt for NormalizedRenderTarget {
     }
 }
 
+#[derive(Debug, thiserror::Error)]
+pub enum MissingRenderTargetInfoError {
+    #[error("RenderTarget::Window missing ({window:?}): Make sure the provided entity has a Window component.")]
+    Window { window: Entity },
+    #[error("RenderTarget::Image missing ({image:?}): Make sure the Image's usages include RenderAssetUsages::MAIN_WORLD.")]
+    Image { image: AssetId<Image> },
+    #[error("RenderTarget::TextureView missing ({texture_view:?}): make sure the texture view handle was not removed.")]
+    TextureView {
+        texture_view: ManualTextureViewHandle,
+    },
+}
+
 /// System in charge of updating a [`Camera`] when its window or projection changes.
 ///
 /// The system detects window creation, resize, and scale factor change events to update the camera
@@ -287,7 +306,7 @@ pub fn camera_system(
     images: Res<Assets<Image>>,
     manual_texture_views: Res<ManualTextureViews>,
     mut cameras: Query<(&mut Camera, &mut Projection)>,
-) {
+) -> Result<(), BevyError> {
     let primary_window = primary_window.iter().next();
 
     let mut changed_window_ids = <HashSet<_>>::default();
@@ -320,25 +339,23 @@ pub fn camera_system(
                 || camera.computed.old_viewport_size != viewport_size
                 || camera.computed.old_sub_camera_view != camera.sub_camera_view)
         {
-            let new_computed_target_info =
-                normalized_target.get_render_target_info(windows, &images, &manual_texture_views);
+            let new_computed_target_info = normalized_target.get_render_target_info(
+                windows,
+                &images,
+                &manual_texture_views,
+            )?;
             // Check for the scale factor changing, and resize the viewport if needed.
             // This can happen when the window is moved between monitors with different DPIs.
             // Without this, the viewport will take a smaller portion of the window moved to
             // a higher DPI monitor.
             if normalized_target.is_changed(&scale_factor_changed_window_ids, &HashSet::default())
-                && let (Some(new_scale_factor), Some(old_scale_factor)) = (
-                    new_computed_target_info
-                        .as_ref()
-                        .map(|info| info.scale_factor),
-                    camera
-                        .computed
-                        .target_info
-                        .as_ref()
-                        .map(|info| info.scale_factor),
-                )
+                && let Some(old_scale_factor) = camera
+                    .computed
+                    .target_info
+                    .as_ref()
+                    .map(|info| info.scale_factor)
             {
-                let resize_factor = new_scale_factor / old_scale_factor;
+                let resize_factor = new_computed_target_info.scale_factor / old_scale_factor;
                 if let Some(ref mut viewport) = camera.viewport {
                     let resize = |vec: UVec2| (vec.as_vec2() * resize_factor).as_uvec2();
                     viewport.physical_position = resize(viewport.physical_position);
@@ -350,12 +367,9 @@ pub fn camera_system(
             // arguments due to a sudden change on the window size to a lower value.
             // If the size of the window is lower, the viewport will match that lower value.
             if let Some(viewport) = &mut camera.viewport {
-                let target_info = &new_computed_target_info;
-                if let Some(target) = target_info {
-                    viewport.clamp_to_size(target.physical_size);
-                }
+                viewport.clamp_to_size(new_computed_target_info.physical_size);
             }
-            camera.computed.target_info = new_computed_target_info;
+            camera.computed.target_info = Some(new_computed_target_info);
             if let Some(size) = camera.logical_viewport_size()
                 && size.x != 0.0
                 && size.y != 0.0
@@ -376,6 +390,7 @@ pub fn camera_system(
             camera.computed.old_sub_camera_view = camera.sub_camera_view;
         }
     }
+    Ok(())
 }
 
 #[derive(Component, Debug)]

release-content/migration-guides/render_target_info_error.md

@@ -0,0 +1,11 @@
+---
+title: "RenderTarget error handling"
+pull_requests: [20503]
+---
+
+`NormalizedRenderTargetExt::get_render_target_info` now returns a `Result`,
+with the `Err` variant indicating which render target (image, window, etc)
+failed to load its metadata.
+
+This should mostly be treated as a hard error, since it indicates the rendering
+state of the app is broken.