AssetMetaMode (#10623)

# Objective

Fixes #10157

## Solution

Add `AssetMetaCheck` resource which can configure when/if asset meta
files will be read:

```rust
app
  // Never attempts to look up meta files. The default meta configuration will be used for each asset.
  .insert_resource(AssetMetaCheck::Never)
  .add_plugins(DefaultPlugins)
```


This serves as a band-aid fix for the issue with wasm's
`HttpWasmAssetReader` creating a bunch of requests for non-existent
meta, which can slow down asset loading (by waiting for the 404
response) and creates a bunch of noise in the logs. This also provides a
band-aid fix for the more serious issue of itch.io deployments returning
403 responses, which results in full failure of asset loads.

If users don't want to include meta files for all deployed assets for
web builds, and they aren't using meta files at all, they should set
this to `AssetMetaCheck::Never`.

If users do want to include meta files for specific assets, they can use
`AssetMetaCheck::Paths`, which will only look up meta for those paths.

Currently, this defaults to `AssetMetaCheck::Always`, which makes this
fully non-breaking for the `0.12.1` release. _**However it _is_ worth
discussing making this `AssetMetaCheck::Never` by default**_, given that
I doubt most people are using meta files without the Asset Processor
enabled. This would be a breaking change, but it would make WASM / Itch
deployments work by default, which is a pretty big win imo. The downside
is that people using meta files _without_ processing would need to
manually enable `AssetMetaCheck::Always`, which is also suboptimal.

When in `AssetMetaCheck::Processed`, the meta check resource is ignored,
as processing requires asset meta files to function.

In general, I don't love adding this knob as things should ideally "just
work" in all cases. But this is the reality of the current situation.

---

## Changelog

- Added `AssetMetaCheck` resource, which can configure when/if asset
meta files will be read

crates/bevy_asset/src/lib.rs

@@ -45,10 +45,12 @@ use bevy_app::{App, First, MainScheduleOrder, Plugin, PostUpdate};
 use bevy_ecs::{
     reflect::AppTypeRegistry,
     schedule::{IntoSystemConfigs, IntoSystemSetConfigs, ScheduleLabel, SystemSet},
+    system::Resource,
     world::FromWorld,
 };
 use bevy_log::error;
 use bevy_reflect::{FromReflect, GetTypeRegistration, Reflect, TypePath};
+use bevy_utils::HashSet;
 use std::{any::TypeId, sync::Arc};
 
 /// Provides "asset" loading and processing functionality. An [`Asset`] is a "runtime value" that is loaded from an [`AssetSource`],
@@ -97,6 +99,20 @@ pub enum AssetMode {
     Processed,
 }
 
+/// Configures how / if meta files will be checked. If an asset's meta file is not checked, the default meta for the asset
+/// will be used.
+// TODO: To avoid breaking Bevy 0.12 users in 0.12.1, this is a Resource. In Bevy 0.13 this should be changed to a field on AssetPlugin (if it is still needed).
+#[derive(Debug, Default, Clone, Resource)]
+pub enum AssetMetaCheck {
+    /// Always check if assets have meta files. If the meta does not exist, the default meta will be used.
+    #[default]
+    Always,
+    /// Only look up meta files for the provided paths. The default meta will be used for any paths not contained in this set.
+    Paths(HashSet<AssetPath<'static>>),
+    /// Never check if assets have meta files and always use the default meta. If meta files exist, they will be ignored and the default meta will be used.
+    Never,
+}
+
 impl Default for AssetPlugin {
     fn default() -> Self {
         Self {
@@ -139,9 +155,16 @@ impl Plugin for AssetPlugin {
                 AssetMode::Unprocessed => {
                     let mut builders = app.world.resource_mut::<AssetSourceBuilders>();
                     let sources = builders.build_sources(watch, false);
-                    app.insert_resource(AssetServer::new(
+                    let meta_check = app
+                        .world
+                        .get_resource::<AssetMetaCheck>()
+                        .cloned()
+                        .unwrap_or_else(AssetMetaCheck::default);
+
+                    app.insert_resource(AssetServer::new_with_meta_check(
                         sources,
                         AssetServerMode::Unprocessed,
+                        meta_check,
                         watch,
                     ));
                 }
@@ -157,6 +180,7 @@ impl Plugin for AssetPlugin {
                             sources,
                             processor.server().data.loaders.clone(),
                             AssetServerMode::Processed,
+                            AssetMetaCheck::Always,
                             watch,
                         ))
                         .insert_resource(processor)
@@ -166,9 +190,10 @@ impl Plugin for AssetPlugin {
                     {
                         let mut builders = app.world.resource_mut::<AssetSourceBuilders>();
                         let sources = builders.build_sources(false, watch);
-                        app.insert_resource(AssetServer::new(
+                        app.insert_resource(AssetServer::new_with_meta_check(
                             sources,
                             AssetServerMode::Processed,
+                            AssetMetaCheck::Always,
                             watch,
                         ));
                     }

crates/bevy_asset/src/processor/mod.rs

@@ -13,7 +13,7 @@ use crate::{
         get_asset_hash, get_full_asset_hash, AssetAction, AssetActionMinimal, AssetHash, AssetMeta,
         AssetMetaDyn, AssetMetaMinimal, ProcessedInfo, ProcessedInfoMinimal,
     },
-    AssetLoadError, AssetPath, AssetServer, AssetServerMode, DeserializeMetaError,
+    AssetLoadError, AssetMetaCheck, AssetPath, AssetServer, AssetServerMode, DeserializeMetaError,
     MissingAssetLoaderForExtensionError,
 };
 use bevy_ecs::prelude::*;
@@ -72,7 +72,12 @@ impl AssetProcessor {
         // The asset processor uses its own asset server with its own id space
         let mut sources = source.build_sources(false, false);
         sources.gate_on_processor(data.clone());
-        let server = AssetServer::new(sources, AssetServerMode::Processed, false);
+        let server = AssetServer::new_with_meta_check(
+            sources,
+            AssetServerMode::Processed,
+            AssetMetaCheck::Always,
+            false,
+        );
         Self { server, data }
     }
 

crates/bevy_asset/src/server/mod.rs

@@ -12,7 +12,7 @@ use crate::{
         MetaTransform, Settings,
     },
     path::AssetPath,
-    Asset, AssetEvent, AssetHandleProvider, AssetId, Assets, DeserializeMetaError,
+    Asset, AssetEvent, AssetHandleProvider, AssetId, AssetMetaCheck, Assets, DeserializeMetaError,
     ErasedLoadedAsset, Handle, LoadedUntypedAsset, UntypedAssetId, UntypedHandle,
 };
 use bevy_ecs::prelude::*;
@@ -54,6 +54,7 @@ pub(crate) struct AssetServerData {
     asset_event_receiver: Receiver<InternalAssetEvent>,
     sources: AssetSources,
     mode: AssetServerMode,
+    meta_check: AssetMetaCheck,
 }
 
 /// The "asset mode" the server is currently in.
@@ -69,13 +70,37 @@ impl AssetServer {
     /// Create a new instance of [`AssetServer`]. If `watch_for_changes` is true, the [`AssetReader`] storage will watch for changes to
     /// asset sources and hot-reload them.
     pub fn new(sources: AssetSources, mode: AssetServerMode, watching_for_changes: bool) -> Self {
-        Self::new_with_loaders(sources, Default::default(), mode, watching_for_changes)
+        Self::new_with_loaders(
+            sources,
+            Default::default(),
+            mode,
+            AssetMetaCheck::Always,
+            watching_for_changes,
+        )
+    }
+
+    /// Create a new instance of [`AssetServer`]. If `watch_for_changes` is true, the [`AssetReader`] storage will watch for changes to
+    /// asset sources and hot-reload them.
+    pub fn new_with_meta_check(
+        sources: AssetSources,
+        mode: AssetServerMode,
+        meta_check: AssetMetaCheck,
+        watching_for_changes: bool,
+    ) -> Self {
+        Self::new_with_loaders(
+            sources,
+            Default::default(),
+            mode,
+            meta_check,
+            watching_for_changes,
+        )
     }
 
     pub(crate) fn new_with_loaders(
         sources: AssetSources,
         loaders: Arc<RwLock<AssetLoaders>>,
         mode: AssetServerMode,
+        meta_check: AssetMetaCheck,
         watching_for_changes: bool,
     ) -> Self {
         let (asset_event_sender, asset_event_receiver) = crossbeam_channel::unbounded();
@@ -85,6 +110,7 @@ impl AssetServer {
             data: Arc::new(AssetServerData {
                 sources,
                 mode,
+                meta_check,
                 asset_event_sender,
                 asset_event_receiver,
                 loaders,
@@ -811,44 +837,57 @@ impl AssetServer {
             AssetServerMode::Processed { .. } => source.processed_reader()?,
         };
         let reader = asset_reader.read(asset_path.path()).await?;
-        match asset_reader.read_meta_bytes(asset_path.path()).await {
-            Ok(meta_bytes) => {
-                // TODO: this isn't fully minimal yet. we only need the loader
-                let minimal: AssetMetaMinimal = ron::de::from_bytes(&meta_bytes).map_err(|e| {
-                    AssetLoadError::DeserializeMeta {
-                        path: asset_path.clone_owned(),
-                        error: Box::new(DeserializeMetaError::DeserializeMinimal(e)),
-                    }
-                })?;
-                let loader_name = match minimal.asset {
-                    AssetActionMinimal::Load { loader } => loader,
-                    AssetActionMinimal::Process { .. } => {
-                        return Err(AssetLoadError::CannotLoadProcessedAsset {
-                            path: asset_path.clone_owned(),
-                        })
-                    }
-                    AssetActionMinimal::Ignore => {
-                        return Err(AssetLoadError::CannotLoadIgnoredAsset {
+        let read_meta = match &self.data.meta_check {
+            AssetMetaCheck::Always => true,
+            AssetMetaCheck::Paths(paths) => paths.contains(asset_path),
+            AssetMetaCheck::Never => false,
+        };
+
+        if read_meta {
+            match asset_reader.read_meta_bytes(asset_path.path()).await {
+                Ok(meta_bytes) => {
+                    // TODO: this isn't fully minimal yet. we only need the loader
+                    let minimal: AssetMetaMinimal =
+                        ron::de::from_bytes(&meta_bytes).map_err(|e| {
+                            AssetLoadError::DeserializeMeta {
+                                path: asset_path.clone_owned(),
+                                error: Box::new(DeserializeMetaError::DeserializeMinimal(e)),
+                            }
+                        })?;
+                    let loader_name = match minimal.asset {
+                        AssetActionMinimal::Load { loader } => loader,
+                        AssetActionMinimal::Process { .. } => {
+                            return Err(AssetLoadError::CannotLoadProcessedAsset {
+                                path: asset_path.clone_owned(),
+                            })
+                        }
+                        AssetActionMinimal::Ignore => {
+                            return Err(AssetLoadError::CannotLoadIgnoredAsset {
+                                path: asset_path.clone_owned(),
+                            })
+                        }
+                    };
+                    let loader = self.get_asset_loader_with_type_name(&loader_name).await?;
+                    let meta = loader.deserialize_meta(&meta_bytes).map_err(|e| {
+                        AssetLoadError::DeserializeMeta {
                             path: asset_path.clone_owned(),
-                        })
-                    }
-                };
-                let loader = self.get_asset_loader_with_type_name(&loader_name).await?;
-                let meta = loader.deserialize_meta(&meta_bytes).map_err(|e| {
-                    AssetLoadError::DeserializeMeta {
-                        path: asset_path.clone_owned(),
-                        error: Box::new(e),
-                    }
-                })?;
+                            error: Box::new(e),
+                        }
+                    })?;
 
-                Ok((meta, loader, reader))
-            }
-            Err(AssetReaderError::NotFound(_)) => {
-                let loader = self.get_path_asset_loader(asset_path).await?;
-                let meta = loader.default_meta();
-                Ok((meta, loader, reader))
+                    Ok((meta, loader, reader))
+                }
+                Err(AssetReaderError::NotFound(_)) => {
+                    let loader = self.get_path_asset_loader(asset_path).await?;
+                    let meta = loader.default_meta();
+                    Ok((meta, loader, reader))
+                }
+                Err(err) => Err(err.into()),
             }
-            Err(err) => Err(err.into()),
+        } else {
+            let loader = self.get_path_asset_loader(asset_path).await?;
+            let meta = loader.default_meta();
+            Ok((meta, loader, reader))
         }
     }