Allow removing and reloading assets with live handles

[cart]

Objective

Fixes #10444

Currently manually removing an asset prevents it from being reloaded while there are still active handles. Doing so will result in a panic, because the storage entry has been marked as "empty / None" but the ID is still assumed to be active by the asset server.

Patterns like images.remove() -> asset_server.reload() and images.remove() -> images.insert() would fail if the handle was still alive.

Solution

Most of the groundwork for this was already laid in Bevy Asset V2. This is largely just a matter of splitting out remove into two separate operations:

• remove_dropped: remove the stored asset, invalidate the internal Assets entry (preventing future insertions with the old id), and recycle the id
• remove_still_alive: remove the stored asset, but leave the entry otherwise untouched (and dont recycle the id).

remove_still_alive and insert can be called any number of times (in any order) for an id until remove_dropped has been called, which will invalidate the id.

From a user-facing perspective, there are no API changes and this is non breaking. The public Assets::remove will internally call remove_still_alive. remove_dropped can only be called by the internal "handle management" system.

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Changelog

• Fix a bug preventing Assets::remove from blocking future inserts for a specific AssetIndex.

[JMS55]

I think there's still the existing issue where the last strong handle drop will infinitely loop for assets that have already been removed, as contains() will always return false, I believe. I think we need to instead query the AssetServer info for this check. I did that in this PR: #10520

bevy/crates/bevy_asset/src/assets.rs

Lines 500 to 503 in 5450faf

	if !assets.contains(id.typed()) {
	not_ready.push(drop_event);
	continue;
	}

[cart]

@JMS55 good call / agreed. Just pushed a fix.