Restructure the logic in AssetServer::load_internal to avoid using weak handles. (#19634)

# Objective

- A step towards #19024.
- The logic here was kinda complex before.

## Solution

- I've restructured the logic here while preserving the behavior (as far
as I can tell).
- We no longer return the handle if it was passed in. The caller should
already have access to it, and the returned handle will be a weak
handle, not a strong handle (which can cause issues). This prevents us
from needing weak handles at all here.
- I verified the callers do not need the return value. The only callsite
that needs the returned handle does not pass in the input_handle
argument.

## Testing

- CI

crates/bevy_asset/src/server/mod.rs

@@ -553,7 +553,9 @@ impl AssetServer {
         path: impl Into<AssetPath<'a>>,
     ) -> Result<UntypedHandle, AssetLoadError> {
         let path: AssetPath = path.into();
-        self.load_internal(None, path, false, None).await
+        self.load_internal(None, path, false, None)
+            .await
+            .map(|h| h.expect("handle must be returned, since we didn't pass in an input handle"))
     }
 
     pub(crate) fn load_unknown_type_with_meta_transform<'a>(
@@ -643,21 +645,25 @@ impl AssetServer {
 
     /// Performs an async asset load.
     ///
-    /// `input_handle` must only be [`Some`] if `should_load` was true when retrieving `input_handle`. This is an optimization to
+    /// `input_handle` must only be [`Some`] if `should_load` was true when retrieving
-    /// avoid looking up `should_load` twice, but it means you _must_ be sure a load is necessary when calling this function with [`Some`].
+    /// `input_handle`. This is an optimization to avoid looking up `should_load` twice, but it
+    /// means you _must_ be sure a load is necessary when calling this function with [`Some`].
+    ///
+    /// Returns the handle of the asset if one was retrieved by this function. Otherwise, may return
+    /// [`None`].
     async fn load_internal<'a>(
         &self,
-        mut input_handle: Option<UntypedHandle>,
+        input_handle: Option<UntypedHandle>,
         path: AssetPath<'a>,
         force: bool,
         meta_transform: Option<MetaTransform>,
-    ) -> Result<UntypedHandle, AssetLoadError> {
+    ) -> Result<Option<UntypedHandle>, AssetLoadError> {
-        let asset_type_id = input_handle.as_ref().map(UntypedHandle::type_id);
+        let input_handle_type_id = input_handle.as_ref().map(UntypedHandle::type_id);
 
         let path = path.into_owned();
         let path_clone = path.clone();
         let (mut meta, loader, mut reader) = self
-            .get_meta_loader_and_reader(&path_clone, asset_type_id)
+            .get_meta_loader_and_reader(&path_clone, input_handle_type_id)
             .await
             .inspect_err(|e| {
                 // if there was an input handle, a "load" operation has already started, so we must produce a "failure" event, if
@@ -674,76 +680,90 @@ impl AssetServer {
         if let Some(meta_transform) = input_handle.as_ref().and_then(|h| h.meta_transform()) {
             (*meta_transform)(&mut *meta);
         }
-        // downgrade the input handle so we don't keep the asset alive just because we're loading it
-        // note we can't just pass a weak handle in, as only strong handles contain the asset meta transform
-        input_handle = input_handle.map(|h| h.clone_weak());
 
-        // This contains Some(UntypedHandle), if it was retrievable
+        let asset_id; // The asset ID of the asset we are trying to load.
-        // If it is None, that is because it was _not_ retrievable, due to
+        let fetched_handle; // The handle if one was looked up/created.
-        //    1. The handle was not already passed in for this path, meaning we can't just use that
+        let should_load; // Whether we need to load the asset.
-        //    2. The asset has not been loaded yet, meaning there is no existing Handle for it
+        if let Some(input_handle) = input_handle {
-        //    3. The path has a label, meaning the AssetLoader's root asset type is not the path's asset type
+            asset_id = Some(input_handle.id());
-        //
+            // In this case, we intentionally drop the input handle so we can cancel loading the
-        // In the None case, the only course of action is to wait for the asset to load so we can allocate the
+            // asset if the handle gets dropped (externally) before it finishes loading.
-        // handle for that type.
+            fetched_handle = None;
-        //
+            // The handle was passed in, so the "should_load" check was already done.
-        // TODO: Note that in the None case, multiple asset loads for the same path can happen at the same time
+            should_load = true;
-        // (rather than "early out-ing" in the "normal" case)
+        } else {
-        // This would be resolved by a universal asset id, as we would not need to resolve the asset type
+            // TODO: multiple asset loads for the same path can happen at the same time (rather than
-        // to generate the ID. See this issue: https://github.com/bevyengine/bevy/issues/10549
+            // "early out-ing" in the "normal" case). This would be resolved by a universal asset
-        let handle_result = match input_handle {
+            // id, as we would not need to resolve the asset type to generate the ID. See this
-            Some(handle) => {
+            // issue: https://github.com/bevyengine/bevy/issues/10549
-                // if a handle was passed in, the "should load" check was already done
-                Some((handle, true))
-            }
-            None => {
-                let mut infos = self.data.infos.write();
-                let result = infos.get_or_create_path_handle_internal(
-                    path.clone(),
-                    path.label().is_none().then(|| loader.asset_type_id()),
-                    HandleLoadingMode::Request,
-                    meta_transform,
-                );
-                unwrap_with_context(result, Either::Left(loader.asset_type_name()))
-            }
-        };
 
-        let handle = if let Some((handle, should_load)) = handle_result {
+            let mut infos = self.data.infos.write();
-            if path.label().is_none() && handle.type_id() != loader.asset_type_id() {
+            let result = infos.get_or_create_path_handle_internal(
+                path.clone(),
+                path.label().is_none().then(|| loader.asset_type_id()),
+                HandleLoadingMode::Request,
+                meta_transform,
+            );
+            match unwrap_with_context(result, Either::Left(loader.asset_type_name())) {
+                // We couldn't figure out the correct handle without its type ID (which can only
+                // happen if we are loading a subasset).
+                None => {
+                    // We don't know the expected type since the subasset may have a different type
+                    // than the "root" asset (which is the type the loader will load).
+                    asset_id = None;
+                    fetched_handle = None;
+                    // If we couldn't find an appropriate handle, then the asset certainly needs to
+                    // be loaded.
+                    should_load = true;
+                }
+                Some((handle, result_should_load)) => {
+                    asset_id = Some(handle.id());
+                    fetched_handle = Some(handle);
+                    should_load = result_should_load;
+                }
+            }
+        }
+        // Verify that the expected type matches the loader's type.
+        if let Some(asset_type_id) = asset_id.map(|id| id.type_id()) {
+            // If we are loading a subasset, then the subasset's type almost certainly doesn't match
+            // the loader's type - and that's ok.
+            if path.label().is_none() && asset_type_id != loader.asset_type_id() {
                 error!(
                     "Expected {:?}, got {:?}",
-                    handle.type_id(),
+                    asset_type_id,
                     loader.asset_type_id()
                 );
                 return Err(AssetLoadError::RequestedHandleTypeMismatch {
                     path: path.into_owned(),
-                    requested: handle.type_id(),
+                    requested: asset_type_id,
                     actual_asset_name: loader.asset_type_name(),
                     loader_name: loader.type_name(),
                 });
             }
-            if !should_load && !force {
+        }
-                return Ok(handle);
+        // Bail out earlier if we don't need to load the asset.
-            }
+        if !should_load && !force {
-            Some(handle)
+            return Ok(fetched_handle);
-        } else {
+        }
-            None
-        };
-        // if the handle result is None, we definitely need to load the asset
 
-        let (base_handle, base_path) = if path.label().is_some() {
+        // We don't actually need to use _base_handle, but we do need to keep the handle alive.
+        // Dropping it would cancel the load of the base asset, which would make the load of this
+        // subasset never complete.
+        let (base_asset_id, _base_handle, base_path) = if path.label().is_some() {
             let mut infos = self.data.infos.write();
             let base_path = path.without_label().into_owned();
-            let (base_handle, _) = infos.get_or_create_path_handle_erased(
+            let base_handle = infos
-                base_path.clone(),
+                .get_or_create_path_handle_erased(
-                loader.asset_type_id(),
+                    base_path.clone(),
-                Some(loader.asset_type_name()),
+                    loader.asset_type_id(),
-                HandleLoadingMode::Force,
+                    Some(loader.asset_type_name()),
-                None,
+                    HandleLoadingMode::Force,
-            );
+                    None,
-            (base_handle, base_path)
+                )
+                .0;
+            (base_handle.id(), Some(base_handle), base_path)
         } else {
-            (handle.clone().unwrap(), path.clone())
+            (asset_id.unwrap(), None, path.clone())
         };
 
         match self
@@ -760,7 +780,7 @@ impl AssetServer {
             Ok(loaded_asset) => {
                 let final_handle = if let Some(label) = path.label_cow() {
                     match loaded_asset.labeled_assets.get(&label) {
-                        Some(labeled_asset) => labeled_asset.handle.clone(),
+                        Some(labeled_asset) => Some(labeled_asset.handle.clone()),
                         None => {
                             let mut all_labels: Vec<String> = loaded_asset
                                 .labeled_assets
@@ -776,16 +796,15 @@ impl AssetServer {
                         }
                     }
                 } else {
-                    // if the path does not have a label, the handle must exist at this point
+                    fetched_handle
-                    handle.unwrap()
                 };
 
-                self.send_loaded_asset(base_handle.id(), loaded_asset);
+                self.send_loaded_asset(base_asset_id, loaded_asset);
                 Ok(final_handle)
             }
             Err(err) => {
                 self.send_asset_event(InternalAssetEvent::Failed {
-                    id: base_handle.id(),
+                    id: base_asset_id,
                     error: err.clone(),
                     path: path.into_owned(),
                 });